How to get a subset of a javascript object's properties


Say I have an object:

elmo = { 
  color: 'red',
  annoying: true,
  height: 'unknown',
  meta: { one: '1', two: '2'}
};

I want to make a new object with a subset of its properties.

 // pseudo code
 subset = elmo.slice('color', 'height')

 //=> { color: 'red', height: 'unknown' }

How may I achieve this?

Using Object Destructuring and Property Shorthand

const object = { a: 5, b: 6, c: 7  };
const picked = (({ a, c }) => ({ a, c }))(object);

console.log(picked); // { a: 5, c: 7 }


From Philipp Kewisch:

This is really just an anonymous function being called instantly. All of this can be found on the Destructuring Assignment page on MDN. Here is an expanded form

let unwrap = ({a, c}) => ({a, c});

let unwrap2 = function({a, c}) { return { a, c }; };

let picked = unwrap({ a: 5, b: 6, c: 7 });

let picked2 = unwrap2({a: 5, b: 6, c: 7})

console.log(picked)
console.log(picked2)


I suggest taking a look at Lodash; it has a lot of great utility functions.

For example pick() would be exactly what you seek:

var subset = _.pick(elmo, ['color', 'height']);

fiddle


If you are using ES6 there is a very concise way to do this using destructuring. Destructuring allows you to easily add on to objects using a spread, but it also allows you to make subset objects in the same way.

const object = {
  a: 'a',
  b: 'b',
  c: 'c',
  d: 'd',
}

// Remove "c" and "d" fields from original object:
const {c, d, ...partialObject} = object;
const subset = {c, d};

console.log(partialObject) // => { a: 'a', b: 'b'}
console.log(subset) // => { c: 'c', d: 'd'};

While it's a bit more verbose, you can accomplish what everyone else was recommending underscore/lodash for 2 years ago, by using Array.prototype.reduce.

var subset = ['color', 'height'].reduce(function(o, k) { o[k] = elmo[k]; return o; }, {});

This approach solves it from the other side: rather than take an object and pass property names to it to extract, take an array of property names and reduce them into a new object.

While it's more verbose in the simplest case, a callback here is pretty handy, since you can easily meet some common requirements, e.g. change the 'color' property to 'colour' on the new object, flatten arrays, etc. -- any of the things you need to do when receiving an object from one service/library and building a new object needed somewhere else. While underscore/lodash are excellent, well-implemented libs, this is my preferred approach for less vendor-reliance, and a simpler, more consistent approach when my subset-building logic gets more complex.

edit: es7 version of the same:

const subset = ['color', 'height'].reduce((a, e) => (a[e] = elmo[e], a), {});

edit: A nice example for currying, too! Have a 'pick' function return another function.

const pick = (...props) => o => props.reduce((a, e) => ({ ...a, [e]: o[e] }), {});

The above is pretty close to the other method, except it lets you build a 'picker' on the fly. e.g.

pick('color', 'height')(elmo);

What's especially neat about this approach, is you can easily pass in the chosen 'picks' into anything that takes a function, e.g. Array#map:

[elmo, grover, bigBird].map(pick('color', 'height'));
// [
//   { color: 'red', height: 'short' },
//   { color: 'blue', height: 'medium' },
//   { color: 'yellow', height: 'tall' },
// ]

There is nothing like that built-in to the core library, but you can use object destructuring to do it...

const {color, height} = sourceObject;
const newObject = {color, height};

You could also write a utility function do it...

const cloneAndPluck = function(sourceObject, keys) {
    const newObject = {};
    keys.forEach((obj, key) => { newObject[key] = sourceObject[key]; });
    return newObject;
};

const subset = cloneAndPluck(elmo, ["color", "height"]);

Libraries such as Lodash also have _.pick().


I am adding this answer because none of the answer used Comma operator.

It's very easy with destructuring assignment and , operator

const object = { a: 5, b: 6, c: 7  };
const picked = ({a,c} = object, {a,c})

console.log(picked);

With a certain enhancement for dynamic properties assignment it could look like this:


ES6 destructuring

Destructuring syntax allows to destructure and recombine an object, with either function parameters or variables.

