Return all properties in JS object to "empty" state

Return all properties in JS object to "empty" state - javascript

I wonder if there's a utility function in any framework like jQuery to truncate a JS object (there is more than one possible result, not sure what is the most useful yet):
{ // original object
foo: "bar",
arr: [1, 2, 3],
sub: {a: 4, b: 5, subsub: {c: 6}} }
{ // truncated, variant 1
foo: "",
arr: [],
sub: {} }
{ // truncated, variant 2
foo: "",
arr: [],
sub: {subsub: {}} }
{ // truncated, variant 3
foo: "",
arr: [],
sub: {a: 0, b: 0, subsub: {c: 0}} }
If no, is there a solution more clever than to recursively iterate over all properties, check types and remove / reset if necessary?

Call the constructor for each property to create an empty object of the same type.
for (var k in obj) {
obj[k] = obj[k] && obj[k].constructor();
}
This will also return numbers to 0, booleans to false, dates to now, and regexps to empty (but NaNs remain NaN).
To do this recursively, retaining object-valued properties but emptying them out too (variant 3):
function truncate(obj) {
for (var k in obj) {
var ctor = obj[k] && obj[k].constructor;
if (ctor === Object) truncate(obj[k]);
else if (ctor) obj[k] = ctor();
}
}
To eliminate numbers (variant 2), add a check:
function truncate(obj) {
for (var k in obj) {
var ctor = obj[k] && obj[k].constructor;
if (ctor === Object) truncate(obj[k]);
else if (ctor === Number) delete obj[k];
else if (ctor) obj[k] = ctor();
}
}

You may be looking for OMIT, part of underscore.js.
omit_.omit(object, *keys) Return a copy of the object, filtered to
omit the blacklisted keys (or array of keys). Alternatively accepts a
predicate indicating which keys to omit.
_.omit({name: 'moe', age: 50, userid: 'moe1'}, 'userid');
=> {name: 'moe', age: 50}
_.omit({name: 'moe', age: 50, userid: 'moe1'}, function(value, key, object) { return _.isNumber(value); });
=> {name: 'moe', userid: 'moe1'}

Object.prototype.clear=function(){
for (prop in this){
if (typeof this[prop]==='string') this[prop]='';
else if (this[prop] instanceof Array) this[prop]=[];
else{
this[prop]={}//variant 1
}
}
return this;
}
.
var obj={
foo:"bar",
arr:[1,2,3],
sub:{b:4,c:5,subsub:{e:6}}
}
obj.clear()//returns {foo:"",arr:[],sub:{}}

Related

Comparing items in ojects in two arrays js [duplicate]

I want to compare 2 arrays of objects in JavaScript code. The objects have 8 total properties, but each object will not have a value for each, and the arrays are never going to be any larger than 8 items each, so maybe the brute force method of traversing each and then looking at the values of the 8 properties is the easiest way to do what I want to do, but before implementing, I wanted to see if anyone had a more elegant solution. Any thoughts?

As serialization doesn't work generally (only when the order of properties matches: JSON.stringify({a:1,b:2}) !== JSON.stringify({b:2,a:1})) you have to check the count of properties and compare each property as well:
const objectsEqual = (o1, o2) =>
Object.keys(o1).length === Object.keys(o2).length
&& Object.keys(o1).every(p => o1[p] === o2[p]);
const obj1 = { name: 'John', age: 33};
const obj2 = { age: 33, name: 'John' };
const obj3 = { name: 'John', age: 45 };
console.log(objectsEqual(obj1, obj2)); // true
console.log(objectsEqual(obj1, obj3)); // false
If you need a deep comparison, you can call the function recursively:
const obj1 = { name: 'John', age: 33, info: { married: true, hobbies: ['sport', 'art'] } };
const obj2 = { age: 33, name: 'John', info: { hobbies: ['sport', 'art'], married: true } };
const obj3 = { name: 'John', age: 33 };
const objectsEqual = (o1, o2) =>
typeof o1 === 'object' && Object.keys(o1).length > 0
? Object.keys(o1).length === Object.keys(o2).length
&& Object.keys(o1).every(p => objectsEqual(o1[p], o2[p]))
: o1 === o2;
console.log(objectsEqual(obj1, obj2)); // true
console.log(objectsEqual(obj1, obj3)); // false
Then it's easy to use this function to compare objects in arrays:
const arr1 = [obj1, obj1];
const arr2 = [obj1, obj2];
const arr3 = [obj1, obj3];
const arraysEqual = (a1, a2) =>
a1.length === a2.length && a1.every((o, idx) => objectsEqual(o, a2[idx]));
console.log(arraysEqual(arr1, arr2)); // true
console.log(arraysEqual(arr1, arr3)); // false

EDIT: You cannot overload operators in current, common browser-based implementations of JavaScript interpreters.
To answer the original question, one way you could do this, and mind you, this is a bit of a hack, simply serialize the two arrays to JSON and then compare the two JSON strings. That would simply tell you if the arrays are different, obviously you could do this to each of the objects within the arrays as well to see which ones were different.
Another option is to use a library which has some nice facilities for comparing objects - I use and recommend MochiKit.
EDIT: The answer kamens gave deserves consideration as well, since a single function to compare two given objects would be much smaller than any library to do what I suggest (although my suggestion would certainly work well enough).
Here is a naïve implemenation that may do just enough for you - be aware that there are potential problems with this implementation:
function objectsAreSame(x, y) {
var objectsAreSame = true;
for(var propertyName in x) {
if(x[propertyName] !== y[propertyName]) {
objectsAreSame = false;
break;
}
}
return objectsAreSame;
}
The assumption is that both objects have the same exact list of properties.
Oh, and it is probably obvious that, for better or worse, I belong to the only-one-return-point camp. :)

Honestly, with 8 objects max and 8 properties max per object, your best bet is to just traverse each object and make the comparisons directly. It'll be fast and it'll be easy.
If you're going to be using these types of comparisons often, then I agree with Jason about JSON serialization...but otherwise there's no need to slow down your app with a new library or JSON serialization code.

I know this is an old question and the answers provided work fine ... but this is a bit shorter and doesn't require any additional libraries ( i.e. JSON ):
function arraysAreEqual(ary1,ary2){
return (ary1.join('') == ary2.join(''));
}

I have worked a bit on a simple algorithm to compare contents of two objects and return an intelligible list of difference. Thought I would share. It borrows some ideas for jQuery, namely the map function implementation and the object and array type checking.
It returns a list of "diff objects", which are arrays with the diff info. It's very simple.
Here it is:
// compare contents of two objects and return a list of differences
// returns an array where each element is also an array in the form:
// [accessor, diffType, leftValue, rightValue ]
//
// diffType is one of the following:
// value: when primitive values at that index are different
// undefined: when values in that index exist in one object but don't in
// another; one of the values is always undefined
// null: when a value in that index is null or undefined; values are
// expressed as boolean values, indicated wheter they were nulls
// type: when values in that index are of different types; values are
// expressed as types
// length: when arrays in that index are of different length; values are
// the lengths of the arrays
//
function DiffObjects(o1, o2) {
// choose a map() impl.
// you may use $.map from jQuery if you wish
var map = Array.prototype.map?
function(a) { return Array.prototype.map.apply(a, Array.prototype.slice.call(arguments, 1)); } :
function(a, f) {
var ret = new Array(a.length), value;
for ( var i = 0, length = a.length; i < length; i++ )
ret[i] = f(a[i], i);
return ret.concat();
};
// shorthand for push impl.
var push = Array.prototype.push;
// check for null/undefined values
if ((o1 == null) || (o2 == null)) {
if (o1 != o2)
return [["", "null", o1!=null, o2!=null]];
return undefined; // both null
}
// compare types
if ((o1.constructor != o2.constructor) ||
(typeof o1 != typeof o2)) {
return [["", "type", Object.prototype.toString.call(o1), Object.prototype.toString.call(o2) ]]; // different type
}
// compare arrays
if (Object.prototype.toString.call(o1) == "[object Array]") {
if (o1.length != o2.length) {
return [["", "length", o1.length, o2.length]]; // different length
}
var diff =[];
for (var i=0; i<o1.length; i++) {
// per element nested diff
var innerDiff = DiffObjects(o1[i], o2[i]);
if (innerDiff) { // o1[i] != o2[i]
// merge diff array into parent's while including parent object name ([i])
push.apply(diff, map(innerDiff, function(o, j) { o[0]="[" + i + "]" + o[0]; return o; }));
}
}
// if any differences were found, return them
if (diff.length)
return diff;
// return nothing if arrays equal
return undefined;
}
// compare object trees
if (Object.prototype.toString.call(o1) == "[object Object]") {
var diff =[];
// check all props in o1
for (var prop in o1) {
// the double check in o1 is because in V8 objects remember keys set to undefined
if ((typeof o2[prop] == "undefined") && (typeof o1[prop] != "undefined")) {
// prop exists in o1 but not in o2
diff.push(["[" + prop + "]", "undefined", o1[prop], undefined]); // prop exists in o1 but not in o2
}
else {
// per element nested diff
var innerDiff = DiffObjects(o1[prop], o2[prop]);
if (innerDiff) { // o1[prop] != o2[prop]
// merge diff array into parent's while including parent object name ([prop])
push.apply(diff, map(innerDiff, function(o, j) { o[0]="[" + prop + "]" + o[0]; return o; }));
}
}
}
for (var prop in o2) {
// the double check in o2 is because in V8 objects remember keys set to undefined
if ((typeof o1[prop] == "undefined") && (typeof o2[prop] != "undefined")) {
// prop exists in o2 but not in o1
diff.push(["[" + prop + "]", "undefined", undefined, o2[prop]]); // prop exists in o2 but not in o1
}
}
// if any differences were found, return them
if (diff.length)
return diff;
// return nothing if objects equal
return undefined;
}
// if same type and not null or objects or arrays
// perform primitive value comparison
if (o1 != o2)
return [["", "value", o1, o2]];
// return nothing if values are equal
return undefined;
}

