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HTTP/2 200 server: GitHub.com content-type: text/html; charset=utf-8 last-modified: Wed, 17 Sep 2025 08:46:20 GMT access-control-allow-origin: * etag: W/"68ca755c-310f7" expires: Tue, 14 Oct 2025 20:33:36 GMT cache-control: max-age=600 content-encoding: gzip x-proxy-cache: MISS x-github-request-id: A365:4FF69:E6A2E:10FFA0:68EEB147 accept-ranges: bytes date: Tue, 14 Oct 2025 20:23:36 GMT via: 1.1 varnish age: 0 x-served-by: cache-bom-vanm7210020-BOM x-cache: MISS x-cache-hits: 0 x-timer: S1760473416.939676,VS0,VE317 vary: Accept-Encoding x-fastly-request-id: ae5e9d6ef40a23ec641413c6cca8a8761108b1b6 content-length: 39077 Web of Things (WoT) Scripting API

The Web of Things is made of entities (Things) that can describe their capabilities in a machine-interpretable Thing Description (TD) and expose these capabilities through the WoT Interface, that is, network interactions modeled as Properties (for reading and writing values), Actions (to execute remote procedures with or without return values) and Events (for signaling notifications).

The main Web of Things (WoT) concepts are described in the [[[wot-architecture11]]] specification.

Scripting is an optional building block in WoT and it is typically used in gateways or browsers that are able to run a WoT Runtime and script management, providing a convenient way to extend WoT support to new types of endpoints and implement WoT applications such as TD Directory.

This specification describes an application programming interface (API) representing the WoT Interface that allows scripts to discover, operate Things and to expose locally defined Things characterized by WoT Interactions specified by a script.

The APIs defined in this document deliberately follow the [[[wot-thing-description11]]] specification closely. It is possible to implement more abstract APIs on top of them, or implementing directly the WoT network facing interface (i.e. the WoT Interface).

This specification is implemented at least by the Eclipse Thingweb project also known as node-wot, which is considered the reference open source implementation at the moment. Check its source code, including examples.

Implementers need to be aware that this specification is considered unstable. Vendors interested in implementing this specification before it eventually reaches the Candidate Recommendation phase should subscribe to the repository and take part in the discussions.

Please contribute to this draft using the GitHub Issues page of the WoT Scripting API repository. For feedback on security and privacy considerations, please use the WoT Security and Privacy Issues.

Introduction

WoT provides layered interoperability based on how Things are used: "consumed" and "exposed", as defined in the [[[wot-architecture11]]] terminology.

By consuming a TD, a client Thing creates a local runtime resource model that allows accessing the Properties, Actions and Events exposed by the server Thing on a remote device.

Exposing a Thing requires:

defining a Thing Description (TD),
then instantiating a software stack that implements the WoT Interface specified by the TD in order to serve requests for accessing the exposed Properties, Actions and Events,
then eventually publishing the Thing Description (for instance to a TD Directory for easier discovery).

This specification describes how to expose and consume Things by a script. Also, it defines a generic API for Thing discovery.

Typically scripts are meant to be used on bridges or gateways that expose and control simpler devices as WoT Things and have means to handle (e.g. install, uninstall, update etc.) and run scripts.

This specification does not make assumptions on how the WoT Runtime handles and runs scripts, including single or multiple tenancy, script deployment and lifecycle management. The API already supports the generic mechanisms that make it possible to implement script management, for instance by exposing a manager Thing whose Actions (action handlers) implement script lifecycle management operations.

Use Case Scenarios

The business use cases listed in the [[wot-usecases]] document may be implemented using this API, based on the scripting use case scenarios described here.

Consuming a Thing

Consume a TD, i.e. create a programmatic object from a Thing Description that exposes WoT Interactions:
- Read the value of a Property or a set of properties.
- Set the value of a Property or a set of properties.
- Observe value changes of a Property.
- Invoke an Action.
- Observe WoT Events emitted by the Thing.
- Introspect the Thing Description.

Exposing a Thing

Exposing the Thing includes generating the protocol bindings in order to access lower level functionality.
Create a local Thing to be exposed, based on Thing Description.
The following use cases can be implemented before creating the Thing by editing the Thing Description:
- Add a Property definition to the Thing.
- Remove a Property definition from the Thing.
- Add an Action definition to the Thing.
- Remove an Action definition from the Thing.
- Add a WoT Event definition to the Thing.
- Remove a WoT Event definition from the Thing.
After evaluating dynamic modifications to Thing Descriptions through several versions of this API, the editors concluded that the simplest way to represent these use cases is to take an existing TD, modify it (i.e. add or remove definitions) and then create a new Thing based on the modified TD.
Emit a WoT Event, i.e. notify all subscribed listeners.
Register service handlers for external requests:
- to retrieve a Property value;
- to update a Property value;
- to observe a Property;
- to unobserve a Property;
- to invoke an Action: take the parameters from the request, execute the defined action, and return the result;
- to subscribe to an Event;
- to unsubscribe from an Event.

Discovery

Discover Things in a network by sending a broadcast request.
Discover Things running in the local WoT Runtime.
Discover nearby Things, for instance connected by NFC or Bluetooth, or within a geo-fence.
Discover Things by sending a discovery request to a given TD Directory.
Discover Things filtered by filters defined on Thing Descriptions
Discover Things filtered by semantic queries.
Stop or suppress an ongoing discovery process.
Optionally specify a timeout to the discovery process after which it is stopped/suppressed.

This specification used to be a Working Draft which was expected to become a W3C Recommendation. However, it is now a WG Note which contains informative statements only. Therefore we need to consider how to deal with the description within this Conformance section.

This specification describes the conformance criteria for the following classes of [= user agent =] (UA).

Due to requirements of small embedded implementations, splitting WoT client and server interfaces was needed. Then, discovery is a distributed application, but typical scenarios have been covered by a generic discovery API in this specification. This resulted in using 3 conformance classes for a UA that implements this API, one for client, one for server, and one for discovery. An application that uses this API can introspect for the presence of the consume(), produce() and discover() methods on the WoT API object in order to determine which conformance class the UA implements.

WoT Consumer UA: Implementations of this conformance class MUST implement the {{ConsumedThing}} interface and the consume() method on the WoT API object.
WoT Producer UA: Implementations of this conformance class MUST implement {{ExposedThing}} interface and the produce() method on the WoT API object.
WoT Discovery UA: Implementations of this conformance class MUST implement the ThingDiscoveryProcess interface, the discover() method, the exploreDirectory() method, and the requestThingDescription() method on the WoT API object.

These conformance classes MAY be implemented in a single UA.

This specification can be used for implementing the WoT Scripting API in multiple programming languages. The interface definitions are specified in [[!WEBIDL]].

The UA may be implemented in the browser, or in a separate runtime environment, such as Node.js or in small embedded runtimes.

Implementations that use ECMAScript executed in a browser to implement the APIs defined in this document MUST implement them in a manner consistent with the ECMAScript Bindings defined in the Web IDL specification [[!WEBIDL]].

Implementations that use TypeScript or ECMAScript in a runtime to implement the APIs defined in this document MUST implement them in a manner consistent with the TypeScript Bindings defined in the TypeScript specification [[!TYPESCRIPT]].

Terminology and conventions

The generic WoT terminology is defined in [[!wot-architecture11]]: Thing, Thing Description (in short TD), Partial TD, Web of Things (in short WoT), WoT Interface, Protocol Bindings, WoT Runtime, Consuming a Thing Description, TD Directory, Property, Action, Event, DataSchema, Form, SecurityScheme, NoSecurityScheme etc.

WoT Interaction is a synonym for Interaction Affordance. An Interaction Affordance (or shortly, affordance) is the term used in [[!wot-thing-description11]] when referring to Thing capabilities, as explained in TD issue 282. However, this term is not well understood outside the TD semantic context. Hence for the sake of readability, this document will use the previous term WoT interaction or, simply, interaction instead.

WoT network interface synonym for WoT Interface.

JSON Schema is defined in these specifications.

{{Promise}}, Error, JSON, JSON.stringify, JSON.parse, internal method and internal slot are defined in [[!ECMASCRIPT]].

The ThingDescription type

      typedef object ThingDescription;

Represents a Thing Description (TD) as defined in [[!wot-thing-description11]]. It is expected to be a parsed JSON object that is validated using JSON Schema validation.

Requesting a Thing Description

Requesting a TD given a URL should be done with the requestThingDescription() method. Alternatively, external methods, such as the Fetch API or an HTTP client library, can be used.

        try {
          const td = await requestThingDescription('https://tds.mythings.biz/sensor11');
          const thing = await WoT.consume(td);
          console.log("Thing name: " + thing.getThingDescription().title);
        } catch (err) {
          console.log("Requesting TD failed", err.message);
        }

Expanding a Thing Description

Note that the [[[wot-thing-description11]]] specification allows using a shortened Thing Description by the means of defaults and requiring clients to expand them with default values specified in the [[[wot-thing-description11]]] specification for the properties that are not explicitly defined in a given TD.

To expand a TD given |td:ThingDescription|, run the following steps:

For each item in the TD default values table from [[!wot-thing-description11]], if the term is not defined in |td|, add the term definition with the default value specified in [[!wot-thing-description11]].

Validating a Thing Description

The [[!wot-thing-description11]] specification defines how a TD should be validated. Therefore, this API expects the {{ThingDescription}} objects be validated before used as parameters. This specification defines a basic TD validation as follows.

To validate a TD given |td:ThingDescription|, run the following steps:

If JSON Schema validation fails on |td|, [= exception/throw =] a {{"TypeError"}} and stop.

Additional steps may be added to fill the default values of mandatory fields.

The WOT namespace

Defines the WoT API object as a singleton and contains the API methods, grouped by conformance classes.

    [SecureContext, Exposed=(Window,Worker)]
    namespace WOT {
      // methods defined in UA conformance classes
    };

The consume() method

      partial namespace WOT {
        Promise<ConsumedThing> consume(ThingDescription td);
      };

Belongs to the WoT Consumer conformance class. Expects an |td:ThingDescription| argument and returns a {{Promise}} that resolves with a {{ConsumedThing}} object that represents a client interface to operate with the Thing. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
Let |thing:ConsumedThing| be a new {{ConsumedThing}} object constructed from |td|.
Set up the WoT Interactions based on introspecting td as explained in [[!wot-thing-description11]] and [[!wot-binding-templates]]. Make a request to the underlying platform to initialize the Protocol Bindings.
Implementations encapsulate the complexity of how to use the Protocol Bindings for implementing WoT interactions. In the future elements of that could be standardized.
[=Resolve=] |promise| with |thing|.

The produce() method

      typedef object ExposedThingInit;
      partial namespace WOT {
        Promise<ExposedThing> produce(ExposedThingInit init);
      };

Belongs to the WoT Producer conformance class. Expects a |init:ExposedThingInit| argument and returns a {{Promise}} that resolves with an {{ExposedThing}} object that extends {{ConsumedThing}} with a server interface, i.e. the ability to define request handlers. The |init:ExposedThingInit| object is an instance of the ExposedThingInit type. Specifically, an ExposedThingInit value is a dictionary used for the initialization of an ExposedThing and it represents a Partial TD as described in the [[!wot-architecture11]]. As such, it has the same structure of a Thing Description but it may omit some information. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
Let |thing:ExposedThing| be a new {{ExposedThing}} object constructed with |init|.
[=Resolve=] |promise| with |thing|.

Expand an ExposedThingInit

To expand an ExposedThingInit given |init:ExposedThingInit| and obtain a valid |td:ThingDescription| as a result, run the following steps:

Run validate an ExposedThingInit on |init|. If that fails, [= exception/throw =] {{SyntaxError}} and stop.
Let |td| be the result of running [=map/clone=] given |init|.
For each |scheme:SecurityScheme| in |td|.["securityDefinitions"], make a request to the underlying platform to check if it is supported by at least one Protocol Binding. If not, then remove |scheme| from |td|.
If |td|.["security"] does not [=map/exist=] in |td|.["securityDefinitions"], then remove security from |td|.
For each |affordance| in |td|.properties, |td|.actions and |td|.events, run the following sub-steps:
1. For each |form:Form| in |affordance|.forms:
  1. If |form|.|contentType:string| is not recognized by the runtime as valid remove |contentType:string| from |form|.
  2. If |form|.|href:URL| has an unknown schema, remove |href| from |form|.
  3. If |form|.|href:URL| is absolute and its authority it is not recognized by the runtime as valid, remove |href| from |form|.
  4. If |form|.|href:URL| is already in use by other ExposedThings, remove |href| from |form|.
Search for missing required properties in |td| accordingly to TD JSON Schema.
The editors find this step vague. It will be improved or removed in the next iteration.
For each |missing| property run these sub-steps:
1. If |missing| is title generate a runtime unique name and assign to title.
2. If |missing| is @context assign the latest supported Thing Description context URI.
3. If |missing| is instance assign the string 1.0.0.
4. If |missing| is forms generate a list of Forms using the available Protocol Bindings and content types encoders. Then assign the obtained list to forms.
5. If |missing| is security assign the label of the first supported SecurityScheme in securityDefinitions field. If no SecurityScheme is found generate a NoSecurityScheme called nosec and assign the string nosec to security.
  The discussion about how to properly generate a value for security is still open. See issue #299
6. If |missing| is href define |formStub| as the partial Form that does not have href. Generate a valid |url:URL| using the first Protocol Binding that satisfy the requirements of |formStub|. Assign |url| to href. If not Protocol Binding can be found remove |formStub| from |td|.
7. Add |missing| to |td| with |value| as value
Run validate a TD on |td|. If that fails re-[= exception/throw =] the error and stop
Return |td|

Validating an ExposedThingInit

To validate an ExposedThingInit given |init:ExposedThingInit|, run the following steps:

Parse TD JSON Schema and load it in object called |exposedThingInitSchema:object|
let |optional:Array| be a list containing the following strings: title, @context, instance, forms, security, and href.
For each property and sub-property |key| in |exposedThingInitSchema| equals to required execute the following steps:
1. if |key| |value| is an Array then remove all its elements equal to the elements in |optional|
2. if |key| |value| is a string then if |value| is equal to one of the elements in |optional| remove |key| from |exposedThingInitSchema|
Return the result of validating an object with JSON Schema given |init| and |exposedThingInitSchema|.
The validating an object with JSON Schema steps are still under discussion. Currently this specification reference to the validation process of JSONSchema. Please follow this document when validating |init| with |exposedThingInitSchema|. Notice that the working group is evaluating an alternative formal approach.