The limitation is that a list of keys is predefined, they cannot be listed as strings, as the question mentions. Destructuring becomes more complicated if a key is non-alphanumeric, e.g. foo_bar.

The downside is that this requires to duplicate a list of keys, this results in verbose code in case a list is long. Since destructuring duplicates object literal syntax in this case, a list can be copied and pasted as is.

The upside is that it's performant solution that is natural to ES6.

IIFE

let subset = (({ foo, bar }) => ({ foo, bar }))(obj); // dupe ({ foo, bar })

Temporary variables

let { foo, bar } = obj;
let subset = { foo, bar }; // dupe { foo, bar }

A list of strings

Arbitrary list of picked keys consists of strings, as the question requires. This allows to not predefine them and use variables that contain key names, like pick(obj, 'foo', someKey, ...moreKeys).

A one-liner becomes shorter with each JS edition.

ES5

var subset = Object.keys(obj)
.filter(function (key) { 
  return ['foo', 'bar'].indexOf(key) >= 0;
})
.reduce(function (obj2, key) {
  obj2[key] = obj[key];
  return obj2;
}, {});

ES6

let subset = Object.keys(obj)
.filter(key => ['foo', 'bar'].indexOf(key) >= 0)
.reduce((obj2, key) => Object.assign(obj2, { [key]: obj[key] }), {});

Or with comma operator:

let subset = Object.keys(obj)
.filter(key => ['foo', 'bar'].indexOf(key) >= 0)
.reduce((obj2, key) => (obj2[key] = obj[key], obj2), {});

ES2019

ECMAScript 2017 has Object.entries and Array.prototype.includes, ECMAScript 2019 has Object.fromEntries, they can be polyfilled when needed and make the task easier:

let subset = Object.fromEntries(
  Object.entries(obj)
  .filter(([key]) => ['foo', 'bar'].includes(key))
)

A one-liner can be rewritten as helper function similar to Lodash pick or omit where the list of keys is passed through arguments:

let pick = (obj, ...keys) => Object.fromEntries(
  Object.entries(obj)
  .filter(([key]) => keys.includes(key))
);

let subset = pick({ foo: 1, qux: 2 }, 'foo', 'bar'); // { foo: 1 }

A note about missing keys

The major difference between destructuring and conventional Lodash-like pick function is that destructuring includes non-existent picked keys with undefined value in a subset:

(({ foo, bar }) => ({ foo, bar }))({ foo: 1 }) // { foo: 1, bar: undefined }

This behaviour may or not be desirable. It cannot be changed for destructuring syntax.

While pick can be changed to include missing keys by iterating a list of picked keys instead:

let inclusivePick = (obj, ...keys) => Object.fromEntries(
  keys.map(key => [key, obj[key]])
);

let subset = inclusivePick({ foo: 1, qux: 2 }, 'foo', 'bar'); // { foo: 1, bar: undefined }

One more solution:

var subset = {
   color: elmo.color,
   height: elmo.height 
}

This looks far more readable to me than pretty much any answer so far, but maybe that's just me!


You can use Lodash also.

var subset = _.pick(elmo ,'color', 'height');

Complementing, let's say you have an array of "elmo"s :

elmos = [{ 
      color: 'red',
      annoying: true,
      height: 'unknown',
      meta: { one: '1', two: '2'}
    },{ 
      color: 'blue',
      annoying: true,
      height: 'known',
      meta: { one: '1', two: '2'}
    },{ 
      color: 'yellow',
      annoying: false,
      height: 'unknown',
      meta: { one: '1', two: '2'}
    }
];

If you want the same behavior, using lodash, you would just:

var subsets = _.map(elmos, function(elm) { return _.pick(elm, 'color', 'height'); });

Destructuring into dynamically named variables is impossible in JavaScript as discussed in this question.

