Monthly Downloads: 0
Programming language: JavaScript
License: MIT License
Tags: JSON     Parsing     Query     Filter     Projection     Fields     Select     Mask    
Latest version: v1.0.4

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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.

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 "/" Prop
  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: 'https://plus.google.com/102817283354809142195/posts/F97fqZwJESL',
  object: {
    objectType: 'note',
      'A picture... of a space ship... launched from earth 40 years ago.',
    attachments: [
        objectType: 'image',
        url: 'http://apod.nasa.gov/apod/ap110908.html',
        image: { height: 284, width: 506 }
  provider: { title: 'Google+' }

Here's what you'll get back:

var expectObj = {
  url: 'https://plus.google.com/102817283354809142195/posts/F97fqZwJESL',
  object: {
      'A picture... of a space ship... launched from earth 40 years ago.',
    attachments: [
        url: 'http://apod.nasa.gov/apod/ap110908.html'

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.

Command Line Interface - CLI

When installed globally using npm i -g json-mask you can use it like:

json-mask "<fields>" <input> [<output>]


Stream from online resource:

curl https://api.myjson.com/bins/krrxw | json-mask "url,object(content,attachments/url)"

Read from file and write to output file:

json-mask "url,object(content,attachments/url)" input.json > output.json

Read from file and print redirect to file:

json-mask "url,object(content,attachments/url)" input.json > output.json



  • https://unpkg.com/json-mask/build/jsonMask.js
  • https://unpkg.com/json-mask/build/jsonMask.min.js



*Note that all licence references and agreements mentioned in the json-mask README section above are relevant to that project's source code only.