The discover() method

      partial namespace WOT {
        Promise<ThingDiscoveryProcess> discover(optional ThingFilter filter = {});
      };

Belongs to the WoT Discovery conformance class. Starts the discovery process that will provide {{ThingDescription}} objects for Thing Descriptions that match an optional |filter:ThingFilter| argument of type {{ThingFilter}}. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
If discovery is not supported by the implementation, [=reject=] |promise| with {{NotSupportedError}} and stop.
Let |discovery:ThingDiscoveryProcess| be a new {{ThingDiscoveryProcess}} object.
Set |discovery|.{{ThingDiscoveryProcess/[[filter]]}} to |filter:ThingFilter|.
Set |discovery|.{{ThingDiscoveryProcess/[[url]]}} to `undefined`.
If filters in general are not supported by the implementation and |filter| is not `undefined` or `null`, [=reject=] |promise| with {{NotSupportedError}} and stop.
If discovery cannot be started by the underlying platform, [=reject=] |promise| with {{OperationError}} and stop.
Request the underlying platform to start the discovery process by any means supported and provisioned in the WoT Runtime for which the script has access to, passing |discovery| to it.
[=Resolve=] |promise| with |discovery|.

Note that the details of the discovery process depend on the underlying implementation which needs to be preconfigured in order to use, for example, the appropriate Introduction methods as defined in the [[[wot-discovery]]] specification. Since the discover() method outputs Thing Descriptions and not URLs, it is expected to cover both the Introduction and the Exploration phase.

The exploreDirectory() method

      partial namespace WOT {
        Promise<ThingDiscoveryProcess> exploreDirectory(USVString url,
            optional ThingFilter filter = {});
      };

Belongs to the WoT Discovery conformance class. Starts the discovery process that given a TD Directory URL, will provide {{ThingDescription}} objects for Thing Descriptions that match an optional |filter:ThingFilter| argument of type {{ThingFilter}}. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
If directory discovery is not supported by the implementation, [=reject=] |promise| with {{NotSupportedError}} and stop.
Let |discovery:ThingDiscoveryProcess| be a new {{ThingDiscoveryProcess}} object.
Set |discovery|.{{ThingDiscoveryProcess/[[url]]}} to |url:USVString|.
Set |discovery|.{{ThingDiscoveryProcess/[[filter]]}} to |filter:ThingFilter|.
Request the underlying platform to start the directory discovery process.
This is a placeholder for more details in the discovery algorithm. Implementations should follow the procedures described in the [[wot-discovery]] and [wot-binding-templates] specifications. Some normative steps are indicated below.
1. If |url| is not a TD Directory or if the underlying implementation cannot support the Protocol Binding indicated by |url|, [=reject=] |promise| with {{NotSupportedError}} and terminate these steps.
2. If filters in general are not supported by the implementation and |filter| is not `undefined` or `null`, [=reject=] |promise| with {{NotSupportedError}} and stop.
3. Run the discovery process given |discovery|.
  From this point on, errors are recorded only on {{ThingDiscoveryProcess/error}}, but don't affect |promise| any longer.
[=Resolve=] |promise| with |discovery|.

The requestThingDescription() method

      partial namespace WOT {
        Promise<ThingDescription> requestThingDescription(USVString url);
      };

Belongs to the WoT Discovery conformance class. Requests a Thing Description from the given URL. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
If requesting a Thing Description is not supported by the implementation, [=reject=] |promise| with {{NotSupportedError}} and stop.
Let |td:ThingDescription| be the result of making a request to the underlying platform to retrieve the Thing Description using the Protocol Binding specified by |url|. If retrieving |td| fails, [=reject=] |promise| with {{NotFoundError}} and stop.
[=Resolve=] |promise| with |td|.

Handling interaction data

As specified in the [[[wot-thing-description11]]] specification, WoT interactions extend DataSchema and include a number of possible Forms, out of which one is selected for the interaction. The Form contains a `contentType` to describe the data. For certain content types, a DataSchema is defined, based on JSON Schema, making possible to represent these contents as JavaScript types and eventually set range constraints on the data.

The InteractionInput type

      typedef any DataSchemaValue;
      typedef (ReadableStream or DataSchemaValue) InteractionInput;

Belongs to the WoT Consumer conformance class and represents the WoT Interaction data provided by application scripts to the UA.

DataSchemaValue is an ECMAScript value that is accepted for DataSchema defined in [[wot-thing-description11]]. The possible values MUST be of type null, boolean, number, string, array, or object.

{{ReadableStream}} is meant to be used for WoT Interactions that don't have a DataSchema in the Thing Description, only a {{Form}}'s `contentType` that can be represented by a stream.

In practice, any ECMAScript value may be used for WoT Interactions that have a DataSchema defined in the Thing Description, or which can be mapped by implementations to the {{Form}}'s `contentType` defined in the Thing Description.

The algorithms in this document specify how exactly input data is used in WoT Interactions.

The InteractionOutput interface

Belongs to the WoT Consumer conformance class. An {{InteractionOutput}} object is always created by the implementations and exposes the data returned from WoT Interactions to application scripts.

This interface exposes a convenience function which should cover the vast majority of IoT use cases: the value() function. Its implementation will inspect the data, parse it if adheres to a DataSchema, or otherwise fail early, leaving the underlying stream undisturbed so that application scripts could attempt reading the stream themselves, or handling the data as {{ArrayBuffer}}.

      [SecureContext, Exposed=(Window,Worker)]
      interface InteractionOutput {
        readonly attribute ReadableStream? data;
        readonly attribute boolean dataUsed;
        readonly attribute Form? form;
        readonly attribute DataSchema? schema;
        Promise<ArrayBuffer> arrayBuffer();
        Promise<DataSchemaValue> value();
      };

The data property represents the raw payload in WoT Interactions as a {{ReadableStream}}, initially `null`.

The dataUsed property tells whether the data stream has been disturbed. Initially `false`.

The form attribute represents the Form selected from the Thing Description for this WoT Interaction, initially `null`.

The schema attribute represents the DataSchema (defined in [[wot-thing-description11]]) of the payload as a {{JSON}} object, initially `null`.

The [[\value]] internal slot represents the parsed value of the WoT Interaction, initially `undefined` (note that `null` is a valid value).

The value() function

Parses the data returned by the WoT Interaction and returns a value with the type described by the interaction DataSchema if that exists, or by the `contentType` of the interaction Form. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If |this|.[[\value]] is not `undefined`, [=resolve=] |promise| with that value and stop.
If |this|.|data| is not a {{ReadableStream}} or if |dataUsed| is `true`, or if |form| is not an {{object}} or if |schema| is `null` or `undefined`, then [=reject=] |promise| with {{NotReadableError}} and stop.
If |form|.|contentType| is not `application/json` and if a mapping is not available in the Protocol Bindings from |form|.|contentType| to [[!JSON-SCHEMA]], [=reject=] |promise| with {{NotSupportedError}} and stop.
Let |reader| be the result of getting a reader from |data|. If that threw an exception, [=reject=] |promise| with that exception and stop.
Let |bytes| be the result of reading all bytes from |data| with |reader|.
Set |dataUsed| to `true`.
If |form|.|contentType| is not `application/json` and if a mapping is available in the Protocol Bindings from |form|.|contentType| to [[!JSON-SCHEMA]], transform |bytes| with that mapping.
Let |json| be the result of running parse JSON from bytes on |bytes|. If that throws, [=reject=] |promise| with that exception and stop.
Set [[\value]] to the result of running check data schema on |json| and |schema|. If that throws, [=reject=] |promise| with that exception and stop.
[=Resolve=] |promise| with [[\value]].

While the {{value()}} function provides built-in validation, we recognize that some use cases may require returning values without validation. In such cases, developers can use alternative patterns, such as directly accessing the underlying data using streams or the {{InteractionOutput/arrayBuffer()}} function (see [[[#validation-arraybuffer-example]]] and [[[#stream-example]]]).

Warning: Disabling validation may introduce risks, particularly when interacting with remote Things, as mismatches in data formats or schema expectations can lead to unforeseen bugs and vulnerabilities. For more details, see the consumer assertions.

The arrayBuffer() function

When invoked, MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If |data| is not {{ReadableStream}} or if |dataUsed| is `true`, [=reject=] |promise| with {{NotReadableError}} and stop.
Let |reader| be the result of getting a reader from |data|. If that threw an exception, [=reject=] |promise| with that exception and stop.
Let |bytes| be the result of reading all bytes from |data| with |reader|.
Set |dataUsed| to `true`.
Let |arrayBuffer| be a new {{ArrayBuffer}} whose contents are |bytes|. If that throws, [=reject=] |promise| with that exception and stop.
[=Resolve=] |promise| with |arrayBuffer|.

The check data schema algorithm

To run the check data schema steps on |payload| and |schema:object|,

If |schema| is `null` or `undefined`, return `undefined`.
If |schema|.|const| is not |undefined| and |schema|.|const| does not equal |payload|, throw {{TypeError}} and stop.
If |schema|.|enum| is not |undefined| and none of the elements in |schema|.|enum| equal |payload|, throw {{TypeError}} and stop.
Let |oneOf| be |schema|.|oneOf|. If |oneOf| is not |undefined|,
1. For each |subSchema| in |oneOf|, run the check data schema steps on |payload| and |subSchema|.
2. If none or more than one of these runs do not throw, throw {{TypeError}} and stop.
Let |type| be |schema|.|type|.
If |type| is `"null"` and if |payload| is not `null`, throw {{TypeError}} and stop, otherwise return `null`.
If |type| is `"boolean"` and |payload| is a falsy value or its byte length is 0, return `false`, otherwise return `true`.
If |type| is `"integer"` or `"number"`,
1. If |payload| is not a number, throw {{TypeError}} and stop.
2. If |form|.|minimum| is defined and |payload| is smaller, or if |form|.|maximum| is defined and |payload| is bigger, throw a {{RangeError}} and stop.
If |type| is `"string"`, return |payload|.
If |type| is `"array"`, run these sub-steps:
1. If |payload| is not an array, throw {{TypeError}} and stop.
2. If |form|.|minItems| is defined and |payload|.|length| is less than that, or if |form|.|maxItems| is defined and |payload|.|length| is more than that, throw {{RangeError}} and stop.
3. Let |payload| be an array of items obtained by running the check data schema steps on each element |item| of |payload| and |schema|.|items|. If this throws at any stage, re-throw that exception and stop.
If |type| is `"object"`, run these sub-steps:
1. If |payload| or |schema|.|properties| is not an {{object}}, throw {{TypeError}} and stop.
2. For each |key| in |payload|:
  1. Let |prop| be |payload|[|key|].
  2. Let |propSchema| be |interaction|.|properties|[|key|].
  3. Let |prop| be the result of running the check data schema steps on |prop| and |propSchema|. If this throws, re-throw that exception and stop.
3. Let |required| be |schema|.|required| if that is an array or an empty array otherwise.
4. For each |key| in |required|, if |key| is not present in |payload|, throw {{SyntaxError}} and stop.
Return |payload|.

