File API

6.2.1. Parameters to slice

When this specification says to normalize parameters for the slice call, the following steps MUST be followed for the start, end, and contentType parameters, which are defined below.

The optional start parameter is a value for the start point of a slice call, and MUST be treated as a byte-order position, with the zeroth position representing the first byte. The following requirements are normative for this parameter, and user agents MUST process slice with start normalized according to the following:

1. If the optional start parameter is not used as a parameter when making this call, let relativeStart be 0.

2. If start is negative, let relativeStart be max((size + start), 0)).

3. Else, let relativeStart be min(start, size).

4. This defines the normalization of the start parameter. When processing the slice call, user agents must normalize start to relativeStart.

The optional end parameter is a value for the end point of a slice call. The following requirements are normative for this parameter, and user agents MUST process slice with end normalized according to the following:

1. If the optional end parameter is not used as a parameter when making this call, let relativeEnd be size.

2. If end is negative, let relativeEnd be max((size + end), 0)

3. Else, let relativeEnd be min(end, size)

4. This defines the normalization of the end parameter. When processing the slice call, user agents must normalize end to relativeEnd.

The optional contentType parameter is used to set a value identical to one that is set with the HTTP/1.1 Content-Type header [HTTP] on the Blob returned by the slice call. The following requirements are normative for this parameter, and user agents MUST process the slice with contentType normalized according to the following:

1. If the contentType parameter is not provided, let relativeContentType be set to the empty string .

2. If the contentType parameter is undefined, let relativeContentType be set to the empty string.

3. Else let relativeContentType be set to contentType.

4. This defines the normalization of the contentType parameter. When processing the slice call, user agents must normalize contentType to relativeContentType.

6.2.2. The slice method

The slice method returns a new Blob object with bytes ranging from the optional start parameter upto but not including the optional end parameter, and with a type attribute that is the value of the optional contentType parameter. It MUST act as follows :

1. Let O be the Blob object on which the slice method is being called.

2. Normalize the parameters start, end, and contentType to relativeStart, relativeEnd, and relativeContentType respectively.

3. Let span be max((relativeEnd - relativeStart), 0).

4. Return a new Blob object A with the following characteristics:

   1. A consists of span consecutive bytes from O, beginning with the byte at byte-order position relativeStart.

   2. A.size = span.

   3. A.type = relativeContentType.

    // obtain input element through DOM
    
    var file = document.getElementById('file').files[0];
    if(file)
    {
      // create an identical copy of file
      // the two calls below are equivalent
      
      var fileClone = file.slice(); 
      var fileClone2 = file.slice(0, file.size);
      
      // slice file into 1/2 chunk starting at middle of file
      // Note the use of negative number
      
      var fileChunkFromEnd = file.slice(-(Math.round(file.size/2)));
      
      // slice file into 1/2 chunk starting at beginning of file
      
      var fileChunkFromStart = file.slice(0, Math.round(file.size/2));
      
      // slice file from beginning till 150 bytes before end
      
      var fileNoMetadata = file.slice(0, -150, "application/experimental");      
    }
    

8.5.1. Multiple Reads

The FileReader interface makes available four asynchronous read methods -- readAsArrayBuffer, readAsBinaryString, readAsText, and readAsDataURL, which read files into memory. If multiple concurrent read methods are called on the same FileReader object, user agents MUST throw an InvalidStateError [DOMCore] on any of the read methods that occur when readyState = LOADING.

8.5.2. The result attribute

On getting, the result attribute returns a Blob's data as a DOMString, or as an ArrayBuffer [TypedArrays], or null, depending on the read method that has been called on the FileReader, and any errors that may have occurred. It can also return partial Blob data. Partial Blob data is the part of the File or Blob that has been read into memory currently; when processing the read methods readAsBinaryString or readAsText, partial Blob data is a DOMString that is incremented as more bytes are loaded (a portion of the total) [ProgressEvents], and when processing readAsArrayBuffer partial Blob data is an ArrayBuffer [TypedArrays] object consisting of the bytes loaded so far (a portion of the total)[ProgressEvents]. The list below is normative for the result attribute and is the conformance criteria for this attribute:

• On getting, if the readyState is EMPTY (no read method has been called) then the result attribute MUST return null.