I tried JSON.stringify() and worked for me.
let array1 = [1,2,{value:'alpha'}] , array2 = [{value:'alpha'},'music',3,4];
JSON.stringify(array1) // "[1,2,{"value":"alpha"}]"
JSON.stringify(array2) // "[{"value":"alpha"},"music",3,4]"
JSON.stringify(array1) === JSON.stringify(array2); // false

There is a optimized code for case when function needs to equals to empty arrays (and returning false in that case)
const objectsEqual = (o1, o2) => {
if (o2 === null && o1 !== null) return false;
return o1 !== null && typeof o1 === 'object' && Object.keys(o1).length > 0 ?
Object.keys(o1).length === Object.keys(o2).length &&
Object.keys(o1).every(p => objectsEqual(o1[p], o2[p]))
: (o1 !== null && Array.isArray(o1) && Array.isArray(o2) && !o1.length &&
!o2.length) ? true : o1 === o2;
}

Here is my attempt, using Node's assert module + npm package object-hash.
I suppose that you would like to check if two arrays contain the same objects, even if those objects are ordered differently between the two arrays.
var assert = require('assert');
var hash = require('object-hash');
var obj1 = {a: 1, b: 2, c: 333},
obj2 = {b: 2, a: 1, c: 444},
obj3 = {b: "AAA", c: 555},
obj4 = {c: 555, b: "AAA"};
var array1 = [obj1, obj2, obj3, obj4];
var array2 = [obj3, obj2, obj4, obj1]; // [obj3, obj3, obj2, obj1] should work as well
// calling assert.deepEquals(array1, array2) at this point FAILS (throws an AssertionError)
// even if array1 and array2 contain the same objects in different order,
// because array1[0].c !== array2[0].c
// sort objects in arrays by their hashes, so that if the arrays are identical,
// their objects can be compared in the same order, one by one
var array1 = sortArrayOnHash(array1);
var array2 = sortArrayOnHash(array2);
// then, this should output "PASS"
try {
assert.deepEqual(array1, array2);
console.log("PASS");
} catch (e) {
console.log("FAIL");
console.log(e);
}
// You could define as well something like Array.prototype.sortOnHash()...
function sortArrayOnHash(array) {
return array.sort(function(a, b) {
return hash(a) > hash(b);
});
}

My practice implementation with sorting, tested and working.
const obj1 = { name: 'John', age: 33};
const obj2 = { age: 33, name: 'John' };
const obj3 = { name: 'John', age: 45 };
const equalObjs = ( obj1, obj2 ) => {
let keyExist = false;
for ( const [key, value] of Object.entries(obj1) ) {
// Search each key in reference object and attach a callback function to
// compare the two object keys
if( Object.keys(obj2).some( ( e ) => e == key ) ) {
keyExist = true;
}
}
return keyExist;
}
console.info( equalObjs( obj1, obj2 ) );
Compare your arrays
// Sort Arrays
var arr1 = arr1.sort(( a, b ) => {
var fa = Object.keys(a);
var fb = Object.keys(b);
if (fa < fb) {
return -1;
}
if (fa > fb) {
return 1;
}
return 0;
});
var arr2 = arr2.sort(( a, b ) => {
var fa = Object.keys(a);
var fb = Object.keys(b);
if (fa < fb) {
return -1;
}
if (fa > fb) {
return 1;
}
return 0;
});
const equalArrays = ( arr1, arr2 ) => {
// If the arrays are different length we an eliminate immediately
if( arr1.length !== arr2.length ) {
return false;
} else if ( arr1.every(( obj, index ) => equalObjs( obj, arr2[index] ) ) ) {
return true;
} else {
return false;
}
}
console.info( equalArrays( arr1, arr2 ) );

I am sharing my compare function implementation as it might be helpful for others:
/*
null AND null // true
undefined AND undefined // true
null AND undefined // false
[] AND [] // true
[1, 2, 'test'] AND ['test', 2, 1] // true
[1, 2, 'test'] AND ['test', 2, 3] // false
[undefined, 2, 'test'] AND ['test', 2, 1] // false
[undefined, 2, 'test'] AND ['test', 2, undefined] // true
[[1, 2], 'test'] AND ['test', [2, 1]] // true
[1, 'test'] AND ['test', [2, 1]] // false
[[2, 1], 'test'] AND ['test', [2, 1]] // true
[[2, 1], 'test'] AND ['test', [2, 3]] // false
[[[3, 4], 2], 'test'] AND ['test', [2, [3, 4]]] // true
[[[3, 4], 2], 'test'] AND ['test', [2, [5, 4]]] // false
[{x: 1, y: 2}, 'test'] AND ['test', {x: 1, y: 2}] // true
1 AND 1 // true
{test: 1} AND ['test', 2, 1] // false
{test: 1} AND {test: 1} // true
{test: 1} AND {test: 2} // false
{test: [1, 2]} AND {test: [1, 2]} // true
{test: [1, 2]} AND {test: [1]} // false
{test: [1, 2], x: 1} AND {test: [1, 2], x: 2} // false
{test: [1, { z: 5 }], x: 1} AND {x: 1, test: [1, { z: 5}]} // true
{test: [1, { z: 5 }], x: 1} AND {x: 1, test: [1, { z: 6}]} // false
*/
function is_equal(x, y) {
const
arr1 = x,
arr2 = y,
is_objects_equal = function (obj_x, obj_y) {
if (!(
typeof obj_x === 'object' &&
Object.keys(obj_x).length > 0
))
return obj_x === obj_y;
return Object.keys(obj_x).length === Object.keys(obj_y).length &&
Object.keys(obj_x).every(p => is_objects_equal(obj_x[p], obj_y[p]));
}
;
if (!( Array.isArray(arr1) && Array.isArray(arr2) ))
return (
arr1 && typeof arr1 === 'object' &&
arr2 && typeof arr2 === 'object'
)
? is_objects_equal(arr1, arr2)
: arr1 === arr2;
if (arr1.length !== arr2.length)
return false;
for (const idx_1 of arr1.keys())
for (const idx_2 of arr2.keys())
if (
(
Array.isArray(arr1[idx_1]) &&
this.is_equal(arr1[idx_1], arr2[idx_2])
) ||
is_objects_equal(arr1[idx_1], arr2[idx_2])
)
{
arr2.splice(idx_2, 1);
break;
}
return !arr2.length;
}

Please try this one:
function used_to_compare_two_arrays(a, b)
{
// This block will make the array of indexed that array b contains a elements
var c = a.filter(function(value, index, obj) {
return b.indexOf(value) > -1;
});
// This is used for making comparison that both have same length if no condition go wrong
if (c.length !== a.length) {
return 0;
} else{
return 1;
}
}

The objectsAreSame function mentioned in #JasonBunting's answer works fine for me. However, there's a little problem: If x[propertyName] and y[propertyName] are objects (typeof x[propertyName] == 'object'), you'll need to call the function recursively in order to compare them.