The create interaction request algorithm

Let |idata| be a new an {{InteractionOutput}} object.
Set |idata|.|form| to |form|, set |idata|.|schema| to |schema|, set |idata.|data| to `null` and set |idata|.{{InteractionOutput/[[value]]}} to `undefined`.
If |source| is a {{ReadableStream}} object, let |idata|.|data| be |source|, return |idata| and stop.
If |schema| and its |type| are defined and not `null`, run these sub-steps:
1. If |type| is `"null"` and |source| is not `"null"`, throw {{TypeError}} and stop.
2. If |type| is `"boolean"` and |source| is a falsy value, set |idata|.{{InteractionOutput/[[value]]}} to `false`, otherwise set it to `true`.
3. If |type| is `"integer"` or `"number"` and |source| is not a number, or if |form|.|minimum| is defined and |source| is smaller, or if |form|.|maximum| is defined and |source| is bigger, throw {{RangeError}} and stop.
4. If |type| is `"string"` and |source| is not a string, let |idata|.{{InteractionOutput/[[value]]}} be the result of running serialize JSON to bytes given |source|. If that is failure, throw {{SyntaxError}} and stop.
5. If |type| is `"array"`, run these sub-steps:
  1. If |source| is not an array, throw a {{TypeError}} and stop.
  2. Let |length| be the length of |source|.
  3. If |form|.|minItems| is defined and |length| is less than that, or if |form|.|maxItems| is defined and |length| is more than that, throw {{RangeError}} and stop.
  4. For each |item| in |source|, let |itemschema| be |schema|.|items| and let |item| be the result of running the create interaction request steps given |item|, |form| and |itemschema|. If this throws, re-throw that exception and stop.
  5. Set |data|.{{InteractionOutput/[[value]]}} to |source|.
6. If |type| is `"object"`, run these sub-steps:
  1. If |source| is not an object, throw {{TypeError}} and stop.
  2. If |schema|.|properties| is not an object, throw {{TypeError}} and stop.
  3. For each |key| in |source|,
    1. Let |value| be |source|[|key|].
    2. Let |propschema| be |properties|.|interactions|[|key|].
    3. Let |value| be the result of running the create interaction request steps on |value|, |form| and |propschema|. If this throws, re-throw that exception and stop.
  4. If |schema|.|required| is an array, for each |item| in |required| check if |item| is a property name in |source|. If an |item| is not found in |source|, throw {{SyntaxError}} and stop.
  5. Set |data|.{{InteractionOutput/[[value]]}} to |source|.
Set |idata|.|data| to a new {{ReadableStream}} created from |idata|.{{InteractionOutput/[[value]]}} internal slot as its underlying source.
Return |idata|.

The parse interaction response algorithm

Let |result| be a new {{InteractionOutput}} object.
Let |result|.|schema| be |schema|.
Let |result|.|form| be |form|.
Let |result|.|data| be a new {{ReadableStream}} with the payload data of |response| as its underlying source.
Let |result|.|dataUsed| be `false`.
Return |result|.

The ActionInteractionOutput interface

Belongs to the WoT Consumer conformance class. An {{ActionInteractionOutput}} object is always created by a consumer implementation and exposes functionality to interact with long running (asynchronous) actions.
Note: The output of a synchronous action MAY be limited to the functionality of a regular {{InteractionOutput}} object, e.g., invoking the `cancel()` method might not have an effect.

This interface exposes functions which will allow cancelling asynchronous actions and query the status of a long running action.

      /**
       * Note: retrieving the result of an action via the implicit InteractionOutput interface
       * will only work after the action has been completed
       */
      [SecureContext, Exposed=(Window,Worker)]
      interface ActionInteractionOutput : InteractionOutput {
        Promise<InteractionOutput> query(
          optional InteractionInput params = {},
          optional InteractionOptions options = {});
        Promise<undefined> cancel(
          optional InteractionInput params = {},
          optional InteractionOptions options = {});
      };

The query() function

Reports the status of an Action, or rejects on error. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not supported (e.g., synchronous action) [=reject=] |promise| with a {{NotSupportedError}} and stop.
Let |res| be the result of making a request to the underlying platform (via the Protocol Bindings) to query the Action status given optional |params| and |options|. If quering |res| fails, [=reject=] |promise| with {{OperationError}} and stop.
[=Resolve=] |promise| with |res|.

The cancel() function

Cancels a running WoT Action, or rejects on error. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not supported (e.g., because the action is synchronous) [=reject=] |promise| with a {{NotSupportedError}} and stop.
Make a request to the underlying platform (via the Protocol Bindings) to cancel the Action given optional |params| and |options|. If cancelling fails, [=reject=] |promise| with {{OperationError}} and stop.
[=Resolve=] |promise|.

Using {{InteractionInput}}, {{InteractionOutput}} and {{ActionInteractionOutput}}

As illustrated in the next pictures, the {{InteractionOutput}} interface is used every time implementations provide data to scripts, while {{InteractionInput}} is used when the scripts pass data to the implementation.

Reading data — Data structures used when reading data

When a {{ConsumedThing}} reads data, it receives it from the implementation as an {{InteractionOutput}} object.

An {{ExposedThing}} read handler provides the read data to the implementation as {{InteractionInput}}.

Writing data — Data structures used when writing data

When a {{ConsumedThing}} writes data, it provides it to the implementation as {{InteractionInput}}.

An {{ExposedThing}} write handler receives data from to implementation as an {{InteractionOutput}} object.

Invoking action — Data structures used when invoking an Action

When a {{ConsumedThing}} invokes an Action, it provides the parameters as {{InteractionInput}} and receives the output of the Action as an {{ActionInteractionOutput}} object.

An {{ExposedThing}} action handler receives arguments from the implementation as an {{InteractionOutput}} object and provides Action output as {{InteractionInput}} to the implementation.

Error handling

The algorithms in this API define the errors to be reported to application scripts.

The errors reported to the other communication end are mapped and encapsulated by the Protocol Bindings.

This topic is still being discussed in Issue #200. A standardized error mapping would be needed in order to ensure consistency in mapping script errors to protocol errors and vice versa. In particular, when algorithms say "error received from the Protocol Bindings", that will be factored out as an explicit error mapping algorithm. Currently, that is encapsulated by implementations.

The ConsumedThing interface

Represents a client API to operate a Thing. Belongs to the WoT Consumer conformance class.

      [SecureContext, Exposed=(Window,Worker)]
      interface ConsumedThing {
        Promise<InteractionOutput> readProperty(DOMString propertyName,
                                    optional InteractionOptions options = {});
        Promise<PropertyReadMap> readAllProperties(
                                    optional InteractionOptions options = {});
        Promise<PropertyReadMap> readMultipleProperties(
                                    sequence<DOMString> propertyNames,
                                    optional InteractionOptions options = {});
        Promise<undefined> writeProperty(DOMString propertyName,
                                    InteractionInput value,
                                    optional InteractionOptions options = {});
        Promise<undefined> writeMultipleProperties(
                                    PropertyWriteMap valueMap,
                                    optional InteractionOptions options = {});
        /*Promise<undefined> writeAllProperties(
                                    PropertyWriteMap valueMap,
                                    optional InteractionOptions options = {});*/
        Promise<ActionInteractionOutput> invokeAction(DOMString actionName,
                                    optional InteractionInput params = {},
                                    optional InteractionOptions options = {});
        Promise<Subscription> observeProperty(DOMString name,
                                    InteractionListener listener,
                                    optional ErrorListener onerror,
                                    optional InteractionOptions options = {});
        Promise<Subscription> subscribeEvent(DOMString name,
                                    InteractionListener listener,
                                    optional ErrorListener onerror,
                                    optional InteractionOptions options = {});
        ThingDescription getThingDescription();
      };
      dictionary InteractionOptions {
        unsigned long formIndex;
        object uriVariables;
        any data;
      };
      [SecureContext, Exposed=(Window,Worker)]
      interface Subscription {
        readonly attribute boolean active;
        Promise<undefined> stop(optional InteractionOptions options = {});
      };
      [SecureContext, Exposed=(Window,Worker)]
      interface PropertyReadMap {
        readonly maplike<DOMString, InteractionOutput>;
      };
      [SecureContext, Exposed=(Window,Worker)]
      interface PropertyWriteMap {
        readonly maplike<DOMString, InteractionInput>;
      };
      callback InteractionListener = undefined(InteractionOutput data);
      callback ErrorListener = undefined(Error error);

The writeAllProperties() method is still under discussion. Meanwhile, use the writeMultipleProperties() method instead.

Internal slots for {{ConsumedThing}}

A {{ConsumedThing}} object has the following internal slots:

Internal Slot	Initial value	Description (non-normative)
[[\td]]	`null`	The Thing Description of the {{ConsumedThing}}.
[[\activeSubscriptions]]	`{}`	An [=ordered map=] [=map/keyed=] on a [=string=] name representing the Event and [=map/value=] is a {{Subscription}} object.
[[\activeObservations]]	`{}`	An [=ordered map=] [=map/keyed=] on a [=string=] name representing a Property and [=map/value=] is a {{Subscription}} object.

Creating `ConsumedThing`

After requesting a Thing Description as a JSON object, one can create a {{ConsumedThing}} object.

To create {{ConsumedThing}} with the {{ThingDescription}} |td:ThingDescription|, run the following steps:

Run the validate a TD steps on |td|. If that fails, [= exception/throw =] {{SyntaxError}} and stop.
Run the expand a TD steps on |td|. If that fails, re-[= exception/throw =] the error and stop.
Let |thing:ConsumedThing| be a new {{ConsumedThing}} object.
Set the internal slot {{ConsumedThing/[[td]]}} of |thing| to |td|.
Return |thing|.

The getThingDescription() method

Returns the {{ConsumedThing/[[td]]}} of the {{ConsumedThing}} object that represents the Thing Description of the {{ConsumedThing}}. Applications may consult the Thing metadata stored in {{ConsumedThing/[[td]]}} in order to introspect its capabilities before interacting with it.

The readProperty() method

Reads a Property value. Takes as arguments |propertyName:string| and optionally |options:InteractionOptions|. It returns a {{Promise}} that resolves with a Property value represented as an {{InteractionOutput}} object or rejects on error. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
Let |interaction| be {{ConsumedThing/[[td]]}}.|properties|.|propertyName|.
If |interaction| is `undefined`, [=reject=] |promise| with a {{NotFoundError}} and stop.
If |option|.|formIndex| is defined, let |form| be the Form associated with |formIndex| in |interaction|.|forms| array, otherwise let |form| be a Form in |interaction|.|forms| whose |op| is `readproperty`, selected by the implementation.
If |form| is failure, [=reject=] |promise| with a {{SyntaxError}} and stop.
Make a request to the underlying platform (via the Protocol Bindings) to retrieve the value of the |propertyName| Property using |form| and the optional URI templates given in |options|.|uriVariables|.
If the request fails, [=reject=] |promise| with the error received from the Protocol Bindings and stop.
Let |response| be the response received to the request.
Let |data| be the result of running parse interaction response on |response|, |form| and |interaction|. If that fails, [=reject=] |promise| with a {{SyntaxError}} and stop.
[=Resolve=] |promise| with |data|.

The readMultipleProperties() method

Reads multiple Property values with one request. Takes as arguments |propertyNames: string sequence| and optionally |options:InteractionOptions|. It returns a {{Promise}} that resolves with a {{PropertyReadMap}} object that maps keys from |propertyNames| to values returned by this algorithm. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
If |option|.|formIndex| is defined, let |form| be the Form associated with |formIndex| in the {{ConsumedThing/[[td]]}}.|forms| array, otherwise let |form| be the Form in {{ConsumedThing/[[td]]}}.|forms| array whose |op| is `readmultipleproperties`, as selected by the implementation.
If |form| is failure, [=reject=] |promise| with a {{SyntaxError}} and stop.
Let |result:object| be an object and for each string |name:string| in |propertyNames| add a property with key |name| and the value `null`.
Make a request to the underlying platform (via the Protocol Bindings) to retrieve the Property values given by |propertyNames| with |form| and optional URI templates given in |options|' |uriVariables|.
If this cannot be done with a single request with the Protocol Bindings, [=reject=] |promise| with a {{NotSupportedError}} and stop.
Process the response and for each |key| in |result|, run the following sub-steps:
1. Let |value| be |result|[|key|].
2. Let |schema| be |this|.{{ConsumedThing/[[td]]}}.|properties|[|key|].
3. Let |property| be the result of running parse interaction response on |value|, |form| and |schema|.
If the above step throws at any point, [=reject=] |promise| with that exception and stop.
[=Resolve=] |promise| with |result|.

