How to concatenate properties from multiple JavaScript objects

I am looking for the best way to "add" multiple JavaScript objects (associative arrays).

For example, given:

a = { "one" : 1, "two" : 2 };
b = { "three" : 3 };
c = { "four" : 4, "five" : 5 };

what is the best way to compute:

{ "one" : 1, "two" : 2, "three" : 3, "four" : 4, "five" : 5 }

ECMAscript 6 introduced Object.assign() to achieve this natively in Javascript.

> The Object.assign() method is used to copy the values of all enumerable own properties from one or more source objects to a target object. It will return the target object.

MDN documentation on Object.assign()

var o1 = { a: 1 };
var o2 = { b: 2 };
var o3 = { c: 3 };

var obj = Object.assign({}, o1, o2, o3);
console.log(obj); // { a: 1, b: 2, c: 3 }

Object.assign is supported in many modern browsers but not yet all of them. Use a transpiler like Babel and Traceur to generate backwards-compatible ES5 JavaScript.

ECMAScript 6 has spread syntax. And now you can do this:

const obj1 = { 1: 11, 2: 22 };
const obj2 = { 3: 33, 4: 44 };
const obj3 = { ...obj1, ...obj2 };

console.log(obj3); // {1: 11, 2: 22, 3: 33, 4: 44}

This should do it:

function collect() {
  var ret = {};
  var len = arguments.length;
  for (var i = 0; i < len; i++) {
    for (p in arguments[i]) {
      if (arguments[i].hasOwnProperty(p)) {
        ret[p] = arguments[i][p];
      }
    }
  }
  return ret;
}

let a = { "one" : 1, "two" : 2 };
let b = { "three" : 3 };
let c = { "four" : 4, "five" : 5 };

let d = collect(a, b, c);
console.log(d);

Output:

{
  "one": 1,
  "two": 2,
  "three": 3,
  "four": 4,
  "five": 5
}

You could use jquery's $.extend like this:

let a = { "one" : 1, "two" : 2 },
    b = { "three" : 3 },
    c = { "four" : 4, "five" : 5 };

let d = $.extend({}, a, b, c)

console.log(d)

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

Underscore has few methods to do this;

1. _.extend(destination, *sources)

Copy all of the properties in the source objects over to the destination object, and return the destination object.

_.extend(a, _.extend(b, c));
=> {"one" : 1, "two" : 2, "three" : 3, "four" : 4, "five" : 5 }

Or

_.extend(a, b);
=> {"one" : 1, "two" : 2, "three" : 3}
_.extend(a, c);
=> {"one" : 1, "two" : 2, "three" : 3, "four" : 4, "five" : 5 }

2. _.defaults(object, *defaults)

Fill in undefined properties in object with values from the defaults objects, and return the object.

_.defaults(a, _.defaults(b, c));
=> {"one" : 1, "two" : 2, "three" : 3, "four" : 4, "five" : 5 }

Or

_.defaults(a, b);
=> {"one" : 1, "two" : 2, "three" : 3}
_.defaults(a, c);
=> {"one" : 1, "two" : 2, "three" : 3, "four" : 4, "five" : 5 }

Shallow-cloning (excluding prototype) or merging of objects is now possible using a shorter syntax than Object.assign().

Spread syntax for object literals was introduced in ECMAScript 2018):

const a = { "one": 1, "two": 2 };
const b = { "three": 3 };
const c = { "four": 4, "five": 5 };

const result = {...a, ...b, ...c};
// Object { "one": 1, "two": 2 , "three": 3, "four": 4, "five": 5 }

Spread (...) operator is supported in many modern browsers but not all of them.

So, it is recommend to use a transpiler like Babel to convert ECMAScript 2015+ code into a backwards compatible version of JavaScript in current and older browsers or environments.