To set keys dynamically, you can use reduce function without mutating object as follows:

const getSubset = (obj, ...keys) => keys.reduce((a, c) => ({ ...a, [c]: obj[c] }), {});

const elmo = { 
  color: 'red',
  annoying: true,
  height: 'unknown',
  meta: { one: '1', two: '2'}
}

const subset = getSubset(elmo, 'color', 'annoying')
console.log(subset)

Should note that you're creating a new object on every iteration though instead of updating a single clone. – mpen

below is a version using reduce with single clone (updating initial value passed in to reduce).

const getSubset = (obj, ...keys) => keys.reduce((acc, curr) => {
  acc[curr] = obj[curr]
  return acc
}, {})

const elmo = { 
  color: 'red',
  annoying: true,
  height: 'unknown',
  meta: { one: '1', two: '2'}
}

const subset = getSubset(elmo, 'annoying', 'height', 'meta')
console.log(subset)


Use pick method of lodash library if you are already using.

var obj = { 'a': 1, 'b': '2', 'c': 3 };

_.pick(object, ['a', 'c']);

// => { 'a': 1, 'c': 3 }

https://lodash.com/docs/4.17.10#pick


TypeScript solution:

export function pick<T extends object, U extends keyof T>(obj: T, paths: Array<U>) {
    return paths.reduce((o, k) => {
        o[k] = obj[k];
        return o;
    }, Object.create(null));
}

The typing information even allows for auto-completion:

Credit to DefinitelyTyped for U extends keyof T trick!


Just another way...

var elmo = { 
  color: 'red',
  annoying: true,
  height: 'unknown',
  meta: { one: '1', two: '2'}
}

var subset = [elmo].map(x => ({
  color: x.color,
  height: x.height
}))[0]

You can use this function with an array of Objects =)


How about:

function sliceObj(obj) {
  var o = {}
    , keys = [].slice.call(arguments, 1);
  for (var i=0; i<keys.length; i++) {
    if (keys[i] in obj) o[keys[i]] = obj[keys[i]];
  }
  return o;
}

var subset = sliceObj(elmo, 'color', 'height');

This works for me in Chrome console. Any problem with this?

var { color, height } = elmo
var subelmo = { color, height }
console.log(subelmo) // {color: "red", height: "unknown"}

Dynamic solution

['color', 'height'].reduce((a,b) => (a[b]=elmo[b],a), {})

let elmo = { 
  color: 'red',
  annoying: true,
  height: 'unknown',
  meta: { one: '1', two: '2'}
};

let subset= ['color', 'height'].reduce((a,b)=> (a[b]=elmo[b],a),{});

console.log( subset );


Destructuring assignment with dynamic properties

This solution not only applies to your specific example but is more generally applicable:

const subset2 = (x, y) => ({[x]:a, [y]:b}) => ({[x]:a, [y]:b});

const subset3 = (x, y, z) => ({[x]:a, [y]:b, [z]:c}) => ({[x]:a, [y]:b, [z]:c});

// const subset4...etc.


const o = {a:1, b:2, c:3, d:4, e:5};


const pickBD = subset2("b", "d");
const pickACE = subset3("a", "c", "e");


console.log(
  pickBD(o), // {b:2, d:4}
  pickACE(o) // {a:1, c:3, e:5}
);

You can easily define subset4 etc. to take more properties into account.


Good-old Array.prototype.reduce:

const selectable = {a: null, b: null};
const v = {a: true, b: 'yes', c: 4};

const r = Object.keys(selectable).reduce((a, b) => {
  return (a[b] = v[b]), a;
}, {});

console.log(r);

this answer uses the magical comma-operator, also: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator

if you want to get really fancy, this is more compact:

const r = Object.keys(selectable).reduce((a, b) => (a[b] = v[b], a), {});

Putting it all together into a reusable function:

const getSelectable = function (selectable, original) {
  return Object.keys(selectable).reduce((a, b) => (a[b] = original[b], a), {})
};

const r = getSelectable(selectable, v);
console.log(r);