The readAllProperties() method

Reads all properties of the Thing with one request. Takes |options:InteractionOptions| as optional argument. It returns a {{Promise}} that resolves with a {{PropertyReadMap}} object that maps keys from Property names to values returned by this algorithm. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
Let |forms| be |subscription|.{{Subscription/[[interaction]]}}.|forms|.
If |forms| is `undefined`, [=reject=] |promise| with a {{SyntaxError}} and stop.
If |option|.|formIndex| is not `undefined` and is less than |forms|.|length|, set |subscription|.{{Subscription/[[form]]}} to |forms|.[|formIndex|].
Otherwise, set |subscription|.{{Subscription/[[form]]}} to a Form in |forms| whose |op| is `"readallproperties"`, as selected by the implementation.
If |subscription|.{{Subscription/[[form]]}} is failure, [=reject=] |promise| with a {{SyntaxError}} and stop.
Make a request to the underlying platform using the Protocol Bindings to retrieve all the Property definitions from the TD given |form| and optional URI templates in |options|.|uriVariables|.
If this cannot be done with a single request with the Protocol Bindings of the Thing, then [=reject=] |promise| with a {{NotSupportedError}} and stop.
If the request fails, [=reject=] |promise| with the error received from the Protocol Bindings and stop.
Process the reply and let |result:object| be an object with the keys and values obtained in the reply.
Process the response and for each |key| in |result|, run the following sub-steps:
1. Let |value| be |result|[|key|].
2. Let |schema| be |this|.{{ConsumedThing/[[td]]}}.|properties|[|key|].
3. Let |property| be the result of running parse interaction response on |value|, |form| and |schema|.
[=Resolve=] |promise| with |result|.

The writeProperty() method

Writes a single Property. Takes as arguments |propertyName:string|, |value:InteractionInput| and optionally |options:InteractionOptions|. It returns a {{Promise}} that resolves on success and rejects on failure. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
Let |interaction| be |this|.{{ConsumedThing/[[td]]}}.|properties|[|propertyName|].
If |interaction| is `undefined`, [=reject=] |promise| with a {{NotFoundError}} and stop.
If |option|.|formIndex| is not `undefined`, let |form| be the Form associated with |formIndex| in the |interaction|.|forms| array, otherwise let |form| be a Form in |interaction|.|forms| whose |op| is `writeproperty`, as selected by the implementation.
If |form| is failure, [=reject=] |promise| with a {{SyntaxError}} and stop.
Let |data| be the result of running the create interaction request steps given |value|, |form| and |interaction|. If that throws, [=reject=] promise with that exception and stop.
Make a request to the underlying platform (via the Protocol Bindings) to write the Property given by |propertyName| using |data:InteractionOutput| and the optional URI templates given in |options|' |uriVariables|.
If the request fails, [=reject=] |promise| with the error received from the Protocol Bindings and stop.
Otherwise [=resolve=] |promise|.

As discussed in Issue #193, the design decision is that write interactions only return success or error, not the written value (optionally). TDs should capture the schema of the Property values, including precision and alternative formats. When a return value is expected from the interaction, an Action should be used instead of a Property.

The writeMultipleProperties() method

Writes a multiple Property values with one request. Takes as arguments |properties:object| - as an object with keys being Property names and values as Property values - and optionally |options:InteractionOptions|. It returns a {{Promise}} that resolves on success and rejects on failure. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
If |option|.|formIndex| is defined, let |form| be the Form associated with |formIndex| in the {{ConsumedThing/[[td]]}}.|forms| array, otherwise let |form| be a Form in {{ConsumedThing/[[td]]}}.|forms| array whose |op| is `writemultipleproperties`, as selected by the implementation.
If |form| is failure, [=reject=] |promise| with a {{SyntaxError}} and stop.
Let |propertyNames| be an array of |string| with as elements the keys of the |properties| object.
For each |name:string| in |propertyNames|, let |property| be |this|.{{ConsumedThing/[[td]]}}.|properties|[|name|].
If |property| is `null` or `undefined` or is not `writeable` [=reject=] |promise| with {{NotSupportedError}} and stop.
Let |result:object| be an object and for each string |name:string| in |propertyNames| add a property with key |name| and let its value be `null`.
Let |schemas:object| be an object and for each |name:string| in |propertyNames| add a property with key |name| and let its value be |this|.{{ConsumedThing/[[td]]}}.|properties|[|name|].
For each key |key:string| in |properties|, run the create interaction request steps given |properties|[|key|], |form| and the value for |schema|[|key|]. If that throws for any |name|, [=reject=] promise with that exception and stop.
Make a single request to the underlying platform (via the Protocol Bindings) to write each Property provided in |properties| with optional URI templates given in |options|' |uriVariables|.
If this cannot be done with a single request with the Protocol Bindings of the Thing, then [=reject=] |promise| with a {{NotSupportedError}} and stop.
If the request fails, return the error received from the Protocol Bindings and stop.
Otherwise [=resolve=] |promise|.

The observeProperty() method

Makes a request for Property value change notifications. Takes as arguments |propertyName:string|, |listener:InteractionListener| and optionally |onerror:ErrorListener| and |options:InteractionOptions|. It returns a {{Promise}} that resolves on success and rejects on failure.

This algorithm allows for only one active {{Subscription}} per Property. If a new {{Subscription}} is made while an existing {{Subscription}} is active the runtime will throw an {{NotAllowedError}}.

The method MUST run the following steps:

Let |thing| be the reference of this {{ConsumedThing}} object.
Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
If |listener| is not a {{Function}}, [=reject=] |promise| with a {{TypeError}} and stop.
If |onerror| is not `null` and is not a {{Function}}, [=reject=] |promise| with a {{TypeError}} and stop.
If |thing|.{{ConsumedThing/[[activeObservations]]}}[|propertyName|] [=map/exists=], [=reject=] |promise| with a {{NotAllowedError}} and stop.
Let |subscription| be a new {{Subscription}} object with its internal slots set as follows:
- Let |subscription|.{{Subscription/[[type]]}} be `"property"`.
- Let |subscription|.{{Subscription/[[name]]}} be |propertyName|.
- Let |subscription|.{{Subscription/[[interaction]]}} be {{ConsumedThing/[[td]]}}.|properties|[|propertyName|].
- Let |subscription|.{{Subscription/[[thing]]}} be |thing|.
- Let |forms| be |subscription|.{{Subscription/[[interaction]]}}.|forms|.
- If |forms| is `undefined`, [=reject=] |promise| with a {{SyntaxError}} and stop.
- If |option|.|formIndex| is not `undefined` and is less than |forms|.|length|, set |subscription|.{{Subscription/[[form]]}} to |forms|.[|formIndex|].
- Otherwise, set |subscription|.{{Subscription/[[form]]}} to a Form in |forms| whose |op| is `"observeproperty"`, as selected by the implementation.
- If |subscription|.{{Subscription/[[form]]}} is failure, [=reject=] |promise| with a {{SyntaxError}} and stop.
- If |subscription|.{{Subscription/[[interaction]]}} is `undefined`, [=reject=] |promise| with a {{NotFoundError}} and stop.
Make a request to the underlying platform to observe the Property identified by |propertyName| with |form| and optional URI templates given in |options|' |uriVariables|.
If the request fails, [=reject=] |promise| with the error received from the Protocol Bindings and stop.
[=map/Set=] |thing|.{{ConsumedThing/[[activeObservations]]}}[|propertyName] to |subscription| and [=resolve=] |promise|.
Whenever the underlying platform detects a notification for this |subscription| [=map/keyed=] on |propertyName| with a new Property value |value|, run the following sub-steps:
- Let |reply| be the result of running parse interaction response with |value|, |subscription|.{{Subscription/[[form]]}} and |subscription|.{{Subscription/[[interaction]]}}. If that throws, [=reject=] |promise| with that exception and stop.
- Invoke |listener| given |reply|.
Whenever the underlying platform detects an error for this subscription, run the following sub-steps:
- If the error is irrecoverable and stops the subscription, set |subscription|.|active| to `false` and suppress further notifications.
- Let |error| be a new {{NetworkError}} and set its |message| to reflect the underlying error condition.
- If |onerror| is a {{Function}}, invoke it given |error|.

The invokeAction() method

Makes a request for invoking an Action and return the result. Takes as arguments |actionName:string|, optionally |params:InteractionInput| and optionally |options:InteractionOptions|. It returns a {{Promise}} that resolves with the result of the Action represented as an {{InteractionOutput}} object, or rejects with an error. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
Let |interaction| be |this|.{{ConsumedThing/[[td]]}}.|actions|[|actionName|].
If |interaction| is not an {{object}}, [=reject=] |promise| with a {{NotFoundError}} and stop.
Let |forms| be |subscription|.{{Subscription/[[interaction]]}}.|forms|.
If |forms| is `undefined`, [=reject=] |promise| with a {{SyntaxError}} and stop.
If |option|.|formIndex| is not `undefined` and is less than |forms|.|length|, set |subscription|.{{Subscription/[[form]]}} to |forms|.[|formIndex|].
Otherwise, set |subscription|.{{Subscription/[[form]]}} to a Form in |forms| whose |op| is `"invokeaction"`, as selected by the implementation.
If |subscription|.{{Subscription/[[form]]}} is failure, [=reject=] |promise| with a {{SyntaxError}} and stop.
Let |args| be the result of running the create interaction request steps on |params|, |form| and |interaction|. If that throws, [=reject=] promise with that exception and stop.
Make a request to the underlying platform (via the Protocol Bindings) to invoke the Action identified by |actionName| given |args| and |options|.|uriVariables|.
If the request fails locally or returns an error over the network, [=reject=] |promise| with the error received from the Protocol Bindings and stop.
Let |value| be the reply returned in the reply.
Let |result| be the result of running parse interaction response with |value|, |form| and |interaction|. If that throws, [=reject=] |promise| with that exception and stop.
[=Resolve=] |promise| with |result|.

The subscribeEvent() method

Makes a request for subscribing to Event notifications. Takes as arguments |eventName:string|, |listener:WoTListener| and optionally |onerror:ErrorListener| and |options:InteractionOptions|. It returns a {{Promise}} to signal success or failure.

This algorithm allows for only one active {{Subscription}} per Event. If a new {{Subscription}} is made while an existing {{Subscription}} is active the runtime will throw an {{NotAllowedError}}.

The method MUST run the following steps:

Let |thing| be the reference of this {{ConsumedThing}} object.
Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
If |listener| is not a {{Function}}, [=reject=] |promise| with a {{TypeError}} and stop.
If |onerror| is not `null` and is not a {{Function}}, [=reject=] |promise| with a {{TypeError}} and stop.
If |thing|.{{ConsumedThing/[[activeSubscriptions]]}}[|eventName|] [=map/exists=], [=reject=] |promise| with a {{NotAllowedError}} and stop.
Let |subscription| be a new {{Subscription}} object with its internal slots set as follows:
- Let |subscription|.{{Subscription/[[type]]}} be `"event"`.
- Let |subscription|.{{Subscription/[[name]]}} be |eventName|.
- Let |subscription|.{{Subscription/[[interaction]]}} be |thing|. {{ConsumedThing/[[td]]}}.|events|[|eventName|].
- If |subscription|.{{Subscription/[[interaction]]}} is `undefined`, [=reject=] |promise| with a {{NotFoundError}} and stop.
- Let |subscription|.{{Subscription/[[thing]]}} be |thing|.
- If |options|.|formIndex| [=map/exists=], then let |subscription|.{{Subscription/[[form]]}} be |thing|.{{Subscription/[[interaction]]}}.|forms|[|formIndex|].
- Otherwise, let |subscription|.{{Subscription/[[form]]}} be an [=implementation-defined=] Form from the |subscription|.{{Subscription/[[interaction]]}}.|forms| array whose |op| is `"subscribeevent"`.
- If |subscription|.{{Subscription/[[form]]}} does not [=map/exist=], [=reject=] |promise| with a {{SyntaxError}} and stop.
Make a request to the underlying platform via the Protocol Bindings to subscribe to the Event identified by |eventName:string| with {{Subscription/[[form]]}}, optional URI templates given in |options|.|uriVariables| and optional subscription data given in |options|.|data|.
If the request fails, [=reject=] |promise| with the error received from the Protocol Bindings and stop.
[=map/Set=] |eventName| to |thing|.{{ConsumedThing/[[activeSubscriptions]]}}[|eventName|] to |subscription|.
[=Resolve=] |promise|.
Whenever the underlying platform detects a notification for the Event |subscription| [=map/keyed=] on |eventName|, run the following sub-steps:
1. Invoke |listener| given the result of running parse interaction response on the data provided with the Event, |subscription|.{{Subscription/[[form]]}} and |subscription|.{{Subscription/[[interaction]]}}.
Whenever the underlying platform detects an error for Event |subscription| [=map/keyed=] on |eventName|, run the following sub-steps:
- If the error is irrecoverable and stops the subscription, set |subscription|.|active| to `false` and suppress further notifications.
- Let |error| be a new {{NetworkError}} and set its |message| to reflect the underlying error condition.
- If |onerror| is a {{Function}}, invoke it given |error|.

The InteractionOptions dictionary

Holds the interaction options that need to be exposed for application scripts according to the Thing Description.

The formIndex property, if defined, represents an application hint for which Form definition, identified by this index, of the TD to use for the given WoT interaction. Implementations SHOULD use the Form with this index for making the interaction, but MAY override this value if the index is not found or not valid. If not defined, implementations SHOULD attempt to use the Form definitions in order of appearance as listed in the TD for the given Wot Interaction.

The uriVariables property if defined, represents the URI template variables to be used with the WoT Interaction that are represented as parsed JSON objects defined in [[!wot-thing-description11]].