• On getting, if an error in reading the File or Blob has occurred (using any read method), then the result attribute MUST return null.

• On getting, if the readAsDataURL read method is used, the result attribute MUST return a DOMString that is a Data URL [DataURL] encoding of the File or Blob's data.

• On getting, if the readAsBinaryString read method is called and no error in reading the File or Blob has occurred, then the result attribute MUST return a DOMString representing the File or Blob's data as a binary string, in which every byte is represented by an integer in the range [0..255]. On getting, while processing the readAsBinaryString read method, the result attribute SHOULD return partial Blob data in binary string format as a DOMString that is incremented as more data is read. User agents MUST return at least one such result, with the final result returned at completion of the read.

• On getting, if the readAsText read method is called and no error in reading the File or Blob has occurred, then the result attribute MUST return a string representing the File or Blob's data as a text string, and SHOULD decode the string into memory in the format specified by the encoding determination. On getting, while processing the readAsText read method, this attibute SHOULD return partial Blob data in the format specified by the encoding determination as a DOMString that is incremented as more data is read. User agents MUST return at least one such result, with the final result returned at completion of the read. See the caveat about partial Blob data being valid within a giving encoding.

• On getting, if the readAsArrayBuffer read method is called and no error in reading the File or Blob has occurred, then the result attribute MUST return an ArrayBuffer [TypedArrays] object. On getting, while processing the readAsArrayBuffer read method, the result attribute SHOULD return partial Blob data as an ArrayBuffer [TypedArrays]. User agents MUST return at least one such result, with the final result returned at completion of the read.

8.5.3. The readAsBinaryString(blob) method

When the readAsBinaryString(blob) method is called, the user agent MUST run the steps below (unless otherwise indicated).

1. If readyState = LOADING throw an InvalidStateError exception [DOMCore] and terminate this algorithm.

   Note: The readAsBinaryString() method returns due to the algorithm being terminated.

2. If an error occurs during reading of the blob parameter, set readyState to DONE and set result to null. Proceed to the error steps below.

   1. Fire a progress event called error. Set the error attribute; on getting, the error attribute MUST be a a DOMError object that indicates the kind of file error that has occurred.

   2. Fire a progress event called loadend.

   3. Terminate this algorithm.

      Note: The readAsBinaryString() method returns due to the algorithm being terminated.

3. If no error has occurred, set readyState to LOADING

4. Fire a progress event called loadstart.

5. Return the readAsBinaryString() method, but continue processing the other steps in this algorithm.

6. Make progress notifications.

7. While processing the read, as data from blob becomes available, user agents SHOULD queue tasks to update the result with partial Blob data represented as a binary string to accompany the firing of progress events until the read is complete. On getting, the result attribute returns partial Blob data representing the number of bytes currently loaded (as a fraction of the total) [ProgressEvents], as a binary string; user agents MUST return at least one such result while processing this read method. The last returned value is at completion of the read.

8. When the blob has been read into memory fully, set readyState to DONE

9. Terminate this algorithm.

8.5.4. The readAsDataURL(blob) method

When the readAsDataURL(blob) method is called, the user agent MUST run the steps below (unless otherwise indicated).

1. If readyState = LOADING throw an InvalidStateError exception [DOMCore] and terminate this algorithm.

   Note: The readAsDataURL() method returns due to the algorithm being terminated.

2. If an error occurs during reading of the blob parameter, OR if a user agent's URL length limitations prevent returning data as a Data URL [DataURL], set readyState to DONE and set result to null. Proceed to the error steps below.

   1. Fire a progress event called error. Set the error attribute; on getting, the error attribute MUST be a a DOMError object that indicates the kind of file error that has occurred.

   2. Fire a progress event called loadend.

   3. Terminate this algorithm.

      Note: The readAsDataURL() method returns due to the algorithm being terminated.

3. If no error has occurred, set readyState to LOADING

4. Fire a progress event called loadstart.

5. Return the readAsDataURL() method, but continue to process the steps in this algorithm.

6. Fire a progress event called progress at completion of the read.

7. Queue a task to update the result attribute with the blob as a DataURL [DataURL] after it has been fully read into memory; on getting, the result attribute returns the (complete) data of blob as a Data URL [DataURL].