not sure about the performance ... will have to test on big objects .. however, this works great for me.. the advantage it has compared to the other solutions is, the objects/array do not have to be in the same order ....
it practically takes the first object in the first array, and scans the second array for every objects .. if it's a match, it will proceed to another
there is absolutely a way for optimization but it's working :)
thx to #ttulka I got inspired by his work ... just worked on it a little bit
const objectsEqual = (o1, o2) => {
let match = false
if(typeof o1 === 'object' && Object.keys(o1).length > 0) {
match = (Object.keys(o1).length === Object.keys(o2).length && Object.keys(o1).every(p => objectsEqual(o1[p], o2[p])))
}else {
match = (o1 === o2)
}
return match
}
const arraysEqual = (a1, a2) => {
let finalMatch = []
let itemFound = []
if(a1.length === a2.length) {
finalMatch = []
a1.forEach( i1 => {
itemFound = []
a2.forEach( i2 => {
itemFound.push(objectsEqual(i1, i2))
})
finalMatch.push(itemFound.some( i => i === true))
})
}
return finalMatch.every(i => i === true)
}
const ar1 = [
{ id: 1, name: "Johnny", data: { body: "Some text"}},
{ id: 2, name: "Jimmy"}
]
const ar2 = [
{name: "Jimmy", id: 2},
{name: "Johnny", data: { body: "Some text"}, id: 1}
]
console.log("Match:",arraysEqual(ar1, ar2))
jsfiddle: https://jsfiddle.net/x1pubs6q/
or just use lodash :))))
const _ = require('lodash')
const isArrayEqual = (x, y) => {
return _.isEmpty(_.xorWith(x, y, _.isEqual));
};

using _.some from lodash: https://lodash.com/docs/4.17.11#some
const array1AndArray2NotEqual =
_.some(array1, (a1, idx) => a1.key1 !== array2[idx].key1
|| a1.key2 !== array2[idx].key2
|| a1.key3 !== array2[idx].key3);

There`s my solution. It will compare arrays which also have objects and arrays. Elements can be stay in any positions.
Example:
const array1 = [{a: 1}, {b: 2}, { c: 0, d: { e: 1, f: 2, } }, [1,2,3,54]];
const array2 = [{a: 1}, {b: 2}, { c: 0, d: { e: 1, f: 2, } }, [1,2,3,54]];
const arraysCompare = (a1, a2) => {
if (a1.length !== a2.length) return false;
const objectIteration = (object) => {
const result = [];
const objectReduce = (obj) => {
for (let i in obj) {
if (typeof obj[i] !== 'object') {
result.push(`${i}${obj[i]}`);
} else {
objectReduce(obj[i]);
}
}
};
objectReduce(object);
return result;
};
const reduceArray1 = a1.map(item => {
if (typeof item !== 'object') return item;
return objectIteration(item).join('');
});
const reduceArray2 = a2.map(item => {
if (typeof item !== 'object') return item;
return objectIteration(item).join('');
});
const compare = reduceArray1.map(item => reduceArray2.includes(item));
return compare.reduce((acc, item) => acc + Number(item)) === a1.length;
};
console.log(arraysCompare(array1, array2));

This is work for me to compare two array of objects without taking into consideration the order of the items
const collection1 = [
{ id: "1", name: "item 1", subtitle: "This is a subtitle", parentId: "1" },
{ id: "2", name: "item 2", parentId: "1" },
{ id: "3", name: "item 3", parentId: "1" },
]
const collection2 = [
{ id: "3", name: "item 3", parentId: "1" },
{ id: "2", name: "item 2", parentId: "1" },
{ id: "1", name: "item 1", subtitle: "This is a subtitle", parentId: "1" },
]
const contains = (arr, obj) => {
let i = arr.length;
while (i--) {
if (JSON.stringify(arr[i]) === JSON.stringify(obj)) {
return true;
}
}
return false;
}
const isEqual = (obj1, obj2) => {
let n = 0
if (obj1.length !== obj2.length) {
return false;
}
for (let i = 0; i < obj1.length; i++) {
if (contains(obj2, obj1[i])) {
n++
}
}
return n === obj1.length
}
console.log(isEqual(collection1,collection2))
if you take into consideration the order of the items use built in function in lodash isEqual

comparing with json is pretty bad. try this package to compare nested arrays and get the difference.
https://www.npmjs.com/package/deep-object-diff

If you stringify them...
type AB = {
nome: string;
}
const a: AB[] = [{ nome: 'Célio' }];
const b: AB[] = [{ nome: 'Célio' }];
console.log(a === b); // false
console.log(JSON.stringify(a) === JSON.stringify(b)); // true

How to update all matching keys of an object with another [duplicate]

I need to be able to merge two (very simple) JavaScript objects at runtime. For example I'd like to:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
obj1.merge(obj2);
//obj1 now has three properties: food, car, and animal
Is there a built in way to do this? I do not need recursion, and I do not need to merge functions, just methods on flat objects.

ECMAScript 2018 Standard Method
You would use object spread:
let merged = {...obj1, ...obj2};
merged is now the union of obj1 and obj2. Properties in obj2 will overwrite those in obj1.
/** There's no limit to the number of objects you can merge.
* Later properties overwrite earlier properties with the same name. */
const allRules = {...obj1, ...obj2, ...obj3};
Here is also the MDN documentation for this syntax. If you're using babel you'll need the #babel/plugin-proposal-object-rest-spread plugin for it to work (This plugin is included in #babel/preset-env, in ES2018).
ECMAScript 2015 (ES6) Standard Method
/* For the case in question, you would do: */
Object.assign(obj1, obj2);
/** There's no limit to the number of objects you can merge.
* All objects get merged into the first object.
* Only the object in the first argument is mutated and returned.
* Later properties overwrite earlier properties with the same name. */
const allRules = Object.assign({}, obj1, obj2, obj3, etc);
(see MDN JavaScript Reference)
Method for ES5 and Earlier
for (var attrname in obj2) { obj1[attrname] = obj2[attrname]; }
Note that this will simply add all attributes of obj2 to obj1 which might not be what you want if you still want to use the unmodified obj1.
If you're using a framework that craps all over your prototypes then you have to get fancier with checks like hasOwnProperty, but that code will work for 99% of cases.
Example function:
/**
* Overwrites obj1's values with obj2's and adds obj2's if non existent in obj1
* #param obj1
* #param obj2
* #returns obj3 a new object based on obj1 and obj2
*/
function merge_options(obj1,obj2){
var obj3 = {};
for (var attrname in obj1) { obj3[attrname] = obj1[attrname]; }
for (var attrname in obj2) { obj3[attrname] = obj2[attrname]; }
return obj3;
}

jQuery also has a utility for this: http://api.jquery.com/jQuery.extend/.
Taken from the jQuery documentation:
// Merge options object into settings object
var settings = { validate: false, limit: 5, name: "foo" };
var options = { validate: true, name: "bar" };
jQuery.extend(settings, options);
// Now the content of settings object is the following:
// { validate: true, limit: 5, name: "bar" }
The above code will mutate the existing object named settings.
If you want to create a new object without modifying either argument, use this:
var defaults = { validate: false, limit: 5, name: "foo" };
var options = { validate: true, name: "bar" };
/* Merge defaults and options, without modifying defaults */
var settings = $.extend({}, defaults, options);
// The content of settings variable is now the following:
// {validate: true, limit: 5, name: "bar"}
// The 'defaults' and 'options' variables remained the same.

The Harmony ECMAScript 2015 (ES6) specifies Object.assign which will do this.
Object.assign(obj1, obj2);
Current browser support is getting better, but if you're developing for browsers that don't have support, you can use a polyfill.

I googled for code to merge object properties and ended up here. However since there wasn't any code for recursive merge I wrote it myself. (Maybe jQuery extend is recursive BTW?) Anyhow, hopefully someone else will find it useful as well.
(Now the code does not use Object.prototype :)
Code
/*
* Recursively merge properties of two objects
*/
function MergeRecursive(obj1, obj2) {
for (var p in obj2) {
try {
// Property in destination object set; update its value.
if ( obj2[p].constructor==Object ) {
obj1[p] = MergeRecursive(obj1[p], obj2[p]);
} else {
obj1[p] = obj2[p];
}
} catch(e) {
// Property in destination object not set; create it and set its value.
obj1[p] = obj2[p];
}
}
return obj1;
}
An example
o1 = { a : 1,
b : 2,
c : {
ca : 1,
cb : 2,
cc : {
cca : 100,
ccb : 200 } } };
o2 = { a : 10,
c : {
ca : 10,
cb : 20,
cc : {
cca : 101,
ccb : 202 } } };
o3 = MergeRecursive(o1, o2);
Produces object o3 like
o3 = { a : 10,
b : 2,
c : {
ca : 10,
cb : 20,
cc : {
cca : 101,
ccb : 202 } } };

Note that underscore.js's extend-method does this in a one-liner:
_.extend({name : 'moe'}, {age : 50});
=> {name : 'moe', age : 50}

Similar to jQuery extend(), you have the same function in AngularJS:
// Merge the 'options' object into the 'settings' object
var settings = {validate: false, limit: 5, name: "foo"};
var options = {validate: true, name: "bar"};
angular.extend(settings, options);

I need to merge objects today, and this question (and answers) helped me a lot. I tried some of the answers, but none of them fit my needs, so I combined some of the answers, added something myself and came up with a new merge function. Here it is:
var merge = function() {
var obj = {},
i = 0,
il = arguments.length,
key;
for (; i < il; i++) {
for (key in arguments[i]) {
if (arguments[i].hasOwnProperty(key)) {
obj[key] = arguments[i][key];
}
}
}
return obj;
};
Some example usages:
var t1 = {
key1: 1,
key2: "test",
key3: [5, 2, 76, 21]
};
var t2 = {
key1: {
ik1: "hello",
ik2: "world",
ik3: 3
}
};
var t3 = {
key2: 3,
key3: {
t1: 1,
t2: 2,
t3: {
a1: 1,
a2: 3,
a4: [21, 3, 42, "asd"]
}
}
};
console.log(merge(t1, t2));
console.log(merge(t1, t3));
console.log(merge(t2, t3));
console.log(merge(t1, t2, t3));
console.log(merge({}, t1, { key1: 1 }));

You can use the object spread syntax to achieve this. It's a part of ES2018 and beyond.
const obj1 = { food: 'pizza', car: 'ford' };
const obj2 = { animal: 'dog' };
const obj3 = { ...obj1, ...obj2 };
console.log(obj3);

The given solutions should be modified to check source.hasOwnProperty(property) in the for..in loops before assigning - otherwise, you end up copying the properties of the whole prototype chain, which is rarely desired...