The support for URI variables comes from the need, exposed by the [[[wot-thing-description11]]] specification, to be able to describe existing RESTful endpoints that use them. However, it should be possible to write a Thing Description that would use Actions for representing this kind of interactions and model the URI variables as action parameters. In that case, implementations can serialize the parameters as URI variables, and therefore, the |options| parameter could be dismissed.

The data property if defined, represents additional opaque data that needs to be passed to the interaction.

The PropertyReadMap type

Represents a map of Property names to an InteractionOutput object that represents the value the Property can take. It is used as a property bag for interactions that involve multiple Properties at once.

The PropertyWriteMap type

Represents a map of Property names to an InteractionInput that represents the value the Property can take. It is used as a property bag for interactions that involve multiple Properties at once.

The InteractionListener callback

User provided callback that is given an argument of type {{InteractionOutput}} and is used for observing Property changes and handling Event notifications. Since subscribing to Events are WoT interactions and might take options or even data, they are not modelled with software events.

The ErrorListener callback

User provided callback that is given an argument of type {{Error}} and is used for conveying critical and non-critical errors from the Protocol Bindings to applications.

The Subscription interface

Represents a subscription to Property change and Event interactions.

The active boolean property denotes if the subscription is active, i.e. it is not stopped because of an error or because of invocation of the stop() method.

Internal slots for {{Subscription}}

A {{Subscription}} object has the following internal slots:

Internal Slot	Initial value	Description (non-normative)
[[\type]]	`null`	Indicates what WoT Interaction the {{Subscription}} refers to. The value can be either `"property"` or `"event"` or `null`.
[[\name]]	`null`	The Property or Event name.
[[\interaction]]	`null`	The Thing Description fragment that describes the WoT interaction.
[[\form]]	`null`	The Form associated with the subscription.
[[\thing]]	`null`	The ConsumedThing associated with the subscription.

The stop() method

Stops delivering notifications for the subscription. It takes an optional parameter |options:InteractionOptions| and returns a {{Promise}}. When invoked, the method MUST execute the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
If |options|' |formIndex| is defined, let |unsubscribeForm| be the Form associated with |formIndex| in {{Subscription/[[interaction]]}}'s |forms| array.
Otherwise let |unsubscribeForm| be the result of running the find a matching unsubscribe form algorithm given {{Subscription/[[form]]}}.
If |unsubscribeForm| is failure, [=reject=] |promise| with a {{SyntaxError}} and stop.
If {{Subscription/[[type]]}} is `"property"`, make a request to the underlying platform via the Protocol Bindings to stop observing the Property identified by {{Subscription/[[name]]}} with |unsubscribeForm| and optional URI templates given in |options|' |uriVariables|.
Otherwise, if {{Subscription/[[type]]}} is `"event"`, make a request to the underlying platform via the Protocol Bindings to unsubscribe from the Event identified by {{Subscription/[[name]]}} with |unsubscribeForm|, with optional URI templates given in |options|' |uriVariables| and optional unsubscribe data given in |options|.|data|.
If the request fails, [=reject=] |promise| with the error received from the Protocol Bindings and stop.
Otherwise:
- set active to `false`.
- if {{Subscription/[[type]]}} is `"event"`, remove {{Subscription/[[name]]}} from {{Subscription/[[thing]]}}.{{ConsumedThing/[[activeSubscriptions]]}} .
- if {{Subscription/[[type]]}} is `"property"`, remove {{Subscription/[[name]]}} from {{Subscription/[[thing]]}}.{{ConsumedThing/[[activeObservations]]}} .
- [=Resolve=] |promise|.
If the underlying platform receives further notifications for this subscription, implementations SHOULD silently suppress them.

Finding an unsubscribe Form

This algorithm is under development and is non-normative. Implementations MAY choose another algorithm to find a matching `unsubscribe` Form to a given `subscribe` Form.

To find a matching unsubscribe form given |subscribeForm| in the context of a {{Subscription}} object, run the following steps:

Let |results| be an empty array.
For each |form| in {{Subscription/[[interaction]]}}.|forms|,
1. Add an internal slot [[\matchLevel]] on |form| and set its value to `0`.
2. If |form|.|op| is `"unobserveproperty"` if {{Subscription/[[type]]}} is `"property"` or if |form|.|op| is `"unsubscribeevent"` if {{Subscription/[[type]]}} is`"event"`,
  1. Set the internal slot [[\matchLevel]] on |form| to 1 and add |form| to |results|.
  2. If |form|.|href| and [[\subscribeForm]].|href| are same origin-domain, increment |form|.[[\matchLevel]].
  3. If |form|.|contentType| is equal to [[\subscribeForm]]'s |contentType| and |form|.[[\matchLevel]] is greater than 2, increment |form|.[[\matchLevel]].
If |results| is empty, return `null` and terminate these steps.
Return the first |form| in |results| that has the highest [[\matchLevel]] value.

ConsumedThing Examples

The next example illustrates how to fetch a TD by URL, create a {{ConsumedThing}}, read metadata (title), read property value, subscribe to property change, subscribe to a WoT event, unsubscribe.

        try {
          let res = await fetch("https://tds.mythings.org/sensor11");
          let td = res.json();
          let thing = new ConsumedThing(td);
          console.log("Thing " + thing.getThingDescription().title + " consumed.");
        } catch (e) {
          console.log("TD fetch error: " + e.message);
        };
        try {
          // subscribe to property change for "temperature"
          await thing.observeProperty("temperature", async (data) => {
            try {
              console.log("Temperature changed to: " + await data.value());
            } catch (error) {
              console.error("Cannot read the observed property temperature");
              console.error(error);
            }
          });
          // subscribe to the "ready" event defined in the TD
          await thing.subscribeEvent("ready", async (eventData) => {
            try {
              console.log("Ready; index: " + await eventData.value());
              // run the "startMeasurement" action defined by TD
              await thing.invokeAction("startMeasurement", { units: "Celsius" });
              console.log("Measurement started.");
            } catch (error) {
              console.error("Cannot read the ready event or startMeasurement failed");
              console.error(error)
            }
          });
        } catch (e) {
          console.log("Error starting measurement.");
        }
        setTimeout(async () => {
          try {
            const temperatureData = await thing.readProperty("temperature")
            const temperature = await temperatureData.value();
            console.log("Temperature: " + temperature);
            await thing.unsubscribe("ready");
            console.log("Unsubscribed from the 'ready' event.");
          } catch (error) {
            console.log("Error in the cleanup function");
          }
        }, 10000);

The following shows an advance usage of {{InteractionOutput}} to read a property without a {{DataSchema}}.

        /*
        * takePicture affordance form:
        * "form": {
        *   "op": "invokeaction",
        *   "href" : "https://camera.example.com:5683/takePicture",
        *   "response": {
        *     "contentType": "image/jpeg",
        *     "contentCoding": "gzip"
        *   }
        *}
        * See https://www.w3.org/TR/wot-thing-description/#example-23
        */
        let response;
        let image;
        try {
          response = await thing.invokeAction("takePicture");
          image = await response.value() // throws NotReadableError --> schema not defined
        } catch(ex) {
          image = await response.arrayBuffer();
          // image: ArrayBuffer [0x1 0x2 0x3 0x5 0x15 0x23 ...]
        }

The {{InteractionOutput/arrayBuffer()}} can be used as a shortcut to skip the validation of the {{value()}} function. See relevant warning for the implications.

          try{
            // output is an InteractionOutput instance
            const value = JSON.parse(Buffer.from(await output.arrayBuffer()).toString())
            // ... custom validation
          } catch(ex) {
            // deal with parsing errors.
          }

Finally, the next two examples shows the usage of a {{ReadableStream}} from an {{InteractionOutput}}.

        /*{
        "video": {
          "description" : "the video stream of this camera",
          "forms": [
            {
              "op": "readproperty",
              "href": "https://camera.example.com/live",
              "subprotocol": "hls"
              "contentType": "video/mp4"
            }
          ]
        }}*/
        const video = await thing.readProperty("video")
        const reader = video.data.getReader()
        reader.read().then(function processVideo({ done, value }) {
          if (done) {
            console.log("live video stopped");
            return;
          }
          const decoded = decode(value)
          UI.show(decoded)
          // Read some more, and call this function again
          return reader.read().then(processText);
        });

Here consider that the JSON object is too big to be read wholly in the memory. Therefore, we use streaming processing to get the total number of the events recorded by the remote Web Thing.

        /*
        * "eventHistory":
        * {
        *   "description" : "A long list of the events recorded by this thing",
        *   "type": "array",
        *   "forms": [
        *     {
        *       "op": "readproperty",
        *       "href": "https://recorder.example.com/eventHistory",
        *     }
        *   ]
        * }
        */
        // Example of streaming processing: counting json objects
        let objectCounter = 0
        const parser = new Parser() //User library for json streaming parsing (i.e. https://github.com/uhop/stream-json/wiki/Parser)
        parser.on('data', data => data.name === 'startObject' && ++objectCounter);
        parser.on('end', () => console.log(`Found ${objectCounter} objects.`));
        const response = await thing.readProperty("eventHistory")
        await response.data.pipeTo(parser);
        // Found N objects

The ExposedThing interface

The {{ExposedThing}} interface is the server API to operate the Thing that allows defining request handlers, Property, Action, and Event interactions.

      [SecureContext, Exposed=(Window,Worker)]
      interface ExposedThing {
        ExposedThing setPropertyReadHandler(DOMString name,
                PropertyReadHandler handler);
        ExposedThing setPropertyWriteHandler(DOMString name,
                PropertyWriteHandler handler);
        ExposedThing setPropertyObserveHandler(DOMString name,
                PropertyReadHandler handler);
        ExposedThing setPropertyUnobserveHandler(DOMString name,
                PropertyReadHandler handler);
        Promise<undefined> emitPropertyChange(DOMString name,
                optional InteractionInput data);
        ExposedThing setActionHandler(DOMString name, ActionHandler action);
        ExposedThing setEventSubscribeHandler(DOMString name,
                EventSubscriptionHandler handler);
        ExposedThing setEventUnsubscribeHandler(DOMString name,
                EventSubscriptionHandler handler);
        Promise<undefined> emitEvent(DOMString name,
                optional InteractionInput data);
        Promise<undefined> expose();
        Promise<undefined> destroy();
        ThingDescription getThingDescription();
      };
      callback PropertyReadHandler = Promise<InteractionInput>(
              optional InteractionOptions options = {});
      callback PropertyWriteHandler = Promise<undefined>(
              InteractionOutput value,
              optional InteractionOptions options = {});
      callback ActionHandler = Promise<InteractionInput>(
              InteractionOutput params,
              optional InteractionOptions options = {});
      callback EventSubscriptionHandler = Promise<undefined>(
              optional InteractionOptions options = {});

Internal slots for ExposedThing

An {{ExposedThing}} object has the following internal slots:

Internal Slot	Initial value	Description (non-normative)
[[\td]]	`null`	The Thing Description of the {{ExposedThing}}.
[[\readHandlers]]	`{}`	A {{Map}} with property names as keys and {{PropertyReadHandler}}s as values
[[\writeHandlers]]	`{}`	A {{Map}} with property names as keys and {{PropertyWriteHandler}}s as values
[[\observeHandlers]]	`{}`	A {{Map}} with property names as keys and {{PropertyReadHandler}}s as values
[[\unobserveHandlers]]	`{}`	A {{Map}} with property names as keys and {{Function}}s as values
[[\actionHandlers]]	`{}`	A {{Map}} with action names as keys and {{ActionHandler}}s as values
[[\subscribeHandlers]]	`{}`	A {{Map}} with event names as keys and {{EventSubscriptionHandler}}s as values
[[\unsubscribeHandlers]]	`{}`	A {{Map}} with event names as keys and {{EventSubscriptionHandler}}s as values
[[\propertyObservers]]	`{}`	A {{Map}} with property names as keys and an {{Array}} of listeners as values
[[\eventListeners]]	`{}`	A {{Map}} with event names as keys and {{Array}} of listeners as values

Creating {{ExposedThing}}

The {{ExposedThing}} interface is created from a full or partial {{ThingDescription}} object.

Note that an existing {{ThingDescription}} object can be optionally modified (for instance by adding or removing elements on its |properties|, |actions| and |events| internal properties) and the resulting object can used for creating an {{ExposedThing}} object. This is the current way of adding and removing Property, Action and Event definitions, as illustrated in the examples.

Before invoking expose(), the {{ExposedThing}} object does not serve any requests. This allows first creating {{ExposedThing}} and then initialize its Properties and service handlers before starting serving requests.

To create an {{ExposedThing}} with the {{ExposedThingInit}} |init:ExposedThingInit|, run the following steps:

If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and stop.
Run the expand an ExposedThingInit steps on |init|. if that fails re-[= exception/throw =] the error and stop. Otherwise store the obtained |td:ThingDescription|
Run the expand a TD steps on |td|. If that fails, re-[= exception/throw =] the error and stop.
Let |thing:ExposedThing| be a new {{ExposedThing}} object.
Set the {{ExposedThing/[[td]]}} of |thing| to |td|.
Return |thing|.