   • Use the blob's type attribute as part of the Data URL if it is available in keeping with the Data URL specification [DataURL] .
   • If the type attribute is not available on the blob return a Data URL without a media-type. [DataURL].
     Note

     Data URLs that do not have media-types [RFC2046] MUST be treated as plain text by conforming user agents. [DataURL].

8. Set readyState to DONE

9. Fire a progress event called load.

10. Fire a progress event called loadend.

11. Terminate this algorithm.

8.5.5. The readAsText(blob, encoding) method

When the readAsText(blob, encoding) method is called (the encoding argument is optional), the user agent MUST run the steps below (unless otherwise indicated).

1. If readyState = LOADING throw an InvalidStateError [DOMCore] and terminate these steps.

   Note: The readAsText() method returns due to the algorithm being terminated.

2. If an error occurs during reading the blob parameter, set readyState to DONE and set result to null. Proceed to the error steps below.

   1. Fire a progress event called error. Set the error attribute; on getting, the error attribute MUST be a a DOMError object that indicates the kind of file error that has occurred.

   2. Fire a progress event called loadend.

   3. Terminate this algorithm.

      Note: The readAsText() method returns due to the algorithm being terminated.

3. If no error has occurred, set readyState to LOADING

4. Fire a progress event called loadstart.

5. Return the readAsText() method, but continue to process the steps in this algorithm

6. Make progress notifications.

7. While processing the read, as data from the blob becomes available, user agents SHOULD queue tasks to update the result with partial Blob data represented as a string in a format determined by the encoding determination until the read is complete, to accompany the firing of progress events. On getting, the result attribute returns partial Blob data representing the number of bytes currently loaded (as a fraction of the total) [ProgressEvents], decoded into memory according to the encoding determination; user agents MUST return at least one such result while processing this read method. The last returned value is at completion of the read.

   Note

   Partial Blob data MUST be returned such that where possible, the bytes read thus far should be valid code points within the encoding; in particular, when executing the encoding determination for Partial Blob data, user agents MUST NOT return the U+FFFD character for bytes that are invalid within an encoding till the entire codepoint has been read. For example:

   Suppose a file resource is to be read in UTF-8, and in hexadecimal the bytes in this file are E3 83 91 E3 83 91, which is effectively 0x30D1 0x30D1. Suppose the first 5 bytes have been read. The result returned here MUST be 0x30D1 and have result.length == 1 , and NOT be 0x30D1 0xFFFD with result.length == 2 . Even though the trailing E3 83 is not a valid code point in UTF-8 at the fifth byte, user agents MUST NOT return a result with such invalid code points replaced with U+FFFD till it can be determined definitively that the codepoint is invalid.

8. When the blob has been read into memory fully, set readyState to DONE

9. Terminate this algorithm.

8.5.6. The readAsArrayBuffer(blob) method

When the readAsArrayBuffer(blob) method is called, the user agent MUST run the steps below (unless otherwise indicated).

1. If readyState = LOADING throw an InvalidStateError exception [DOMCore] and terminate these steps.

   Note: The readAsArrayBuffer() method returns due to the algorithm being terminated.

2. If an error occurs during reading the blob parameter, set readyState to DONE and set result to null. Proceed to the error steps below.

   1. Fire a progress event called error. Set the error attribute; on getting, the error attribute MUST be a a DOMError that indicates the kind of file error that has occurred.

   2. Fire a progress event called loadend.

   3. Terminate this algorithm.

      Note: The readAsArrayBuffer() method returns due to the algorithm being terminated.

3. If no error has occurred, set readyState to LOADING

4. Fire a progress event called loadstart.

5. Return the readAsArrayBuffer() method, but continue to process the steps in this algorithm.

6. Make progress notifications.

7. While processing the read, as data from the blob becomes available, user agents SHOULD queue tasks to update the result with partial Blob data as an ArrayBuffer [TypedArrays] object containing the bytes read until the read is complete, to accompany the firing of progress events. On getting, the result attribute returns partial Blob data representing the number of bytes currently loaded (as a fraction of the total) [ProgressEvents], as an ArrayBuffer [TypedArrays] object; user agents MUST return at least one such ArrayBuffer [TypedArrays] while processing this read method. The last returned value is at completion of the read.

