underscore has a where function that does just this

var _ = require(underscore);



var json = '[{user: a, age: 20}, {user: b, age: 30}, {user: c, age: 40}]';



var users = JSON.parse(json);



var filtered = _.where(users, {user: a});



// => [{user: a, age: 20}]

Another utility library, Lo-Dash, has a where function that operates identically.

You can add underscore to your project using

$ npm install --save underscore

or lodash

$ npm install --save lodash

If you only care about the where function, lodash offers it as a separate module

// only install lodash.where

$ npm install --save lodash.where

To use it in your project

var where = require(lodash.where);



// ...

var filtered = where(users, {user: a});

Even if you use a library to do this, a better approach is probably to setup a chain of streams that handles all of your data processing in smaller modules.

Without knowing what you actually want to do, I've created this as an example. For the purposes of this code, maybe think of a debug logging stream or something.

json-parser.js

input: string (JSON)

output: object

var Transform = require(stream).Transform;



function JsonParser() {

  Transform.call(this, {objectMode: true});

  this._transform = function _transform(json, enc, done) {

    try {

      this.push(JSON.parse(json));

    }

    catch (e) {

      return done(e);

    }

    done();

  }

}



JsonParser.prototype = Object.create(Transform.prototype, {

  constructor: {

    value: JsonParser

  }

});



module.exports = JsonParser;

obj-filter.js

input: object

output: object (result of where(data, filters))

var Transform = require(stream).Transform;

var where = require(lodash.where);



function ObjFilter(filters) {

  Transform.call(this, {objectMode: true});

  this._transform = function _transform(obj, enc, done) {

    this.push(where(obj, filters));

    done();

  }

}



ObjFilter.prototype = Object.create(Transform.prototype, {

  constructor: {

    value: ObjFilter

  }

});



module.exports = ObjFilter;

stringifier.js

input: object

output: string (JSON)

var Transform = require(stream).Transform;



function Stringifier() {

  Transform.call(this, {objectMode: true});

  this._transform = function _transform(obj, enc, done) {

    this.push(JSON.stringify(obj));

    done();

  }

}



Stringifier.prototype = Object.create(Transform.prototype, {

  constructor: {

    value: Stringifier

  }

});



module.exports = Stringifier;

app.js

// modules

var JsonParser = require(json-parser);

var ObjFilter = require(obj-filter);

var Stringifier = require(stringifier);



// run

var parser = new JsonParser();



// setup stream chain

parser.pipe(new ObjFilter({user: a}))

      .pipe(new Stringifier())

      .pipe(process.stdout);



// send example json in

parser.write('[{user: a, age: 20}, {user: b, age: 30}, {user: c, age: 40}]');



// output

// => [{user:a,age:20}]

Here, I made a Stringifier stream that converts objects back into JSON so that we can see them dumped into the console, though you could easily create any streams you needed to handle the operations that your app requires. Your stream end points will likely not end up in writing to the console.

As a last note, you would probably create a database stream that accepts some sort of query options and emits json. You would pipe that stream directly into parser.

Anyway, I hope this gives you a better idea of how to process data in node.js.