The getThingDescription() method

Returns the {{ExposedThing/[[td]]}} of the {{ExposedThing}} object that represents the Thing Description of the Thing. Applications may consult the Thing metadata stored in {{ExposedThing/[[td]]}} in order to introspect its capabilities before interacting with it.

The PropertyReadHandler callback

A function that is called when an external request for reading a Property is received and defines what to do with such requests. It returns a {{Promise}} and resolves with an {{ReadableStream}} object or an ECMAScript value conforming to DataSchema, or rejects with an error.

The setPropertyReadHandler() method

Note that there is no need to register handlers for handling requests for reading multiple or all Properties. The request and reply are transmitted in a single network request, but the ExposedThing may implement them using multiple calls to the single read handler.

The |handler| callback function should implement reading a Property and SHOULD be called by implementations when a request for reading a Property is received from the underlying platform.

There MUST be at most one handler for any given Property, so newly added handlers MUST replace the previous handlers. If no handler is initialized for any given Property, implementations SHOULD implement a default property read handler based on the Thing Description provided in the {{ExposedThing/[[td]]}} internal slot.

When the method is invoked given |name:string| and |handler:PropertyReadHandler|, implementations MUST run the following steps:

If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and stop.
If {{ExposedThing/[[td]]}}.|properties|[|name|] does not [=map/exist=], [= exception/throw =] {{NotFoundError}} and stop.
Set |this|.{{ExposedThing/[[readHandlers]]}}[|name|] to |handler|.

Handling requests for reading a Property

When a network request for reading Property |name:string| is received by the implementation with |options:InteractionOptions|, run the following steps:

If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and stop.
If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and stop.
Let |value| be the result of running the read server property steps with |name:string| and |options:InteractionOptions|:
1. Let |interaction| be {{ExposedThing/[[td]]}}.|properties|.|name|.
2. If a Property with |name| does not exist, throw {{NotFoundError}} and stop.
3. Let |handler:function| be `null`.
4. If there is a user provided {{PropertyReadHandler}} in {{ExposedThing/[[readHandlers]]}} internal slot for |interaction|, let |handler| be that.
5. Otherwise, if there is a default read handler provided by the implementation, let |handler| be that.
6. If |handler| is `null`, throw {{NotSupportedError}} and stop.
7. Let |value| be the result of invoking |handler| given |options|. If that fails, throw the error and stop.
8. Return |value|.
  The |value| returned here SHOULD either conform to DataSchema or it SHOULD be an {{ReadableStream}} object created by the |handler|.
If the previous step has thrown an error, send the error back with the reply created by following the Protocol Bindings and stop.
Serialize and add the returned |value| to the reply created by following the Protocol Bindings.

Handling requests for reading multiple Properties

If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and stop.
If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and stop.
For each property with key |name| defined in |propertyNames|,
1. Let |value| be the result of running the read server property steps on |name| and |options|. If that throws, send back the error in the reply created by following the Protocol Bindings and stop.
2. Set |propertyNames|.|name| to |value|.
Reply to the request by sending a single reply created from |propertyNames| according to the Protocol Bindings.

Handling requests for reading all Properties

When a network request for reading all Properties is received with |options:InteractionOptions|, run the following steps:

If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and stop.
If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and stop.
Let |properties| be an object created with all properties defined in the Thing with values set to `null`.
Run the read multiple properties steps on |properties| and |options|.

The setPropertyObserveHandler() method

The |handler| callback function should implement reading a Property and [=resolve=] with an {{InteractionOutput}} object or [=reject=] with an error.

When the method is invoked given |name:string| and |handler:PropertyReadHandler|, implementations MUST run the following steps:

If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and stop.
If |this|.{{ExposedThing/[[td]]}}.|properties|[|name|] does not [=map/exist=], [= exception/throw =] {{NotFoundError}} and stop.
Set |this|{{ExposedThing/[[observeHandlers]]}}[|name|] to |handler|.

Handling Property observe requests

When a network request for observing a Property |name:string| is received by the implementation with |options:InteractionOptions|, run the following steps:

If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and stop.
If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and stop.
If |this|.{{ExposedThing/[[td]]}}.|properties|[|name|] does not [=map/exist=], send back a {{NotFoundError}} in the reply and stop.
Internally save the request sender information together with |options| and |this|.{{ExposedThing/[[propertyObservers]]}}[|name|], in order to be able to notify about Property value changes.

Every time the value of |property| changes, `emitPropertyChange()` needs to be explicitly called by the application script.

The setPropertyUnobserveHandler() method

The |handler| callback function should implement what to do when an unobserve request is received by the implementation.

When the method is invoked given |name:string| and |handler:PropertyReadHandler|, implementations MUST run the following steps:

If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and stop.
If |this|.{{ExposedThing/[[td]]}}.|properties|[|name|] does not [=map/exist=], [= exception/throw =] {{NotFoundError}} and stop.
Set |this|.{{ExposedThing/[[unobserveHandlers]]}}[|name|] to |handler|.

Handling Property unobserve requests

When a network request for unobserving a Property |name:string| with |options:InteractionOptions| is received by the implementation, run the following steps:

If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and stop.
If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and stop.
If |this|.{{ExposedThing/[[td]]}}.|properties|[|name|] does not [=map/exist=], send back a {{NotFoundError}} in the reply and stop.
Let |handler| be |this|.{{ExposedThing/[[unobserveHandlers]]}}[|name|];
If |handler| is a {{Function}}, invoke that given |options|, then send back a reply following the Protocol Bindings and stop.
Otherwise, if |this|.{{ExposedThing/[[propertyObservers]]}}[|name|] [=map/exists=], remove it from |this|.{{ExposedThing/[[propertyObservers]]}}, send back a reply as defined in the Protocol Bindings and stop.
Otherwise, send back a {{NotFoundError}} in the reply as defined in the Protocol Bindings and stop.

The emitPropertyChange() method

Takes the |name:string| argument, denoting a Property name, and optionally the |data:InteractionInput| argument. Triggers emitting a notification to all observers of the specified Property with the data provided by the |data| argument, or if not provided, it is obtained by the observe handler or the by read handler associated with that Property. The method MUST run the following steps:

Let |promise:Promise| be a new {{Promise}}.
Return |promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
Let |name| be the first argument.
Let |property| be |this|.{{ExposedThing/[[td]]}}.|properties|[|name|].
If |property| is `undefined`, [=reject=] |promise| with {{NotFoundError}} and stop.
Let |data| be the second argument.
If |data| is `undefined`, run the following sub-steps:
1. Let |handler:function| be `null.`
2. If |name| does not [=map/exist=] in {{ExposedThing/[[readHandlers]]}}, [=reject=] |promise| and stop.
3. Let |handler| be {{ExposedThing/[[readHandlers]]}}[|name|].
4. If |handler| is `null` or `undefined`, [=reject=] |promise| and stop.
5. Let |handled| be the result of invoking |handler| given `null`.
6. If |handled| is [=rejected=], then [=reject=] |promise| and stop.
7. Otherwise if |handled| [=resolved=] with |value|, let |data| be |value|.
For each |observer| in {{ExposedThing/[[propertyObservers]]}}[|name|], run the following sub-steps:
1. Let |options| be the interaction options saved with |observer|.
2. Request the underlying platform to create a |reply| from |data| and |options| according to the Protocol Bindings.
  This clause needs expanding and/or refer to an algorithm in [[wot-binding-templates]].
3. Send |reply| to |observer|.
[=Resolve=] |promise|.

The PropertyWriteHandler callback

A function that is called when an external request for writing a Property is received and defines what to do with such requests. Takes as argument |value:InteractionOutput| and returns a {{Promise}}, resolved when the value of the Property - identified by the name provided when setting the handler has been updated -, or rejects with an error if the property is not found or the value cannot be updated.

Note that the code in this callback function can read the property before updating it in order to find out the old value, if needed. Therefore the old value is not provided to this function.

The value is provided by implementations as an {{InteractionOutput}} object in order to be able to represent values that are not described by a DataSchema, such as streams.

The setPropertyWriteHandler() method

Note that even for readonly Properties it is possible to specify a write handler, as explained in Issue 199. In this case, the write handler may define in an application-specific way to fail the request.

There MUST be at most one write handler for any given Property, so newly added handlers MUST replace the previous handlers. If no write handler is initialized for any given Property, implementations SHOULD implement default property update if the Property is writeable and notifying observers on change if the Property is observable, based on the Thing Description.

When the method is invoked given |name:string| and |handler:PropertyWriteHandler|, implementations MUST run the following steps:

If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and stop.
If |this|.{{ExposedThing/[[td]]}}.|properties|[|name|] does not [=map/exist=], [= exception/throw =] {{NotFoundError}} and stop.
Set |this|.{{ExposedThing/[[writeHandlers]]}}[|name|] to |handler|.

Handling requests for writing a Property

If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and stop.
If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and stop.
Let |interaction| be |this|.{{ExposedThing/[[td]]}}.|properties|[|name|].
If |interaction| is `undefined`, return a {{NotFoundError}} in the reply and stop.
Let |handler:function| be |this|.{{ExposedThing/[[writeHandlers]]}}[|name|].
If |handler| is `undefined` and if there is a default write handler provided by the implementation, let |handler| be that.
If |handler| is `undefined`, send back a {{NotSupportedError}} with the reply and stop.
Let |promise| be the result of invoking |handler| given |name| and |options|. If it fails, return the error in the reply and stop.
If |mode| is `"single"`, reply to the request reporting success, following the Protocol Bindings and stop.

Handling requests for writing multiple Properties

When a network request for writing multiple Properties given in an object |propertyNames| is received with |options:InteractionOptions|, run the following steps:

If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and stop.
If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and stop.
For each property with key |name| and value |value| defined in |propertyNames|, run the update property steps with |name|, |value|, |options| and |mode| set to `"multiple"`. If that fails, reply to the request with that error and stop.
Reply to the request by sending a single reply according to the Protocol Bindings.

The ActionHandler callback

A function that is called when an external request for invoking an Action is received and defines what to do with such requests. It is invoked given |params:InteractionOutput| and optionally with an |options:InteractionOptions| object. It returns a {{Promise}} that rejects with an error or resolves with the value returned by the Action as {{InteractionInput}}.

Application scripts MAY return a {{ReadableStream}} object from an {{ActionHandler}}. Implementations will then use the stream for constructing the Action's response.

The setActionHandler() method

The |action| callback function will implement an Action and SHOULD be called by implementations when a request for invoking the Action is received from the underlying platform.

There MUST be at most one handler for any given Action, so newly added handlers MUST replace the previous handlers.

When the method is invoked given |name:string| and |action:ActionHandler|, run the following steps:

If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and stop.
Let |interaction| be |this|.{{ExposedThing/[[td]]}}.|actions|[|name|].
If |interaction| is `undefined`, [= exception/throw =] a {{NotFoundError}} and stop.
Set |this|.{{ExposedThing/[[actionHandlers]]}}[|name|] to |action|.

Handling Action requests

If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and stop.
If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and stop.
Let |interaction| be |this|.{{ExposedThing/[[td]]}}.|properties|[|name|].
If |interaction| is `undefined`, return a {{NotFoundError}} in the reply and stop.
Let |handler:function| be |this|.{{ExposedThing/[[actionHandlers]]}}[|name|].
If |handler| is `undefined`, return a {{NotSupportedError}} with the reply created by following the Protocol Bindings and stop.
Let |promise| be the result of invoking |handler| given |name|, |inputs| and |options|.
If |promise| rejects, send the error with the reply and stop.
When |promise| resolves with |data:InteractionInput|, use |data| to create and send the reply according to the Protocol Bindings.

The EventSubscriptionHandler callback

A function that is called when an external request for subscribing to an Event is received and defines what to do with such requests. It is invoked given an |options:InteractionOptions| object provided by the implementation and coming from subscribers. It returns a {{Promise}} that rejects with an error or resolves when the subscription is accepted.

The setEventSubscribeHandler() method

The |handler| callback function SHOULD implement what to do when an subscribe request is received, for instance necessary initializations. Note that the handler for emitting Events is set separately.

There MUST be at most one event subscribe handler for any given Event, so newly added handlers MUST replace the previous handlers.

When the method is invoked given |name:string| and |handler:EventSubscriptionHandler|, run the following steps:

If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and stop.
Let |interaction| be |this|.{{ExposedThing/[[td]]}}.|events|[|name|].
If |interaction| is `undefined`, [= exception/throw =] a {{NotFoundError}} and stop.
Set |this|.{{ExposedThing/[[subscribeHandlers]]}}[|name|] to |handler|.
Return `this`.