Merge properties of N objects in one line of code
An Object.assign method is part of the ECMAScript 2015 (ES6) standard and does exactly what you need. (IE not supported)
var clone = Object.assign({}, obj);
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.
Read more...
The polyfill to support older browsers:
if (!Object.assign) {
Object.defineProperty(Object, 'assign', {
enumerable: false,
configurable: true,
writable: true,
value: function(target) {
'use strict';
if (target === undefined || target === null) {
throw new TypeError('Cannot convert first argument to object');
}
var to = Object(target);
for (var i = 1; i < arguments.length; i++) {
var nextSource = arguments[i];
if (nextSource === undefined || nextSource === null) {
continue;
}
nextSource = Object(nextSource);
var keysArray = Object.keys(nextSource);
for (var nextIndex = 0, len = keysArray.length; nextIndex < len; nextIndex++) {
var nextKey = keysArray[nextIndex];
var desc = Object.getOwnPropertyDescriptor(nextSource, nextKey);
if (desc !== undefined && desc.enumerable) {
to[nextKey] = nextSource[nextKey];
}
}
}
return to;
}
});
}

The following two are probably a good starting point. lodash also has a customizer function for those special needs!
_.extend (http://underscorejs.org/#extend)
_.merge (https://lodash.com/docs#merge)

Here's my stab which
Supports deep merge
Does not mutate arguments
Takes any number of arguments
Does not extend the object prototype
Does not depend on another library (jQuery, MooTools, Underscore.js, etc.)
Includes check for hasOwnProperty
Is short :)
/*
Recursively merge properties and return new object
obj1 <- obj2 [ <- ... ]
*/
function merge () {
var dst = {}
,src
,p
,args = [].splice.call(arguments, 0)
;
while (args.length > 0) {
src = args.splice(0, 1)[0];
if (toString.call(src) == '[object Object]') {
for (p in src) {
if (src.hasOwnProperty(p)) {
if (toString.call(src[p]) == '[object Object]') {
dst[p] = merge(dst[p] || {}, src[p]);
} else {
dst[p] = src[p];
}
}
}
}
}
return dst;
}
Example:
a = {
"p1": "p1a",
"p2": [
"a",
"b",
"c"
],
"p3": true,
"p5": null,
"p6": {
"p61": "p61a",
"p62": "p62a",
"p63": [
"aa",
"bb",
"cc"
],
"p64": {
"p641": "p641a"
}
}
};
b = {
"p1": "p1b",
"p2": [
"d",
"e",
"f"
],
"p3": false,
"p4": true,
"p6": {
"p61": "p61b",
"p64": {
"p642": "p642b"
}
}
};
c = {
"p1": "p1c",
"p3": null,
"p6": {
"p62": "p62c",
"p64": {
"p643": "p641c"
}
}
};
d = merge(a, b, c);
/*
d = {
"p1": "p1c",
"p2": [
"d",
"e",
"f"
],
"p3": null,
"p5": null,
"p6": {
"p61": "p61b",
"p62": "p62c",
"p63": [
"aa",
"bb",
"cc"
],
"p64": {
"p641": "p641a",
"p642": "p642b",
"p643": "p641c"
}
},
"p4": true
};
*/

Just by the way, what you're all doing is overwriting properties, not merging...
This is how JavaScript objects area really merged: Only keys in the to object which are not objects themselves will be overwritten by from. Everything else will be really merged. Of course you can change this behaviour to not overwrite anything which exists like only if to[n] is undefined, etc...:
var realMerge = function (to, from) {
for (n in from) {
if (typeof to[n] != 'object') {
to[n] = from[n];
} else if (typeof from[n] == 'object') {
to[n] = realMerge(to[n], from[n]);
}
}
return to;
};
Usage:
var merged = realMerge(obj1, obj2);

Object.assign()
ECMAScript 2015 (ES6)
This is a new technology, part of the ECMAScript 2015 (ES6) standard.
This technology's specification has been finalized, but check the compatibility table for usage and implementation status in various browsers.
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.
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 }
console.log(o1); // { a: 1, b: 2, c: 3 }, target object itself is changed.

For not-too-complicated objects you could use JSON:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog', car: 'chevy'}
var objMerge;
objMerge = JSON.stringify(obj1) + JSON.stringify(obj2);
// {"food": "pizza","car":"ford"}{"animal":"dog","car":"chevy"}
objMerge = objMerge.replace(/\}\{/, ","); // \_ replace with comma for valid JSON
objMerge = JSON.parse(objMerge); // { food: 'pizza', animal: 'dog', car: 'chevy'}
// Of same keys in both objects, the last object's value is retained_/
Mind you that in this example "}{" must not occur within a string!

There's a library called deepmerge on GitHub: That seems to be getting some traction. It's a standalone, available through both the npm and bower package managers.
I would be inclined to use or improve on this instead of copy-pasting code from answers.

The best way for you to do this is to add a proper property that is non-enumerable using Object.defineProperty.
This way you will still be able to iterate over your objects properties without having the newly created "extend" that you would get if you were to create the property with Object.prototype.extend.
Hopefully this helps:
Object.defineProperty(Object.prototype, "extend", {
enumerable: false,
value: function(from) {
var props = Object.getOwnPropertyNames(from);
var dest = this;
props.forEach(function(name) {
if (name in dest) {
var destination = Object.getOwnPropertyDescriptor(from, name);
Object.defineProperty(dest, name, destination);
}
});
return this;
}
});
Once you have that working, you can do:
var obj = {
name: 'stack',
finish: 'overflow'
}
var replacement = {
name: 'stock'
};
obj.extend(replacement);
I just wrote a blog post about it here: http://onemoredigit.com/post/1527191998/extending-objects-in-node-js

You can simply use jQuery extend
var obj1 = { val1: false, limit: 5, name: "foo" };
var obj2 = { val2: true, name: "bar" };
jQuery.extend(obj1, obj2);
Now obj1 contains all the values of obj1 and obj2

Prototype has this:
Object.extend = function(destination,source) {
for (var property in source)
destination[property] = source[property];
return destination;
}
obj1.extend(obj2) will do what you want.

Wow.. this is the first StackOverflow post I've seen with multiple pages. Apologies for adding another "answer"
ES5 & Earlier
This method is for ES5 & Earlier - there are plenty of other answers addressing ES6.
I did not see any "deep" object merging utilizing the arguments property. Here is my answer - compact & recursive, allowing unlimited object arguments to be passed:
function extend() {
for (var o = {}, i = 0; i < arguments.length; i++) {
// Uncomment to skip arguments that are not objects (to prevent errors)
// if (arguments[i].constructor !== Object) continue;
for (var k in arguments[i]) {
if (arguments[i].hasOwnProperty(k)) {
o[k] = arguments[i][k].constructor === Object
? extend(o[k] || {}, arguments[i][k])
: arguments[i][k];
}
}
}
return o;
}
Example
/**
* Extend objects
*/
function extend() {
for (var o = {}, i = 0; i < arguments.length; i++) {
for (var k in arguments[i]) {
if (arguments[i].hasOwnProperty(k)) {
o[k] = arguments[i][k].constructor === Object
? extend(o[k] || {}, arguments[i][k])
: arguments[i][k];
}
}
}
return o;
}
/**
* Example
*/
document.write(JSON.stringify(extend({
api: 1,
params: {
query: 'hello'
}
}, {
params: {
query: 'there'
}
})));
// outputs {"api": 1, "params": {"query": "there"}}
This answer is now but a drop in the ocean ...

