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JSON Mask Build Status NPM version js-standard-style

This is a tiny language and an engine for selecting specific parts of a JS object, hiding/masking the rest.

var mask = require('json-mask')
mask({p: {a: 1, b: 2}, z: 1}, 'p/a,z')  // {p: {a: 1}, z: 1} 

The main difference between JSONPath / JSONSelect and this engine is that JSON Mask preserves the structure of the original input object. Instead of returning an array of selected sub-elements (e.g. [{a: 1}, {z: 1}] from example above), it filters-out the parts of the object that you don't need, keeping the structure unchanged: {p: {a: 1}, z: 1}.

This is important because JSON Mask was designed with HTTP resources in mind, the structure of which I didn't want to change after the unwanted fields were masked / filtered.

If you've used the Google APIs, and provided a ?fields= query-string to get a Partial Response, you've already used this language. The desire to have partial responses in my own Node.js-based HTTP services was the reason I wrote JSON Mask.

For express users, there's an express-partial-response middleware. It will integrate with your existing services with no additional code if you're using res.json() or res.jsonp(). And if you're already using koa check out the koa-json-mask middleware.

This library has no dependencies. It works in Node as well as in the browser:

browser support

Note: the 1.5KB (gz), or 4KB (uncompressed) browser build is in the /build folder.


The syntax is loosely based on XPath:

  • a,b,c comma-separated list will select multiple fields
  • a/b/c path will select a field from its parent
  • a(b,c) sub-selection will select many fields from a parent
  • a/*/c the star * wildcard will select all items in a field

Take a look at test/index-test.js for examples of all of these and more.


  Props ::= Prop | Prop "," Props
   Prop ::= Object | Array
 Object ::= NAME | NAME "/" Object
  Array ::= NAME "(" Props ")"
   NAME ::= ? all visible characters ?


Identify the fields you want to keep:

var fields = 'url,object(content,attachments/url)'

From this sample object:

var originalObj = {
  id: 'z12gtjhq3qn2xxl2o224exwiqruvtda0i',
  url: '',
  object: {
    objectType: 'note',
    content: 'A picture... of a space ship... launched from earth 40 years ago.',
    attachments: [{
      objectType: 'image',
      url: '',
      image: {height: 284, width: 506}
  provider: {title: 'Google+'}

Here's what you'll get back:

var expectObj = {
  url: '',
  object: {
    content: 'A picture... of a space ship... launched from earth 40 years ago.',
    attachments: [{
      url: ''

Let's test that:

var mask = require('json-mask')
var assert = require('assert')
var maskedObj = mask(originalObj, fields)
assert.deepEqual(maskedObj, expectObj)

Partial Responses Server Example

Here's an example of using json-mask to implement the Google API Partial Response

var http = require('http')
var url = require('url')
var mask = require('json-mask')
var server
server = http.createServer(function (req, res) {
  var fields = url.parse(req.url, true).query.fields
  var data = {
    firstName: 'Mohandas',
    lastName: 'Gandhi',
    aliases: [{
      firstName: 'Mahatma',
      lastName: 'Gandhi'
    }, {
      firstName: 'Bapu'
  res.writeHead(200, {'Content-Type': 'application/json'})
  res.end(JSON.stringify(mask(data, fields)))

Let's test it:

$ curl 'http://localhost:4000'
$ # Let's just get the first name
$ curl 'http://localhost:4000?fields=lastName'
$ # Now, let's just get the first names directly as well as from aliases
$ curl 'http://localhost:4000?fields=firstName,aliases(firstName)'

Note: a few more examples are in the /example folder.



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bower install json-mask