This is the equivalent code Babel will generate for you:

"use strict";

var _extends = Object.assign || function(target) {
  for (var i = 1; i < arguments.length; i++) {
    var source = arguments[i];
    for (var key in source) {
      if (Object.prototype.hasOwnProperty.call(source, key)) {
        target[key] = source[key];
      }
    }
  }
  return target;
};
    
var a = { "one": 1, "two": 2 };
var b = { "three": 3 };
var c = { "four": 4, "five": 5 };

var result = _extends({}, a, b, c);
// Object { "one": 1, "two": 2 , "three": 3, "four": 4, "five": 5 }

Why should the function be restricted to 3 arguments? Also, check for hasOwnProperty.

function Collect() {
	var o={};
	for(var i=0;i<arguments.length;i++) {
	  var arg=arguments[i];
	  if(typeof arg != "object") continue;
	  for(var p in arg) {
	    if(arg.hasOwnProperty(p)) o[p] = arg[p];
	  }
	}
	return o;
}

It's easy using ES7 spread operator for an object, in your browser console put

({ name: "Alex", ...(true  ? { age: 19 } : { })}) // {name: "Alex", age: 19}
({ name: "Alex", ...(false ? { age: 19 } : { })}) // {name: "Alex",        }

To merge a dynamic number of objects, we can use Object.assign with spread syntax.

const mergeObjs = (...objs) => Object.assign({}, ...objs);

The above function accepts any number of objects, merging all of their properties into a new object with properties from later objects overwriting those from previous objects.

Demo:

const mergeObjs = (...objs) => Object.assign({}, ...objs);
const a = {prop: 1, prop2: '2'},
      b = {prop3: 3, prop4: [1,2,3,4]}
      c = {prop5: 5},
      d = {prop6: true, prop7: -1},
      e = {prop1: 2};
const abcd = mergeObjs(a,b,c,d);
console.log("Merged a,b,c,d:", abcd);
const abd = mergeObjs(a,b,d);
console.log("Merged a,b,d:", abd);
const ae = mergeObjs(a,e);//prop1 from e will overwrite prop1 from a
console.log("Merged a,e:", ae);

To merge an array of objects, a similar method may be applied.

const mergeArrayOfObjs = arr => Object.assign({}, ...arr);

Demo:

const mergeArrayOfObjs = arr => Object.assign({}, ...arr);
const arr = [
  {a: 1, b: 2},
  {c:1, d:3},
  {abcd: [1,2,3,4], d: 4}
];
const merged = mergeArrayOfObjs(arr);
console.log(merged);

function Collect(a, b, c) {
    for (property in b)
        a[property] = b[property];

    for (property in c)
        a[property] = c[property];

    return a;
}

Notice: Existing properties in previous objects will be overwritten.

ES6 ++

The question is adding various different objects into one.

let obj = {};
const obj1 = { foo: 'bar' };
const obj2 = { bar: 'foo' };
Object.assign(obj, obj1, obj2);
//output => {foo: 'bar', bar: 'foo'};

lets say you have one object with multiple keys that are objects:

let obj = {
  foo: { bar: 'foo' },
  bar: { foo: 'bar' }
}

this was the solution I found (still have to foreach :/)

let objAll = {};

Object.values(obj).forEach(o => {
  objAll = {...objAll, ...o};
});

By doing this we can dynamically add ALL object keys into one.

// Output => { bar: 'foo', foo: 'bar' }

Probably, the fastest, efficient and more generic way is this (you can merge any number of objects and even copy to the first one ->assign):

function object_merge(){
    for (var i=1; i<arguments.length; i++)
       for (var a in arguments[i])
         arguments[0][a] = arguments[i][a];
   return arguments[0];
}

It also allows you to modify the first object as it passed by reference. If you don't want this but want to have a completely new object containing all properties, then you can pass {} as the first argument.

var object1={a:1,b:2};
var object2={c:3,d:4};
var object3={d:5,e:6};
var combined_object=object_merge(object1,object2,object3); 

combined_object and object1 both contain the properties of object1,object2,object3.

var object1={a:1,b:2};
var object2={c:3,d:4};
var object3={d:5,e:6};
var combined_object=object_merge({},object1,object2,object3); 

In this case, the combined_object contains the properties of object1,object2,object3 but object1 is not modified.

Check here: https://jsfiddle.net/ppwovxey/1/

Note: JavaScript objects are passed by reference.

Simplest: spread operators

var obj1 = {a: 1}
var obj2 = {b: 2}
var concat = { ...obj1, ...obj2 } // { a: 1, b: 2 }

function collect(a, b, c){
	var d = {};
	
	for(p in a){
		d[p] = a[p];
	}
	for(p in b){
		d[p] = b[p];
	}
	for(p in c){
		d[p] = c[p];
	}
	
    return d;
}