Just if anyone is using Google Closure Library:
goog.require('goog.object');
var a = {'a': 1, 'b': 2};
var b = {'b': 3, 'c': 4};
goog.object.extend(a, b);
// Now object a == {'a': 1, 'b': 3, 'c': 4};
Similar helper function exists for array:
var a = [1, 2];
var b = [3, 4];
goog.array.extend(a, b); // Extends array 'a'
goog.array.concat(a, b); // Returns concatenation of array 'a' and 'b'

**Merging objects is simple using Object.assign or the spread ... operator **
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog', car: 'BMW' }
var obj3 = {a: "A"}
var mergedObj = Object.assign(obj1,obj2,obj3)
// or using the Spread operator (...)
var mergedObj = {...obj1,...obj2,...obj3}
console.log(mergedObj);
The objects are merged from right to left, this means that objects which have identical properties as the objects to their right will be overriden.
In this example obj2.car overrides obj1.car

I extended David Coallier's method:
Added the possibility to merge multiple objects
Supports deep objects
override parameter (that's detected if the last parameter is a boolean)
If override is false, no property gets overridden but new properties will be added.
Usage:
obj.merge(merges... [, override]);
Here is my code:
Object.defineProperty(Object.prototype, "merge", {
enumerable: false,
value: function () {
var override = true,
dest = this,
len = arguments.length,
props, merge, i, from;
if (typeof(arguments[arguments.length - 1]) === "boolean") {
override = arguments[arguments.length - 1];
len = arguments.length - 1;
}
for (i = 0; i < len; i++) {
from = arguments[i];
if (from != null) {
Object.getOwnPropertyNames(from).forEach(function (name) {
var descriptor;
// nesting
if ((typeof(dest[name]) === "object" || typeof(dest[name]) === "undefined")
&& typeof(from[name]) === "object") {
// ensure proper types (Array rsp Object)
if (typeof(dest[name]) === "undefined") {
dest[name] = Array.isArray(from[name]) ? [] : {};
}
if (override) {
if (!Array.isArray(dest[name]) && Array.isArray(from[name])) {
dest[name] = [];
}
else if (Array.isArray(dest[name]) && !Array.isArray(from[name])) {
dest[name] = {};
}
}
dest[name].merge(from[name], override);
}
// flat properties
else if ((name in dest && override) || !(name in dest)) {
descriptor = Object.getOwnPropertyDescriptor(from, name);
if (descriptor.configurable) {
Object.defineProperty(dest, name, descriptor);
}
}
});
}
}
return this;
}
});
Examples and TestCases:
function clone (obj) {
return JSON.parse(JSON.stringify(obj));
}
var obj = {
name : "trick",
value : "value"
};
var mergeObj = {
name : "truck",
value2 : "value2"
};
var mergeObj2 = {
name : "track",
value : "mergeObj2",
value2 : "value2-mergeObj2",
value3 : "value3"
};
assertTrue("Standard", clone(obj).merge(mergeObj).equals({
name : "truck",
value : "value",
value2 : "value2"
}));
assertTrue("Standard no Override", clone(obj).merge(mergeObj, false).equals({
name : "trick",
value : "value",
value2 : "value2"
}));
assertTrue("Multiple", clone(obj).merge(mergeObj, mergeObj2).equals({
name : "track",
value : "mergeObj2",
value2 : "value2-mergeObj2",
value3 : "value3"
}));
assertTrue("Multiple no Override", clone(obj).merge(mergeObj, mergeObj2, false).equals({
name : "trick",
value : "value",
value2 : "value2",
value3 : "value3"
}));
var deep = {
first : {
name : "trick",
val : "value"
},
second : {
foo : "bar"
}
};
var deepMerge = {
first : {
name : "track",
anotherVal : "wohoo"
},
second : {
foo : "baz",
bar : "bam"
},
v : "on first layer"
};
assertTrue("Deep merges", clone(deep).merge(deepMerge).equals({
first : {
name : "track",
val : "value",
anotherVal : "wohoo"
},
second : {
foo : "baz",
bar : "bam"
},
v : "on first layer"
}));
assertTrue("Deep merges no override", clone(deep).merge(deepMerge, false).equals({
first : {
name : "trick",
val : "value",
anotherVal : "wohoo"
},
second : {
foo : "bar",
bar : "bam"
},
v : "on first layer"
}));
var obj1 = {a: 1, b: "hello"};
obj1.merge({c: 3});
assertTrue(obj1.equals({a: 1, b: "hello", c: 3}));
obj1.merge({a: 2, b: "mom", d: "new property"}, false);
assertTrue(obj1.equals({a: 1, b: "hello", c: 3, d: "new property"}));
var obj2 = {};
obj2.merge({a: 1}, {b: 2}, {a: 3});
assertTrue(obj2.equals({a: 3, b: 2}));
var a = [];
var b = [1, [2, 3], 4];
a.merge(b);
assertEquals(1, a[0]);
assertEquals([2, 3], a[1]);
assertEquals(4, a[2]);
var o1 = {};
var o2 = {a: 1, b: {c: 2}};
var o3 = {d: 3};
o1.merge(o2, o3);
assertTrue(o1.equals({a: 1, b: {c: 2}, d: 3}));
o1.b.c = 99;
assertTrue(o2.equals({a: 1, b: {c: 2}}));
// checking types with arrays and objects
var bo;
a = [];
bo = [1, {0:2, 1:3}, 4];
b = [1, [2, 3], 4];
a.merge(b);
assertTrue("Array stays Array?", Array.isArray(a[1]));
a = [];
a.merge(bo);
assertTrue("Object stays Object?", !Array.isArray(a[1]));
a = [];
a.merge(b);
a.merge(bo);
assertTrue("Object overrides Array", !Array.isArray(a[1]));
a = [];
a.merge(b);
a.merge(bo, false);
assertTrue("Object does not override Array", Array.isArray(a[1]));
a = [];
a.merge(bo);
a.merge(b);
assertTrue("Array overrides Object", Array.isArray(a[1]));
a = [];
a.merge(bo);
a.merge(b, false);
assertTrue("Array does not override Object", !Array.isArray(a[1]));
My equals method can be found here: Object comparison in JavaScript

In MooTools, there's Object.merge():
Object.merge(obj1, obj2);

var firstObject = {
key1 : 'value1',
key2 : 'value2'
};
var secondObject={
...firstObject,
key3 : 'value3',
key4 : 'value4',
key5 : 'value5'
}
console.log(firstObject);
console.log(secondObject);

In Ext JS 4 it can be done as follows:
var mergedObject = Ext.Object.merge(object1, object2)
// Or shorter:
var mergedObject2 = Ext.merge(object1, object2)
See merge( object ) : Object.

var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
// result
result: {food: "pizza", car: "ford", animal: "dog"}
Using jQuery.extend() - Link
// Merge obj1 & obj2 to result
var result1 = $.extend( {}, obj1, obj2 );
Using _.merge() - Link
// Merge obj1 & obj2 to result
var result2 = _.merge( {}, obj1, obj2 );
Using _.extend() - Link
// Merge obj1 & obj2 to result
var result3 = _.extend( {}, obj1, obj2 );
Using Object.assign() ECMAScript 2015 (ES6) - Link
// Merge obj1 & obj2 to result
var result4 = Object.assign( {}, obj1, obj2 );
Output of all
obj1: { animal: 'dog' }
obj2: { food: 'pizza', car: 'ford' }
result1: {food: "pizza", car: "ford", animal: "dog"}
result2: {food: "pizza", car: "ford", animal: "dog"}
result3: {food: "pizza", car: "ford", animal: "dog"}
result4: {food: "pizza", car: "ford", animal: "dog"}