8. When the blob has been read into memory fully, set readyState to DONE

9. Terminate this algorithm.

8.5.7. The abort() method

When the abort() method is called, the user agent MUST run the steps below:

1. If readyState = EMPTY or if readyState = DONE set result to null and terminate this overall set of steps without doing anything else.

2. If readyState = LOADING set readyState to DONE and result to null.

3. If there are any tasks from the object's FileReader task source in one of the task queues, then remove those tasks.

4. Terminate any steps while processing a read method.

5. Fire a progress event called abort

6. Fire a progress event called loadend

7. Terminate this algorithm.

8.5.8. Blob Parameters

Many methods in this specification take mandatory Blob parameters.

blob

This is a Blob argument used to call all four asynchronous read methods on FileReader and all four synchronous read methods on FileReaderSync; it is also used to call the createObjectURL method. This argument MUST be a reference to a single File in a FileList or a Blob object not obtained from the file system that is in scope of the global object from which the method call was made.

8.5.9. Determining Encoding

When reading blob objects using the readAsText() read method, the optional encoding string parameter MUST be a name or an alias of a character set used on the Internet [IANACHARSET], or else is considered invalid. The following encoding determination steps MUST be followed:

1. Decode the blob using the encoding parameter, if provided and if valid, and terminate this set of steps. If invalid, or if not provided, or if the user agent cannot determine the encoding, go to the next step.

2. Decode the blob using the Charset Parameter [RFC2046] of the blob argument's type attribute, if it has one. If it does not have one, or if the one provided is not a character set used on the Internet [IANACHARSET], go to the next step.

   Example

   If a given blob has a type attribute of text/plain;charset=UTF-8 then decode the blob using UTF-8.

3. Let charset be null.

4. For each of the rows in the following table, starting with the first one and going down, if the first bytes of blob match the bytes given in the first column, then let charset be the encoding given in the cell in the second column of that row. If there is no match charset remains null.

   Bytes in Hexadecimal	Description
   FE FF	UTF-16BE BOM
   FF FE	UTF-16LE BOM
   EF BB BF	UTF-8 BOM

5. If charset is null let charset be UTF-8.

6. Return the result of decoding the blob using charset; on getting, the result attribute of the FileReader object returns a string in charset format. The synchronous readAsText method of the FileReaderSync object returns a string in charset format. Replace bytes or sequences of bytes that are not valid according to the charset with a single U+FFFD character [Unicode]. When processing Partial Blob Data, use the encoding caveat, if applicable.

8.5.10. Events

When this specification says to make progress notifications for a read method, the following steps MUST be followed:

1. While the read method is processing, queue a task to fire a progress event called progress at the FileReader object about every 50ms or for every byte read into memory, whichever is least frequent. At least one event called progress MUST fire before load is fired, and at 100% completion of the read operation; if 100% of blob can be read into memory in less than 50ms, user agents MUST fire a progress event called progress at completion.

   Example

   If a given implementation uses buffers of size 65536 bytes to read files, and thus limits reading operations to that size, and a read method is called on a file that is 65537 bytes, then that implementation MUST fire one progress event for the 65536 first bytes, one progress event for the 65537th byte (which is at completion of read), one load event and one loadend event.

2. When the data from the blob has been completely read into memory, queue a task to fire a progress event called load at the FileReader object.

3. When the data from the blob has been completely read into memory, queue a task to fire a progress event called loadend at the FileReader object.

When this specification says to fire a progress event called e (for some ProgressEvent e at a FileReader reader), the following are normative:

• The progress event e does not bubble. e.bubbles MUST be false [DOMCore]

• The progress event e is NOT cancelable. e.cancelable MUST be false [DOMCore]

• The term "fire an event" is defined in DOM Core [DOMCore]. Progress Events are defined in Progress Events [ProgressEvents].

9.1.1. Constructors

When the FileReaderSync() constructor is invoked, the user agent MUST return a new FileReaderSync object.

In environments where the global object is represented by a WorkerGlobalScope object, the FileReaderSync constructor MUST be available.

9.1.2. The readAsBinaryString method

When the readAsBinaryString(blob) method is called, the following steps MUST be followed:

1. If an error occurs during reading the blob parameter, throw the appropriate exception. Terminate these overall steps.

2. If no error has occurred, read blob into memory. Return the data contents of blob as a binary string.

9.1.3. The readAsText method

When the readAsText(blob, encoding) method is called (the encoding argument is optional), the following steps MUST be followed:

1. If an error occurs during reading of the blob parameter, throw the appropriate exception. Terminate these overall steps.

