# JSON Mask [](http://travis-ci.org/nemtsov/json-mask) [](https://www.npmjs.com/package/json-mask) [](http://standardjs.com/)
This is a tiny language and an engine for selecting specific parts of a JS object, hiding/masking the rest.
```js
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](https://developers.google.com/+/api/#partial-responses), 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](http://expressjs.com/) users, there's an
[express-partial-response](https://github.com/nemtsov/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](https://github.com/koajs/koa.git)
check out the [koa-json-mask](https://github.com/nemtsov/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.
## Syntax
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.
## Grammar
```
Props ::= Prop | Prop "," Props
Prop ::= Object | Array
Object ::= NAME | NAME "/" Object
Array ::= NAME "(" Props ")"
NAME ::= ? all visible characters ?
```
## Examples
Identify the fields you want to keep:
```js
var fields = 'url,object(content,attachments/url)';
```
From this sample object:
```js
var originalObj = {
id: 'z12gtjhq3qn2xxl2o224exwiqruvtda0i',
url: 'https://plus.google.com/102817283354809142195/posts/F97fqZwJESL',
object: {
objectType: 'note',
content:
'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:
```js
var expectObj = {
url: 'https://plus.google.com/102817283354809142195/posts/F97fqZwJESL',
object: {
content:
'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:
```js
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](https://developers.google.com/+/api/#partial-responses)
```js
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)));
});
server.listen(4000);
```
Let's test it:
```bash
$ curl 'http://localhost:4000'
{"firstName":"Mohandas","lastName":"Gandhi","aliases":[{"firstName":"Mahatma","lastName":"Gandhi"},{"firstName":"Bapu"}]}
$ # Let's just get the first name
$ curl 'http://localhost:4000?fields=lastName'
{"lastName":"Gandhi"}
$ # Now, let's just get the first names directly as well as from aliases
$ curl 'http://localhost:4000?fields=firstName,aliases(firstName)'
{"firstName":"Mohandas","aliases":[{"firstName":"Mahatma"},{"firstName":"Bapu"}]}
```
**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 "" [
This is a tiny language and an engine for selecting specific parts of a JS object, hiding/masking the rest.
```js
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](https://developers.google.com/+/api/#partial-responses), 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](http://expressjs.com/) users, there's an
[express-partial-response](https://github.com/nemtsov/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](https://github.com/koajs/koa.git)
check out the [koa-json-mask](https://github.com/nemtsov/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.
## Syntax
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.
## Grammar
```
Props ::= Prop | Prop "," Props
Prop ::= Object | Array
Object ::= NAME | NAME "/" Object
Array ::= NAME "(" Props ")"
NAME ::= ? all visible characters ?
```
## Examples
Identify the fields you want to keep:
```js
var fields = 'url,object(content,attachments/url)';
```
From this sample object:
```js
var originalObj = {
id: 'z12gtjhq3qn2xxl2o224exwiqruvtda0i',
url: 'https://plus.google.com/102817283354809142195/posts/F97fqZwJESL',
object: {
objectType: 'note',
content:
'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:
```js
var expectObj = {
url: 'https://plus.google.com/102817283354809142195/posts/F97fqZwJESL',
object: {
content:
'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:
```js
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](https://developers.google.com/+/api/#partial-responses)
```js
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)));
});
server.listen(4000);
```
Let's test it:
```bash
$ curl 'http://localhost:4000'
{"firstName":"Mohandas","lastName":"Gandhi","aliases":[{"firstName":"Mahatma","lastName":"Gandhi"},{"firstName":"Bapu"}]}
$ # Let's just get the first name
$ curl 'http://localhost:4000?fields=lastName'
{"lastName":"Gandhi"}
$ # Now, let's just get the first names directly as well as from aliases
$ curl 'http://localhost:4000?fields=firstName,aliases(firstName)'
{"firstName":"Mohandas","aliases":[{"firstName":"Mahatma"},{"firstName":"Bapu"}]}
```
**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 "