Handling Event subscribe requests

When an Event subscription request for |name| is received by the underlying platform with optional |options:InteractionOptions|, run the following steps:

If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and stop.
If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and stop.
Let |interaction| be |this|.{{ExposedThing/[[td]]}}.|events|[|name|].
If |interaction| is `undefined`, send back a {{NotFoundError}} and stop.
If |this|.{{ExposedThing/[[subscribeHandlers]]}}[|name|] is a {{Function}}, invoke it given |options| and stop.
Otherwise implement the default subscriber mechanism:
1. Let |subscriber| be a tuple formed of |options| (from which |uriVariables| and |data| may be used) and the subscriber information needed to create an Event notification response.
2. Set |this|.{{ExposedThing/[[eventListeners]]}}[|name|] to |subscriber|.

The setEventUnsubscribeHandler() method

The |handler| callback function SHOULD implement what to do when an unsubscribe request is received.

There MUST be at most one handler for any given Event, so newly added handlers MUST replace the previous handlers.

When the method is invoked given |name:string| and |handler:EventSubscriptionHandler|, run the following steps:

If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and stop.
Let |interaction| be |this|.{{ExposedThing/[[td]]}}.|events|[|name|].
If |interaction| is `undefined`, [= exception/throw =] a {{NotFoundError}} and stop.
Set |this|.{{ExposedThing/[[unsubscribeHandlers]]}}[|name|] to |handler|.
Return `this`.

Handling Event unsubscribe requests

When an Event unsubscribe request for |name| is received by the underlying platform optionally with |options:InteractionOptions|, run the following steps:

If this operation is not supported, send back a {{NotSupportedError}} according to the Protocol Bindings and stop.
If this operation is not allowed, send back a {{NotAllowedError}} according to the Protocol Bindings and stop.
Let |interaction| be |this|.{{ExposedThing/[[td]]}}.|events|[|name|].
If |interaction| is `undefined`, send back a {{NotFoundError}} and stop.
If |this|.{{ExposedThing/[[unsubscribeHandlers]]}}[|name|] [=map/exists=] and is a {{Function}}, invoke it given |options| and stop.
Otherwise |name| [=map/exists] in |this|.{{ExposedThing/[[eventListeners]]}}, [=map/remove=] |name|.
Return `this`.

Handling Events

When an Event with name |name| is emitted with |data:InteractionInput| by the emitEvent() method, run the following steps:

Let |listeners| be {{ExposedThing/[[eventListeners]]}}.|name|.
For each |subscriber| in |listeners|, run the following sub-steps:
1. Create an Event notification |response| according to the Protocol Bindings from |data| and |subscriber|, including its |options|.
2. If |data| is `undefined`, assume that the notification |response| will contain an empty data payload as specified by Protocol Bindings.
3. If the underlying protocol stack permits conveying event errors and if an error condition has been detected by the UA, create |response| as an error notification according to the Protocol Bindings, using |data|, |subscriber| and its |options|.
  The error reporting is protocol specific and it is encapsulated by implementations. On the client end, the error listener passed with the subscription will be invoked if the client UA detects the error.
4. Send |response| to the subscriber identified by |subscriber|.

The emitEvent() method

Takes as arguments |name:string| denoting an Event name and optionally |data:InteractionInput|. Triggers emitting the Event with the optional data. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
Let |interaction| be {{ExposedThing/[[td]]}}.|events|.|name|.
If an Event with the name |name| is not found, [=reject=] |promise| with {{NotFoundError}} and stop.
Make a request to the underlying platform to emit an Event with optional |data|. Call the handling events steps.

The expose() method

Start serving external requests for the Thing, so that WoT Interactions using Properties, Actions and Events will be possible. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
Run the expand a TD steps on the {{ExposedThing/[[td]]}}.
Run the validate a TD on {{ExposedThing/[[td]]}}. If that fails, [=reject=] |promise| with a {{TypeError}} and stop.
For each |key| in {{ExposedThing/[[td]]}}.|properties| initialize |this|.{{ExposedThing/[[propertyObservers]]}}.|key| to an empty {{Array}} in order to store observe request data needed to notify the observers on value changes.
For each |key| in |this|.{{ExposedThing/[[td]]}}.|events| initialize |this|.{{ExposedThing/[[eventListeners]]}}.|key| to an empty {{Array}} in order to store subscribe request data needed to notify the subscribers on event emission.
Set up the WoT Interactions based on introspecting {{ExposedThing/[[td]]}} as explained in [[!wot-thing-description11]] and [[!wot-binding-templates]]. Make a request to the underlying platform to initialize the Protocol Bindings and then start serving external requests for WoT Interactions (read, write and observe Properties, invoke Actions and manage Event subscriptions), based on the Protocol Bindings. Implementations MAY reject this step for any reason (e.g. if they want to enforce further checks and constraints on interaction forms).
If there was an error during the request, [=reject=] |promise| with an {{Error}} object |error| with |error|.|message| set to the error code seen by the Protocol Bindings and stop.
Otherwise [=resolve=] |promise| and stop.

The destroy() method

Stop serving external requests for the Thing and destroy the object. Note that eventual unregistering should be done before invoking this method. The method MUST run the following steps:

Return a {{Promise}} |promise:Promise| and execute the next steps [=in parallel=].
If invoking this method is not allowed for the current scripting context for security reasons, [=reject=] |promise| with a {{SecurityError}} and stop.
Make a request to the underlying platform to stop serving external requests for WoT Interactions, based on the Protocol Bindings.
If there was an error during the request, [=reject=] |promise| with an {{Error}} object |error| with its |message| set to the error code seen by the Protocol Bindings and stop.
Otherwise [=resolve=] |promise| and stop.

ExposedThing Examples

The next example illustrates how to create an {{ExposedThing}} based on a partial TD object constructed beforehand.

        try {
          let temperaturePropertyDefinition = {
            type: "number",
            minimum: -50,
            maximum: 10000
          };
          let tdFragment = {
            properties: {
              temperature: temperaturePropertyDefinition
            },
            actions: {
              reset: {
                description: "Reset the temperature sensor",
                input: {
                  temperature: temperatureValueDefinition
                },
                output: null,
                forms: []
              },
            },
            events: {
              onchange: temperatureValueDefinition
            }
          };
          let thing1 = await WOT.produce(tdFragment);
          // initialize Properties
          await thing1.writeProperty("temperature", 0);
          // add service handlers
          thing1.setPropertyReadHandler("temperature", () => {
             return readLocalTemperatureSensor();  // Promise
          });
          // start serving requests
          await thing1.expose();
        } catch (err) {
           console.log("Error creating ExposedThing: " + err);
        }

The next example illustrates how to add or modify a Property definition on an existing {{ExposedThing}}: take its |td| property, add or modify it, then create another {{ExposedThing}} with that.

        try {
          // create a deep copy of thing1's TD
          let instance = JSON.parse(JSON.stringify(thing1.td));
          const statusValueDefinition = {
            type: "object",
            properties: {
              brightness: {
                type: "number",
                minimum: 0.0,
                maximum: 100.0,
                required: true
              },
              rgb: {
                type: "array",
                "minItems": 3,
                "maxItems": 3,
                items : {
                    "type" : "number",
                    "minimum": 0,
                    "maximum": 255
                }
              }
          };
          instance["name"] = "mySensor";
          instance.properties["brightness"] = {
            type: "number",
            minimum: 0.0,
            maximum: 100.0,
            required: true,
          };
          instance.properties["status"] = statusValueDefinition;
          instance.actions["getStatus"] = {
            description: "Get status object",
            input: null,
            output: {
              status : statusValueDefinition;
            },
            forms: [...]
          };
          instance.events["onstatuschange"] = statusValueDefinition;
          instance.forms = [...];  // update
          var thing2 = new ExposedThing(instance);
          // TODO: add service handlers
          await thing2.expose();
          });
        } catch (err) {
           console.log("Error creating ExposedThing: " + err);
        }

The following will cover a set of examples for the generation of a Thing Description from an ExposedThingInit using expand an ExposedThingInit steps. As hypothesis the runtime supports HTTP and COAP protocol bindings and it is hosted at 192.168.0.1.

The next example shows how to exploit a ExposedThingInit to create a simple Thing Description with one Property with the default values.

          {
            "properties" : {
              "temperature":{}
            }
          }

          {
            "@context": [
              "https://www.w3.org/2019/wot/td/v1"
            ],
            "title": "Thing-123",
            "securityDefinitions": {
              "no_sec": {
                "scheme": "nosec"
              }
            },
            "security": "no_sec",
            "properties": {
              "temperature": {
                "forms": [{
                  "href": "https://192.168.0.1:8080/properties/temperature",
                  "contentType": "application/json"
                },
                {
                "href": "coap://192.168.0.1:9090/properties/temperature",
                "contentType": "application/json"
                }]
              }
            }
          }

TODO: add more examples where the ExposedThingInit contains suggested values that are replaced by the algorithm.

The ThingDiscoveryProcess interface

Discovery is a distributed application that requires provisioning and support from participating network nodes (clients, servers, directory services). This API models the client side of typical discovery schemes supported by various IoT deployments.

The {{ThingDiscoveryProcess}} object provides the properties and methods controlling the discovery process and returning the results.

      [SecureContext, Exposed=(Window,Worker)]
      interface ThingDiscoveryProcess {
        constructor(optional ThingFilter filter = {});
        readonly attribute boolean done;
        readonly attribute Error? error;
        undefined stop();
        async iterable<ThingDescription>;
      };

The {{ThingDiscoveryProcess}} object has the following internal slots.

Internal Slot	Initial value	Description (non-normative)
[[\filter]]	`undefined`	The {{ThingFilter}} object used in discovery.
[[\url]]	`undefined`	A {{URL}} representing the TD Directory when used within the `exploreDirectory()` method.

The done property is `true` if the discovery has been stopped or completed with no more results to report.

The error property represents the last error that occurred during the discovery process. Typically used for critical errors that stop discovery.

The {{ThingDiscoveryProcess}} object implements the async iterator concept.

Constructing {{ThingDiscoveryProcess}}

To create {{ThingDiscoveryProcess}} with a |filter:ThingFilter|, run the following steps:

If |filter| is not an object or `null`, [= exception/throw =] a {{TypeError}} and stop.
Let |discovery:ThingDiscoveryProcess| be a new {{ThingDiscoveryProcess}} object.
Set |discovery|.{{ThingDiscoveryProcess/[[filter]]}} to |filter|.
Set |discovery|.{{ThingDiscoveryProcess/done}} to `false`.
Set |discovery|.{{ThingDiscoveryProcess/error}} to `null`.
Return |discovery|.

The ThingFilter dictionary

Represents an object containing the constraints for discovering Things as key-value pairs.

        dictionary ThingFilter {
          object? fragment;
          
        };

The fragment property represents a template object used for matching property by property against discovered Things.

The |query:DOMString| property was temporarily removed from ThingFilter, until it is standardized in the WoT Discovery task force. It represented a query string accepted by the implementation, for instance a SPARQL or JSON query. Support was to be implemented locally in the WoT Runtime or remotely as a service in a TD Directory.

The |url:USVString| property was removed. It used to represent the target entity serving the discovery request, for instance the URL of a TD Directory, or the URL of a directly targeted Thing, but these are implemented by dedicated methods now.

The discovery process algorithm

Describes what to do during a discovery process that has already started. The algorithm, given |discovery:ThingDiscoveryProcess|, runs the following steps:

Whenever a new |link| to a Thing Description is discovered and provided by the underlying platform, run the following sub-steps:
1. Retrieve |td:ThingDescription| as a JSON object, using the Protocol Binding used by the underlying discovery process (as specified by |link|). In the case of an HTTP(S) Binding, this process could use the Fetch API for the |td|'s retrieval.
2. If that fails, set the |discovery|.{{ThingDiscoveryProcess/error}} property to {{SyntaxError}}, discard |td| and continue the discovery process.
Whenever a Thing Description |td| is discovered and provided by the underlying platform or by the previous step, run the following sub-steps:
At this point implementations MAY control the flow of the discovery process (depending on memory constraints, for instance queue the results, or temporarily stop discovery if the queue is getting too large, or resume discovery when the queue is emptied sufficiently). These steps are run for each discovered/fetched |td|.
1. Let |fragment| be |discovery|.{{ThingDiscoveryProcess/[[filter]]}}.{{ThingFilter/fragment}}.
2. If |fragment| is an {{object}}, then for each |key| defined in it:
  1. Check if that |key| [=map/exists=] in |json| and |json|[|key| is equal to |fragment|.|key|.
  2. If this is fails in any checks, discard |td| and continue the discovery process.
3. Yield |td| using {{Symbol/asyncIterator}}.
  Improve this step using proper asyncIterator terminology.
Whenever an error occurs during the discovery process, run the following sub-steps:
The last error is retained. Implementations MAY choose to stop the discovery process if they consider it should be reported.
1. Let |error| be a new {{Error}} object. Set |error|.|name| to `"DiscoveryError"`.
2. If there was an error code or message provided by the Protocol Bindings, set |error|.|message| to that value as string.
3. Set |discovery|.{{ThingDiscoveryProcess/error}} to |error|.
4. If the error is irrecoverable and discovery has been stopped by the underlying platform, or if the implementation decided to stop the discovery process and report the error, set |discovery|.{{ThingDiscoveryProcess/done}} to `true` and terminate these steps.