  1. convert arguments to array

  2. use Array.forEach() to pick the property

    Object.prototype.pick = function(...args) {
       var obj = {};
       args.forEach(k => obj[k] = this[k])
       return obj
    }
    var a = {0:"a",1:"b",2:"c"}
    var b = a.pick('1','2')  //output will be {1: "b", 2: "c"}
    

function splice()
{
    var ret = new Object();

    for(i = 1; i < arguments.length; i++)
        ret[arguments[i]] = arguments[0][arguments[i]];

    return ret;
}

var answer = splice(elmo, "color", "height");

Try

const elmo={color:"red",annoying:!0,height:"unknown",meta:{one:"1",two:"2"}};

const {color, height} = elmo; newObject = ({color, height});

console.log(newObject); //{ color: 'red', height: 'unknown' }

Adding my 2 cents to Ivan Nosov answer:

In my case I needed many keys to be 'sliced' out of the object so it's becoming ugly very fast and not a very dynamic solution:

const object = { a: 5, b: 6, c: 7, d: 8, aa: 5, bb: 6, cc: 7, dd: 8, aaa: 5, bbb: 6, ccc: 7, ddd: 8, ab: 5, bc: 6, cd: 7, de: 8  };
const picked = (({ a, aa, aaa, ab, c, cc, ccc, cd }) => ({ a, aa, aaa, ab, c, cc, ccc, cd }))(object);

console.log(picked);

So here is a dynamic solution using eval:

const slice = (k, o) => eval(`(${k} => ${k})(o)`);


const object    = { a: 5, b: 6, c: 7, d: 8, aa: 5, bb: 6, cc: 7, dd: 8, aaa: 5, bbb: 6, ccc: 7, ddd: 8, ab: 5, bc: 6, cd: 7, de: 8  };
const sliceKeys = '({ a, aa, aaa, ab, c, cc, ccc, cd })';

console.log( slice(sliceKeys, object) );

Note: though the original question asked was for javascript, it can be done jQuery by below solution

you can extend jquery if you want here is the sample code for one slice:

jQuery.extend({
  sliceMe: function(obj, str) {
      var returnJsonObj = null;
    $.each( obj, function(name, value){
        alert("name: "+name+", value: "+value);
        if(name==str){
            returnJsonObj = JSON.stringify("{"+name+":"+value+"}");
        }

    });
      return returnJsonObj;
  }
});

var elmo = { 
  color: 'red',
  annoying: true,
  height: 'unknown',
  meta: { one: '1', two: '2'}
};


var temp = $.sliceMe(elmo,"color");
alert(JSON.stringify(temp));

here is the fiddle for same: http://jsfiddle.net/w633z/


I want to mention that very good curation here:

pick-es2019.js

Object.fromEntries(
  Object.entries(obj)
  .filter(([key]) => ['whitelisted', 'keys'].includes(key))
);

pick-es2017.js

Object.entries(obj)
.filter(([key]) => ['whitelisted', 'keys'].includes(key))
.reduce((obj, [key, val]) => Object.assign(obj, { [key]: val }), {});

pick-es2015.js

Object.keys(obj)
.filter((key) => ['whitelisted', 'keys'].indexOf(key) >= 0)
.reduce((newObj, key) => Object.assign(newObj, { [key]: obj[key] }), {})

omit-es2019.js

Object.fromEntries(
  Object.entries(obj)
  .filter(([key]) => !['blacklisted', 'keys'].includes(key))
);

omit-es2017.js

Object.entries(obj)
.filter(([key]) => !['blacklisted', 'keys'].includes(key))
.reduce((obj, [key, val]) => Object.assign(obj, { [key]: val }), {});

omit-es2015.js

Object.keys(obj)
.filter((key) => ['blacklisted', 'keys'].indexOf(key) < 0)
.reduce((newObj, key) => Object.assign(newObj, { [key]: obj[key] }), {})

I've got the same problem and solved it easily by using the following libs:

object.pick

https://www.npmjs.com/package/object.pick

pick({a: 'a', b: 'b', c: 'c'}, ['a', 'b'])
//=> {a: 'a', b: 'b'}

object.omit

https://www.npmjs.com/package/object.omit

omit({a: 'a', b: 'b', c: 'c'}, ['a', 'c'])
//=> { b: 'b' }