This Express Middleware will allow you to send a subset of a JSON object instead of the entire object from your HTTP services. To do so, your services will begin accepting the ?fields= query-string that, using a simple language, will specify which fields and sub-fields to keep and which to ignore.

If you've used the Google APIs, provided a ?fields= query-string to get a Partial Response, and wanted to do the same for your own server, now you can do so with this middleware.

Underneath, this middleware uses json-mask. Use it directly without this middleware if you need more flexibility.

npm install express-partial-response

const express = require('express');
const partialResponse = require('express-partial-response');
const app = express();
app.use(partialResponse());
app.get('/', (req, res) => {
  res.json({
    firstName: 'Mohandas',
    lastName: 'Gandhi',
    aliases: [
      {
        firstName: 'Mahatma',
        lastName: 'Gandhi'
      },
      {
        firstName: 'Bapu'
      }
    ]
  });
});
app.listen(4000);

Let's test it:

$ curl 'https://localhost:4000'
{"firstName":"Mohandas","lastName":"Gandhi","aliases":[{"firstName":"Mahatma","lastName":"Gandhi"},{"firstName":"Bapu"}]}
$ # Let's just get the first name
$ curl 'https://localhost:4000?fields=lastName'
{"lastName":"Gandhi"}
$ # Now, let's just get the first names directly as well as from aliases
$ curl 'https://localhost:4000?fields=firstName,aliases(firstName)'
{"firstName":"Mohandas","aliases":[{"firstName":"Mahatma"},{"firstName":"Bapu"}]}

Note: take a look at /example.

Look at json-mask for the available syntax of the fields param.

query specifies the query-string to use. Defaults to fields

app.use(
  partialResponse({
    query: 'filter'
  })
);

MIT. See LICENSE