Based on Markus' and vsync' answer, this is an expanded version. The function takes any number of arguments. It can be used to set properties on DOM nodes and makes deep copies of values. However, the first argument is given by reference.
To detect a DOM node, the isDOMNode() function is used (see Stack Overflow question JavaScript isDOM — How do you check if a JavaScript Object is a DOM Object?)
It was tested in Opera 11, Firefox 6, Internet Explorer 8 and Google Chrome 16.
Code
function mergeRecursive() {
// _mergeRecursive does the actual job with two arguments.
var _mergeRecursive = function (dst, src) {
if (isDOMNode(src) || typeof src !== 'object' || src === null) {
return dst;
}
for (var p in src) {
if (!src.hasOwnProperty(p))
continue;
if (src[p] === undefined)
continue;
if ( typeof src[p] !== 'object' || src[p] === null) {
dst[p] = src[p];
} else if (typeof dst[p]!=='object' || dst[p] === null) {
dst[p] = _mergeRecursive(src[p].constructor===Array ? [] : {}, src[p]);
} else {
_mergeRecursive(dst[p], src[p]);
}
}
return dst;
}
// Loop through arguments and merge them into the first argument.
var out = arguments[0];
if (typeof out !== 'object' || out === null)
return out;
for (var i = 1, il = arguments.length; i < il; i++) {
_mergeRecursive(out, arguments[i]);
}
return out;
}
Some examples
Set innerHTML and style of a HTML Element
mergeRecursive(
document.getElementById('mydiv'),
{style: {border: '5px solid green', color: 'red'}},
{innerHTML: 'Hello world!'});
Merge arrays and objects. Note that undefined can be used to preserv values in the lefthand array/object.
o = mergeRecursive({a:'a'}, [1,2,3], [undefined, null, [30,31]], {a:undefined, b:'b'});
// o = {0:1, 1:null, 2:[30,31], a:'a', b:'b'}
Any argument not beeing a JavaScript object (including null) will be ignored. Except for the first argument, also DOM nodes are discarded. Beware that i.e. strings, created like new String() are in fact objects.
o = mergeRecursive({a:'a'}, 1, true, null, undefined, [1,2,3], 'bc', new String('de'));
// o = {0:'d', 1:'e', 2:3, a:'a'}
If you want to merge two objects into a new (without affecting any of the two) supply {} as first argument
var a={}, b={b:'abc'}, c={c:'cde'}, o;
o = mergeRecursive(a, b, c);
// o===a is true, o===b is false, o===c is false
Edit (by ReaperSoon):
To also merge arrays
function mergeRecursive(obj1, obj2) {
if (Array.isArray(obj2)) { return obj1.concat(obj2); }
for (var p in obj2) {
try {
// Property in destination object set; update its value.
if ( obj2[p].constructor==Object ) {
obj1[p] = mergeRecursive(obj1[p], obj2[p]);
} else if (Array.isArray(obj2[p])) {
obj1[p] = obj1[p].concat(obj2[p]);
} else {
obj1[p] = obj2[p];
}
} catch(e) {
// Property in destination object not set; create it and set its value.
obj1[p] = obj2[p];
}
}
return obj1;
}

It seems like this should be all you need:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
var obj3 = { ...obj1, ...obj2 }
After that obj3 should now have the following value:
{food: "pizza", car: "ford", animal: "dog"}
Try it out here:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
var obj3 = { ...obj1, ...obj2 }
console.log(obj3);

You should use lodash's defaultsDeep
_.defaultsDeep({ 'user': { 'name': 'barney' } }, { 'user': { 'name': 'fred', 'age': 36 } });
// → { 'user': { 'name': 'barney', 'age': 36 } }

Simple push array JS [duplicate]

I need to be able to merge two (very simple) JavaScript objects at runtime. For example I'd like to:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
obj1.merge(obj2);
//obj1 now has three properties: food, car, and animal
Is there a built in way to do this? I do not need recursion, and I do not need to merge functions, just methods on flat objects.

ECMAScript 2018 Standard Method
You would use object spread:
let merged = {...obj1, ...obj2};
merged is now the union of obj1 and obj2. Properties in obj2 will overwrite those in obj1.
/** There's no limit to the number of objects you can merge.
* Later properties overwrite earlier properties with the same name. */
const allRules = {...obj1, ...obj2, ...obj3};
Here is also the MDN documentation for this syntax. If you're using babel you'll need the #babel/plugin-proposal-object-rest-spread plugin for it to work (This plugin is included in #babel/preset-env, in ES2018).
ECMAScript 2015 (ES6) Standard Method
/* For the case in question, you would do: */
Object.assign(obj1, obj2);
/** There's no limit to the number of objects you can merge.
* All objects get merged into the first object.
* Only the object in the first argument is mutated and returned.
* Later properties overwrite earlier properties with the same name. */
const allRules = Object.assign({}, obj1, obj2, obj3, etc);
(see MDN JavaScript Reference)
Method for ES5 and Earlier
for (var attrname in obj2) { obj1[attrname] = obj2[attrname]; }
Note that this will simply add all attributes of obj2 to obj1 which might not be what you want if you still want to use the unmodified obj1.
If you're using a framework that craps all over your prototypes then you have to get fancier with checks like hasOwnProperty, but that code will work for 99% of cases.
Example function:
/**
* Overwrites obj1's values with obj2's and adds obj2's if non existent in obj1
* #param obj1
* #param obj2
* #returns obj3 a new object based on obj1 and obj2
*/
function merge_options(obj1,obj2){
var obj3 = {};
for (var attrname in obj1) { obj3[attrname] = obj1[attrname]; }
for (var attrname in obj2) { obj3[attrname] = obj2[attrname]; }
return obj3;
}

jQuery also has a utility for this: http://api.jquery.com/jQuery.extend/.
Taken from the jQuery documentation:
// Merge options object into settings object
var settings = { validate: false, limit: 5, name: "foo" };
var options = { validate: true, name: "bar" };
jQuery.extend(settings, options);
// Now the content of settings object is the following:
// { validate: true, limit: 5, name: "bar" }
The above code will mutate the existing object named settings.
If you want to create a new object without modifying either argument, use this:
var defaults = { validate: false, limit: 5, name: "foo" };
var options = { validate: true, name: "bar" };
/* Merge defaults and options, without modifying defaults */
var settings = $.extend({}, defaults, options);
// The content of settings variable is now the following:
// {validate: true, limit: 5, name: "bar"}
// The 'defaults' and 'options' variables remained the same.

The Harmony ECMAScript 2015 (ES6) specifies Object.assign which will do this.
Object.assign(obj1, obj2);
Current browser support is getting better, but if you're developing for browsers that don't have support, you can use a polyfill.

I googled for code to merge object properties and ended up here. However since there wasn't any code for recursive merge I wrote it myself. (Maybe jQuery extend is recursive BTW?) Anyhow, hopefully someone else will find it useful as well.
(Now the code does not use Object.prototype :)
Code
/*
* Recursively merge properties of two objects
*/
function MergeRecursive(obj1, obj2) {
for (var p in obj2) {
try {
// Property in destination object set; update its value.
if ( obj2[p].constructor==Object ) {
obj1[p] = MergeRecursive(obj1[p], obj2[p]);
} else {
obj1[p] = obj2[p];
}
} catch(e) {
// Property in destination object not set; create it and set its value.
obj1[p] = obj2[p];
}
}
return obj1;
}
An example
o1 = { a : 1,
b : 2,
c : {
ca : 1,
cb : 2,
cc : {
cca : 100,
ccb : 200 } } };
o2 = { a : 10,
c : {
ca : 10,
cb : 20,
cc : {
cca : 101,
ccb : 202 } } };
o3 = MergeRecursive(o1, o2);
Produces object o3 like
o3 = { a : 10,
b : 2,
c : {
ca : 10,
cb : 20,
cc : {
cca : 101,
ccb : 202 } } };

Note that underscore.js's extend-method does this in a one-liner:
_.extend({name : 'moe'}, {age : 50});
=> {name : 'moe', age : 50}

Similar to jQuery extend(), you have the same function in AngularJS:
// Merge the 'options' object into the 'settings' object
var settings = {validate: false, limit: 5, name: "foo"};
var options = {validate: true, name: "bar"};
angular.extend(settings, options);

I need to merge objects today, and this question (and answers) helped me a lot. I tried some of the answers, but none of them fit my needs, so I combined some of the answers, added something myself and came up with a new merge function. Here it is:
var merge = function() {
var obj = {},
i = 0,
il = arguments.length,
key;
for (; i < il; i++) {
for (key in arguments[i]) {
if (arguments[i].hasOwnProperty(key)) {
obj[key] = arguments[i][key];
}
}
}
return obj;
};
Some example usages:
var t1 = {
key1: 1,
key2: "test",
key3: [5, 2, 76, 21]
};
var t2 = {
key1: {
ik1: "hello",
ik2: "world",
ik3: 3
}
};
var t3 = {
key2: 3,
key3: {
t1: 1,
t2: 2,
t3: {
a1: 1,
a2: 3,
a4: [21, 3, 42, "asd"]
}
}
};
console.log(merge(t1, t2));
console.log(merge(t1, t3));
console.log(merge(t2, t3));
console.log(merge(t1, t2, t3));
console.log(merge({}, t1, { key1: 1 }));

You can use the object spread syntax to achieve this. It's a part of ES2018 and beyond.
const obj1 = { food: 'pizza', car: 'ford' };
const obj2 = { animal: 'dog' };
const obj3 = { ...obj1, ...obj2 };
console.log(obj3);

The given solutions should be modified to check source.hasOwnProperty(property) in the for..in loops before assigning - otherwise, you end up copying the properties of the whole prototype chain, which is rarely desired...