The stop() method

Stops or suppresses the discovery process. It might not be supported by all discovery methods and endpoints, however, any further discovery results or errors will be discarded and the discovery is marked as done. The method MUST run the following steps:

If invoking this method is not allowed for the current scripting context for security reasons, [= exception/throw =] a {{SecurityError}} and stop.
Request the underlying platform to stop the discovery process. If this returns an error, or if it is not possible, for instance when discovery is based on open ended multicast requests, the implementation SHOULD discard subsequent discovered items.
Set the {{ThingDiscoveryProcess/done}} property to `true`.

Discovery Examples

The following example finds {{ThingDescription}} objects of Things that are exposed by local hardware, regardless how many instances of WoT Runtime it is running Using the {{Symbol/asyncIterator}} provided by the Discovery object, we can iterate asynchronously over the results and perform operations with the obtained {{ThingDescription}} objects.

        let url = "https://mythings.com/thing1";
        let td = await WOT.requestThingDescription(url);
        console.log("Found Thing Description for " + td.title);

The next example finds {{ThingDescription}} objects of Things listed in a TD Directory service. We set a timeout for safety.

        let discovery = await WOT.exploreDirectory("https://directory.wotservice.org");
        setTimeout( () => {
            discovery.stop();
            console.log("Discovery stopped after timeout.");
          },
          3000);
        for await (const td of discovery) {
          console.log("Found Thing Description for " + td.title);
          let thing = new ConsumedThing(td);
          console.log("Thing name: " + thing.getThingDescription().title);
        };
        if (discovery.error) {
          console.log("Discovery stopped because of an error: " + error.message);
        }

The next example is for a generic discovery, by any means provisioned to the WOT runtime, including local Things, if any is available.

        let discovery = await WOT.discover();
        setTimeout( () => {
            discovery.stop();
            console.log("Stopped open-ended discovery");
          },
          10000);
        for await (const td of discovery) {
          console.log("Found Thing Description for " + td.title);
        };
        if (discovery.error) {
          console.log("Discovery stopped because of an error: " + error.message);
        }

Security and Privacy

A detailed discussion of security and privacy considerations for the Web of Things, including a threat model that can be adapted to various circumstances, is presented in the informative document [[!wot-security]]. This section discusses only security and privacy risks and possible mitigations directly relevant to the scripts and WoT Scripting API.

A suggested set of best practices to improve security for WoT devices and services has been documented in [[!wot-security]]. That document may be updated as security measures evolve. Following these practices does not guarantee security, but it might help avoid commonly known vulnerabilities.

The WoT security risks and possible mitigations are concerning the following groups:

Implementors of WoT Runtimes that do not implement a Scripting Runtime. The [[!wot-architecture11]] document provides generic security guidelines for this group.
Implementors of the WoT Scripting API in a WoT Scripting Runtime. This is the main scope and is covered in the Scripting Runtime Security and Privacy Risks sub-section that contains normative text regarding security.
WoT script developers, covered in the Script Security and Privacy Risks sub-section that contains informative recommendations concerning security.

Scripting Runtime Security and Privacy Risks

This section is normative and contains specific risks relevant for the WoT Scripting Runtime.

Corrupted Input Security and Privacy Risk

A typical way to compromise any process is to send it a corrupted input via one of the exposed interfaces. This can be done to a script instance using WoT interface it exposes.

Mitigation:: Implementors of this API SHOULD perform validation on all script inputs. In addition to input validation, fuzzing should be used to verify that the input processing is done correctly. There are many tools and techniques in existence to do such validation. More details can be found in [[!wot-security]].

Physical Device Direct Access Security and Privacy Risk

In case a script is compromised or misbehaving, the underlying physical device (and potentially surrounded environment) can be damaged if a script can use directly exposed native device interfaces. If such interfaces lack safety checks on their inputs, they might bring the underlying physical device (or environment) to an unsafe state (i.e. device overheats and explodes).

Mitigation:: The WoT Scripting Runtime SHOULD avoid directly exposing the native device interfaces to the script developers. Instead, a WoT Scripting Runtime should provide a hardware abstraction layer for accessing the native device interfaces. Such hardware abstraction layer should refuse to execute commands that might put the device (or environment) to an unsafe state. Additionally, in order to reduce the damage to a physical WoT device in cases a script gets compromised, it is important to minimize the number of interfaces that are exposed or accessible to a particular script based on its functionality.

Provisioning and Update Security Risk

If the WoT Scripting Runtime supports post-manufacturing provisioning or updates of scripts, WoT Scripting Runtime or any related data (including security credentials), it can be a major attack vector. An attacker can try to modify any above described element during the update or provisioning process or simply provision attacker's code and data directly.

Mitigation:: Post-manufacturing provisioning or update of scripts, WoT Scripting Runtime or any related data should be done in a secure fashion. A set of recommendations for secure update and post-manufacturing provisioning can be found in [[!wot-security]].

Security Credentials Storage Security and Privacy Risk

Typically the WoT Scripting Runtime needs to store the security credentials that are provisioned to a WoT device to operate in WoT network. If an attacker can compromise the confidentiality or integrity of these credentials, then it can obtain access to the WoT assets, impersonate WoT things or devices or create Denial-Of-Service (DoS) attacks.

Mitigation:: The WoT Scripting Runtime should securely store the provisioned security credentials, guaranteeing their integrity and confidentiality. In case there are more than one tenant on a single WoT-enabled device, a WoT Scripting Runtime should guarantee isolation of each tenant provisioned security credentials. Additionally, in order to minimize a risk that provisioned security credentials get compromised, the WoT Scripting Runtime should not expose any API for scripts to query the provisioned security credentials.

Script Security and Privacy Risks

This section describes specific risks relevant for script developers.

Corrupted Script Input Security and Privacy Risk

A script instance may receive data formats defined by the TD, or data formats defined by the applications. While the WoT Scripting Runtime SHOULD perform validation on all input fields defined by the TD, scripts may be still exploited by input data.

Mitigation:: Script developers should perform validation on all application defined script inputs. In addition to input validation, fuzzing could be used to verify that the input processing is done correctly. There are many tools and techniques in existence to do such validation. More details can be found in [[!wot-security]].

Denial of Service (DoS) Security Risk

If a script performs heavy functional processing on received requests before the request is authenticated, it presents a great risk for Denial-of-Service (DoS) attacks.

Mitigation:: Scripts should avoid heavy functional processing without prior successful authentication of requestor. The set of recommended authentication mechanisms can be found in [[!wot-security]].

API design rationale

API rationale usually belongs to a separate document, but in the WoT case the complexity of the context justifies including basic rationale here.

Approaches to WoT application development

The WoT Interest Group and Working Group have explored different approaches to application development for WoT that have been all implemented and tested.

No Scripting API

It is possible to develop WoT applications that only use the WoT network interface, typically exposed by a WoT gateway that presents a RESTful API towards clients and implements IoT protocol plugins that communicate with supported IoT deployments. One such implementation is the Mozilla WebThings platform.

Simple Scripting API

WoT Things show good synergy with software objects, so a Thing can be represented as a software object, with Properties represented as object properties, Actions as methods, and Events as events. In addition, metadata is stored in special properties. Consuming and exposing is done with factory methods that produce a software object that directly represents a remote Thing and its interactions. One such implementation is the Arena Web Hub project.

In the next example, a Thing that represents interactions with a lock would look like the following: the |status| property and the open() method are directly exposed on the object.

          let lock = await WoT.consume('https://td.my.com/lock-00123');
          console.log(lock.status);
          lock.open('withThisKey');

This API, aligned with the [[[wot-thing-description11]]] specification

Since the direct mapping of Things to software objects have had some challenges, this specification takes another approach that exposes software objects to represent the Thing metadata as data property and the WoT interactions as methods. One implementation is node-wot in the Eclipse ThingWeb project, which is the current reference implementation of the API specified in this document.

The same example now would look like the following: the |status| property and the open() method are represented indirectly.

          const lockTd = await WoT.requestThingDescription('https://td.my.com/lock-00123');
          const lock = WoT.consume(lockTd);
          console.log(lock.readProperty('status'));
          lock.invokeAction('open', 'withThisKey');

In conclusion, the WoT WG decided to explore the third option that closely follows the [[[wot-thing-description11]]] specification. Based on this, a simple API can also be implemented. Since Scripting is an optional module in WoT, this leaves room for applications that only use the WoT network interface. Therefore all three approaches above are supported by the [[[wot-thing-description11]]] specification.

Moreover, the WoT network interface can be implemented in many languages and runtimes. Consider this API an example for what needs to be taken into consideration when designing a Scripting API for WoT.

Requesting and validating a TD

The current version of this specification defines a new `requestThingDescription` method that simplifies the process of retrieving and validating a Thing Description. However, it only covers simple use cases that do not require additional parameters or special HTTP headers for the retrieval.

More sophisticated use cases need to be covered by external methods, such as the Fetch API or an HTTP client library, which offer already standardized options on specifying fetch details. In these cases, the user is required to perform validation manually as described by the [[[wot-thing-description11]]] specification.

In the future, `requestThingDescription()` might be extended with an `options` argument, including frequently used `fetch` options. Please open an issue to request support for options.

Observers

Earlier drafts used the Observer construct, but since it has not become standard, a new design was needed that was light enough for embedded implementations. Therefore observing Property changes and handling WoT Events is done with callback registrations.

Using Events

This API ended up not using software events at all, for the following reasons:

Subscription to WoT Events may be different from handling software events (subscription might need parameters, might involve security tokens etc).
Most implementations are for Node.js and browser implementations will likely be libraries (because possible dependency management issues in native implementations), using Events has been challenging.
Observing Property changes and handling WoT Events is done with the solution above.

Polymorphic functions

The reason to use function names like readProperty(), readMultipleProperties() etc. instead of a generic polymorphic read() function is that the current names map exactly to the "op" vocabulary from the Form definition in the [[[wot-thing-description11]]] specification.

Changes

The following is a list of major changes to the document. Major versions of this specification are the following:

First Public Working Draft September 2017.
Working Draft April 2018.
Working Draft November 2018.
Working draft October 2019.
This version, introducing the following major changes:
- Added support for `formIndex`, `InteractionData` including streams.
- Cleaned up the specification prose, aligned terminology and used modern ReSpec.
- Improved algorithm descriptions, included illustrative figures.

For a complete list of changes, see the github change log. You can also view the recently closed issues.

Full Web IDL

Acknowledgements

Special thanks to former editor Johannes Hund (until August 2017, when at Siemens AG) and Kazuaki Nimura (until December 2018) for developing this specification. Also, the editors would like to thank Dave Raggett, Matthias Kovatsch, Michael Koster, Elena Reshetova, Michael McCool as well as the other WoT WG members for their comments, contributions and guidance.

Original Source | Taken Source

Introduction

Use Case Scenarios

Consuming a Thing

Exposing a Thing

Discovery

Terminology and conventions

The ThingDescription type

Requesting a Thing Description

Expanding a Thing Description

Validating a Thing Description

The WOT namespace

The consume() method

The produce() method

Expand an ExposedThingInit

Validating an ExposedThingInit

The discover() method

The exploreDirectory() method

The requestThingDescription() method

Handling interaction data

The InteractionInput type

The InteractionOutput interface

The value() function

The arrayBuffer() function

The check data schema algorithm

The create interaction request algorithm

The parse interaction response algorithm

The ActionInteractionOutput interface

The query() function

The cancel() function

Using {{InteractionInput}}, {{InteractionOutput}} and {{ActionInteractionOutput}}

Error handling

The ConsumedThing interface

Internal slots for {{ConsumedThing}}

Creating ConsumedThing

The getThingDescription() method

The readProperty() method

The readMultipleProperties() method

The readAllProperties() method

The writeProperty() method

The writeMultipleProperties() method

The observeProperty() method

The invokeAction() method

The subscribeEvent() method

The InteractionOptions dictionary

The PropertyReadMap type

The PropertyWriteMap type

The InteractionListener callback

The ErrorListener callback

The Subscription interface

Internal slots for {{Subscription}}

The stop() method

Finding an unsubscribe Form

ConsumedThing Examples

The ExposedThing interface

Internal slots for ExposedThing

Creating {{ExposedThing}}

The getThingDescription() method

The PropertyReadHandler callback

The setPropertyReadHandler() method

Handling requests for reading a Property

Handling requests for reading multiple Properties

Handling requests for reading all Properties

The setPropertyObserveHandler() method

Handling Property observe requests

The setPropertyUnobserveHandler() method

Handling Property unobserve requests

The emitPropertyChange() method

The PropertyWriteHandler callback

The setPropertyWriteHandler() method

Handling requests for writing a Property

Handling requests for writing multiple Properties

The ActionHandler callback

The setActionHandler() method

Handling Action requests

The EventSubscriptionHandler callback

The setEventSubscribeHandler() method

Handling Event subscribe requests

The setEventUnsubscribeHandler() method

Handling Event unsubscribe requests

Handling Events

Creating `ConsumedThing`