2. If no error has occurred, read blob into memory. Return the data contents of blob using the encoding determination algorithm.

9.1.4. The readAsDataURL method

When the readAsDataURL(blob) method is called, the following steps MUST be followed:

1. If an error occurs during reading of the blob parameter, throw the appropriate exception. Terminate these overall steps.

2. If no error has occurred, read blob into memory. Return the data contents of blob as a Data URL [DataURL]

   • Use the blob's type attribute as part of the Data URL if it is available in keeping with the Data URL specification [DataURL] .
   • If the type attribute is not available on the blob return a Data URL without a media-type. [DataURL].
     Note

     Data URLs that do not have media-types [RFC2046] MUST be treated as plain text by conforming user agents. [DataURL].

9.1.5. The readAsArrayBuffer method

When the readAsArrayBuffer(blob) method is called, the following steps MUST be followed:

1. If an error occurs during reading the blob parameter, throw the appropriate exception. Terminate these overall steps.

2. If no error has occurred, read blob into memory. Return the data contents of blob as an ArrayBuffer [TypedArrays]

11.3.1. The Opaque String

Opaque strings MUST NOT include any reserved characters from [RFC3986] without percent-encoding them; these characters MUST be percent-encoded. Opaque strings MUST be globally unique. Such strings SHOULD only use characters in the ranges U+002A to U+002B, U+002D to U+002E, U+0030 to U+0039, U+0041 to U+005A, U+005E to U+007E [Unicode], and MUST be at least 36 characters long. UUID is one potential option available to user agents for use with Blob URIs as opaque strings, and their use is strongly encouraged. UUIDs are defined in [RFC4122]. For an ABNF of UUID, see Appendix A.

11.7.1. Blob Protocol Version

The version of the blob: protocol request and response messages is indicated by the "blob/1.0" string, which MUST be used in request messages and in response messages, and must be used just as the HTTP version in HTTP messages is used [HTTP]. See blob: protocol examples.

11.7.2. 200 OK

This response [HTTP] MUST be used if the request has succeeded, namely the blob: URI has been requested with a GET, satisfies the origin requirement, and satisfies the lifetime requirement. If this response code is used, the user agent MUST also use a Content-Type header [HTTP] with a value equal to the Blob object's type attribute. See blob: protocol examples.

11.7.3. 500 Error Condition

This response [HTTP] MUST be used if:

• Any request method other than GET is used to dereference the URL.

• The request violates the origin requirement. In this case, the response SHOULD be accompanied by clarifying text alongside the 500 response, e.g. "500 Origin Violation."

• The request does not satisfy the lifetime requirement. In this case, the response SHOULD be accompanied by clarifying text alongside the 500 response, e.g. "500 Expired URI."

• The underlying resource has changed, moved, been deleted or has become invalid. In this case, the response SHOULD be accompanied by clarifying text alongside the 500 response, e.g. "500 Invalid Resource."

• The permissions surrounding the underlying resource do not permit access. In this case, the response SHOULD be accompanied by clarifying text alongside the 500 response, e.g. "500 Access Violation."

• A security error has occurred. In this case, the response SHOULD be accompanied by clarifying text alongside the 500 response, e.g. "500 Security Violation."

This response MAY be accompanied by additional messages in the response indicating why the Blob resource could not be served. See blob: protocol examples.

11.7.4. Request and Response Headers

This section provides sample exchanges between web applications and user agents using the blob: protocol. A request can be triggered using HTML markup of the sort <img src="blob:550e8400-e29b-41d4-a716-446655440000">, after a web application calls URL.createObjectURL on a given blob, which returns blob:550e8400-e29b-41d4-a716-446655440000 to dereference that blob. These examples merely illustrate the protocol; web developers are not likely to interact with all the headers, but the getAllResponseHeaders() method of XMLHttpRequest, if used, will show relevant response headers [XHR2].

	GET 550e8400-e29b-41d4-a716-446655440000 blob/1.0
	

	blob/1.0 200 OK
	Content-Type: image/jpeg
	....
	

	blob/1.0 500 Error Condition
	This file cannot be read.