Merge properties of N objects in one line of code
An Object.assign method is part of the ECMAScript 2015 (ES6) standard and does exactly what you need. (IE not supported)
var clone = Object.assign({}, obj);
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.
Read more...
The polyfill to support older browsers:
if (!Object.assign) {
Object.defineProperty(Object, 'assign', {
enumerable: false,
configurable: true,
writable: true,
value: function(target) {
'use strict';
if (target === undefined || target === null) {
throw new TypeError('Cannot convert first argument to object');
}
var to = Object(target);
for (var i = 1; i < arguments.length; i++) {
var nextSource = arguments[i];
if (nextSource === undefined || nextSource === null) {
continue;
}
nextSource = Object(nextSource);
var keysArray = Object.keys(nextSource);
for (var nextIndex = 0, len = keysArray.length; nextIndex < len; nextIndex++) {
var nextKey = keysArray[nextIndex];
var desc = Object.getOwnPropertyDescriptor(nextSource, nextKey);
if (desc !== undefined && desc.enumerable) {
to[nextKey] = nextSource[nextKey];
}
}
}
return to;
}
});
}

The following two are probably a good starting point. lodash also has a customizer function for those special needs!
_.extend (http://underscorejs.org/#extend)
_.merge (https://lodash.com/docs#merge)

Here's my stab which
Supports deep merge
Does not mutate arguments
Takes any number of arguments
Does not extend the object prototype
Does not depend on another library (jQuery, MooTools, Underscore.js, etc.)
Includes check for hasOwnProperty
Is short :)
/*
Recursively merge properties and return new object
obj1 <- obj2 [ <- ... ]
*/
function merge () {
var dst = {}
,src
,p
,args = [].splice.call(arguments, 0)
;
while (args.length > 0) {
src = args.splice(0, 1)[0];
if (toString.call(src) == '[object Object]') {
for (p in src) {
if (src.hasOwnProperty(p)) {
if (toString.call(src[p]) == '[object Object]') {
dst[p] = merge(dst[p] || {}, src[p]);
} else {
dst[p] = src[p];
}
}
}
}
}
return dst;
}
Example:
a = {
"p1": "p1a",
"p2": [
"a",
"b",
"c"
],
"p3": true,
"p5": null,
"p6": {
"p61": "p61a",
"p62": "p62a",
"p63": [
"aa",
"bb",
"cc"
],
"p64": {
"p641": "p641a"
}
}
};
b = {
"p1": "p1b",
"p2": [
"d",
"e",
"f"
],
"p3": false,
"p4": true,
"p6": {
"p61": "p61b",
"p64": {
"p642": "p642b"
}
}
};
c = {
"p1": "p1c",
"p3": null,
"p6": {
"p62": "p62c",
"p64": {
"p643": "p641c"
}
}
};
d = merge(a, b, c);
/*
d = {
"p1": "p1c",
"p2": [
"d",
"e",
"f"
],
"p3": null,
"p5": null,
"p6": {
"p61": "p61b",
"p62": "p62c",
"p63": [
"aa",
"bb",
"cc"
],
"p64": {
"p641": "p641a",
"p642": "p642b",
"p643": "p641c"
}
},
"p4": true
};
*/

Just by the way, what you're all doing is overwriting properties, not merging...
This is how JavaScript objects area really merged: Only keys in the to object which are not objects themselves will be overwritten by from. Everything else will be really merged. Of course you can change this behaviour to not overwrite anything which exists like only if to[n] is undefined, etc...:
var realMerge = function (to, from) {
for (n in from) {
if (typeof to[n] != 'object') {
to[n] = from[n];
} else if (typeof from[n] == 'object') {
to[n] = realMerge(to[n], from[n]);
}
}
return to;
};
Usage:
var merged = realMerge(obj1, obj2);

Object.assign()
ECMAScript 2015 (ES6)
This is a new technology, part of the ECMAScript 2015 (ES6) standard.
This technology's specification has been finalized, but check the compatibility table for usage and implementation status in various browsers.
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.
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 }
console.log(o1); // { a: 1, b: 2, c: 3 }, target object itself is changed.

For not-too-complicated objects you could use JSON:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog', car: 'chevy'}
var objMerge;
objMerge = JSON.stringify(obj1) + JSON.stringify(obj2);
// {"food": "pizza","car":"ford"}{"animal":"dog","car":"chevy"}
objMerge = objMerge.replace(/\}\{/, ","); // \_ replace with comma for valid JSON
objMerge = JSON.parse(objMerge); // { food: 'pizza', animal: 'dog', car: 'chevy'}
// Of same keys in both objects, the last object's value is retained_/
Mind you that in this example "}{" must not occur within a string!

There's a library called deepmerge on GitHub: That seems to be getting some traction. It's a standalone, available through both the npm and bower package managers.
I would be inclined to use or improve on this instead of copy-pasting code from answers.

The best way for you to do this is to add a proper property that is non-enumerable using Object.defineProperty.
This way you will still be able to iterate over your objects properties without having the newly created "extend" that you would get if you were to create the property with Object.prototype.extend.
Hopefully this helps:
Object.defineProperty(Object.prototype, "extend", {
enumerable: false,
value: function(from) {
var props = Object.getOwnPropertyNames(from);
var dest = this;
props.forEach(function(name) {
if (name in dest) {
var destination = Object.getOwnPropertyDescriptor(from, name);
Object.defineProperty(dest, name, destination);
}
});
return this;
}
});
Once you have that working, you can do:
var obj = {
name: 'stack',
finish: 'overflow'
}
var replacement = {
name: 'stock'
};
obj.extend(replacement);
I just wrote a blog post about it here: http://onemoredigit.com/post/1527191998/extending-objects-in-node-js

You can simply use jQuery extend
var obj1 = { val1: false, limit: 5, name: "foo" };
var obj2 = { val2: true, name: "bar" };
jQuery.extend(obj1, obj2);
Now obj1 contains all the values of obj1 and obj2

Prototype has this:
Object.extend = function(destination,source) {
for (var property in source)
destination[property] = source[property];
return destination;
}
obj1.extend(obj2) will do what you want.

Wow.. this is the first StackOverflow post I've seen with multiple pages. Apologies for adding another "answer"
ES5 & Earlier
This method is for ES5 & Earlier - there are plenty of other answers addressing ES6.
I did not see any "deep" object merging utilizing the arguments property. Here is my answer - compact & recursive, allowing unlimited object arguments to be passed:
function extend() {
for (var o = {}, i = 0; i < arguments.length; i++) {
// Uncomment to skip arguments that are not objects (to prevent errors)
// if (arguments[i].constructor !== Object) continue;
for (var k in arguments[i]) {
if (arguments[i].hasOwnProperty(k)) {
o[k] = arguments[i][k].constructor === Object
? extend(o[k] || {}, arguments[i][k])
: arguments[i][k];
}
}
}
return o;
}
Example
/**
* Extend objects
*/
function extend() {
for (var o = {}, i = 0; i < arguments.length; i++) {
for (var k in arguments[i]) {
if (arguments[i].hasOwnProperty(k)) {
o[k] = arguments[i][k].constructor === Object
? extend(o[k] || {}, arguments[i][k])
: arguments[i][k];
}
}
}
return o;
}
/**
* Example
*/
document.write(JSON.stringify(extend({
api: 1,
params: {
query: 'hello'
}
}, {
params: {
query: 'there'
}
})));
// outputs {"api": 1, "params": {"query": "there"}}
This answer is now but a drop in the ocean ...

Just if anyone is using Google Closure Library:
goog.require('goog.object');
var a = {'a': 1, 'b': 2};
var b = {'b': 3, 'c': 4};
goog.object.extend(a, b);
// Now object a == {'a': 1, 'b': 3, 'c': 4};
Similar helper function exists for array:
var a = [1, 2];
var b = [3, 4];
goog.array.extend(a, b); // Extends array 'a'
goog.array.concat(a, b); // Returns concatenation of array 'a' and 'b'

**Merging objects is simple using Object.assign or the spread ... operator **
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog', car: 'BMW' }
var obj3 = {a: "A"}
var mergedObj = Object.assign(obj1,obj2,obj3)
// or using the Spread operator (...)
var mergedObj = {...obj1,...obj2,...obj3}
console.log(mergedObj);
The objects are merged from right to left, this means that objects which have identical properties as the objects to their right will be overriden.
In this example obj2.car overrides obj1.car

I extended David Coallier's method:
Added the possibility to merge multiple objects
Supports deep objects
override parameter (that's detected if the last parameter is a boolean)
If override is false, no property gets overridden but new properties will be added.
Usage:
obj.merge(merges... [, override]);
Here is my code:
Object.defineProperty(Object.prototype, "merge", {
enumerable: false,
value: function () {
var override = true,
dest = this,
len = arguments.length,
props, merge, i, from;
if (typeof(arguments[arguments.length - 1]) === "boolean") {
override = arguments[arguments.length - 1];
len = arguments.length - 1;
}
for (i = 0; i < len; i++) {
from = arguments[i];
if (from != null) {
Object.getOwnPropertyNames(from).forEach(function (name) {
var descriptor;
// nesting
if ((typeof(dest[name]) === "object" || typeof(dest[name]) === "undefined")
&& typeof(from[name]) === "object") {
// ensure proper types (Array rsp Object)
if (typeof(dest[name]) === "undefined") {
dest[name] = Array.isArray(from[name]) ? [] : {};
}
if (override) {
if (!Array.isArray(dest[name]) && Array.isArray(from[name])) {
dest[name] = [];
}
else if (Array.isArray(dest[name]) && !Array.isArray(from[name])) {
dest[name] = {};
}
}
dest[name].merge(from[name], override);
}
// flat properties
else if ((name in dest && override) || !(name in dest)) {
descriptor = Object.getOwnPropertyDescriptor(from, name);
if (descriptor.configurable) {
Object.defineProperty(dest, name, descriptor);
}
}
});
}
}
return this;
}
});
Examples and TestCases:
function clone (obj) {
return JSON.parse(JSON.stringify(obj));
}
var obj = {
name : "trick",
value : "value"
};
var mergeObj = {
name : "truck",
value2 : "value2"
};
var mergeObj2 = {
name : "track",
value : "mergeObj2",
value2 : "value2-mergeObj2",
value3 : "value3"
};
assertTrue("Standard", clone(obj).merge(mergeObj).equals({
name : "truck",
value : "value",
value2 : "value2"
}));
assertTrue("Standard no Override", clone(obj).merge(mergeObj, false).equals({
name : "trick",
value : "value",
value2 : "value2"
}));
assertTrue("Multiple", clone(obj).merge(mergeObj, mergeObj2).equals({
name : "track",
value : "mergeObj2",
value2 : "value2-mergeObj2",
value3 : "value3"
}));
assertTrue("Multiple no Override", clone(obj).merge(mergeObj, mergeObj2, false).equals({
name : "trick",
value : "value",
value2 : "value2",
value3 : "value3"
}));
var deep = {
first : {
name : "trick",
val : "value"
},
second : {
foo : "bar"
}
};
var deepMerge = {
first : {
name : "track",
anotherVal : "wohoo"
},
second : {
foo : "baz",
bar : "bam"
},
v : "on first layer"
};
assertTrue("Deep merges", clone(deep).merge(deepMerge).equals({
first : {
name : "track",
val : "value",
anotherVal : "wohoo"
},
second : {
foo : "baz",
bar : "bam"
},
v : "on first layer"
}));
assertTrue("Deep merges no override", clone(deep).merge(deepMerge, false).equals({
first : {
name : "trick",
val : "value",
anotherVal : "wohoo"
},
second : {
foo : "bar",
bar : "bam"
},
v : "on first layer"
}));
var obj1 = {a: 1, b: "hello"};
obj1.merge({c: 3});
assertTrue(obj1.equals({a: 1, b: "hello", c: 3}));
obj1.merge({a: 2, b: "mom", d: "new property"}, false);
assertTrue(obj1.equals({a: 1, b: "hello", c: 3, d: "new property"}));
var obj2 = {};
obj2.merge({a: 1}, {b: 2}, {a: 3});
assertTrue(obj2.equals({a: 3, b: 2}));
var a = [];
var b = [1, [2, 3], 4];
a.merge(b);
assertEquals(1, a[0]);
assertEquals([2, 3], a[1]);
assertEquals(4, a[2]);
var o1 = {};
var o2 = {a: 1, b: {c: 2}};
var o3 = {d: 3};
o1.merge(o2, o3);
assertTrue(o1.equals({a: 1, b: {c: 2}, d: 3}));
o1.b.c = 99;
assertTrue(o2.equals({a: 1, b: {c: 2}}));
// checking types with arrays and objects
var bo;
a = [];
bo = [1, {0:2, 1:3}, 4];
b = [1, [2, 3], 4];
a.merge(b);
assertTrue("Array stays Array?", Array.isArray(a[1]));
a = [];
a.merge(bo);
assertTrue("Object stays Object?", !Array.isArray(a[1]));
a = [];
a.merge(b);
a.merge(bo);
assertTrue("Object overrides Array", !Array.isArray(a[1]));
a = [];
a.merge(b);
a.merge(bo, false);
assertTrue("Object does not override Array", Array.isArray(a[1]));
a = [];
a.merge(bo);
a.merge(b);
assertTrue("Array overrides Object", Array.isArray(a[1]));
a = [];
a.merge(bo);
a.merge(b, false);
assertTrue("Array does not override Object", !Array.isArray(a[1]));
My equals method can be found here: Object comparison in JavaScript

In MooTools, there's Object.merge():
Object.merge(obj1, obj2);

var firstObject = {
key1 : 'value1',
key2 : 'value2'
};
var secondObject={
...firstObject,
key3 : 'value3',
key4 : 'value4',
key5 : 'value5'
}
console.log(firstObject);
console.log(secondObject);

In Ext JS 4 it can be done as follows:
var mergedObject = Ext.Object.merge(object1, object2)
// Or shorter:
var mergedObject2 = Ext.merge(object1, object2)
See merge( object ) : Object.

var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
// result
result: {food: "pizza", car: "ford", animal: "dog"}
Using jQuery.extend() - Link
// Merge obj1 & obj2 to result
var result1 = $.extend( {}, obj1, obj2 );
Using _.merge() - Link
// Merge obj1 & obj2 to result
var result2 = _.merge( {}, obj1, obj2 );
Using _.extend() - Link
// Merge obj1 & obj2 to result
var result3 = _.extend( {}, obj1, obj2 );
Using Object.assign() ECMAScript 2015 (ES6) - Link
// Merge obj1 & obj2 to result
var result4 = Object.assign( {}, obj1, obj2 );
Output of all
obj1: { animal: 'dog' }
obj2: { food: 'pizza', car: 'ford' }
result1: {food: "pizza", car: "ford", animal: "dog"}
result2: {food: "pizza", car: "ford", animal: "dog"}
result3: {food: "pizza", car: "ford", animal: "dog"}
result4: {food: "pizza", car: "ford", animal: "dog"}

Based on Markus' and vsync' answer, this is an expanded version. The function takes any number of arguments. It can be used to set properties on DOM nodes and makes deep copies of values. However, the first argument is given by reference.
To detect a DOM node, the isDOMNode() function is used (see Stack Overflow question JavaScript isDOM — How do you check if a JavaScript Object is a DOM Object?)
It was tested in Opera 11, Firefox 6, Internet Explorer 8 and Google Chrome 16.
Code
function mergeRecursive() {
// _mergeRecursive does the actual job with two arguments.
var _mergeRecursive = function (dst, src) {
if (isDOMNode(src) || typeof src !== 'object' || src === null) {
return dst;
}
for (var p in src) {
if (!src.hasOwnProperty(p))
continue;
if (src[p] === undefined)
continue;
if ( typeof src[p] !== 'object' || src[p] === null) {
dst[p] = src[p];
} else if (typeof dst[p]!=='object' || dst[p] === null) {
dst[p] = _mergeRecursive(src[p].constructor===Array ? [] : {}, src[p]);
} else {
_mergeRecursive(dst[p], src[p]);
}
}
return dst;
}
// Loop through arguments and merge them into the first argument.
var out = arguments[0];
if (typeof out !== 'object' || out === null)
return out;
for (var i = 1, il = arguments.length; i < il; i++) {
_mergeRecursive(out, arguments[i]);
}
return out;
}
Some examples
Set innerHTML and style of a HTML Element
mergeRecursive(
document.getElementById('mydiv'),
{style: {border: '5px solid green', color: 'red'}},
{innerHTML: 'Hello world!'});
Merge arrays and objects. Note that undefined can be used to preserv values in the lefthand array/object.
o = mergeRecursive({a:'a'}, [1,2,3], [undefined, null, [30,31]], {a:undefined, b:'b'});
// o = {0:1, 1:null, 2:[30,31], a:'a', b:'b'}
Any argument not beeing a JavaScript object (including null) will be ignored. Except for the first argument, also DOM nodes are discarded. Beware that i.e. strings, created like new String() are in fact objects.
o = mergeRecursive({a:'a'}, 1, true, null, undefined, [1,2,3], 'bc', new String('de'));
// o = {0:'d', 1:'e', 2:3, a:'a'}
If you want to merge two objects into a new (without affecting any of the two) supply {} as first argument
var a={}, b={b:'abc'}, c={c:'cde'}, o;
o = mergeRecursive(a, b, c);
// o===a is true, o===b is false, o===c is false
Edit (by ReaperSoon):
To also merge arrays
function mergeRecursive(obj1, obj2) {
if (Array.isArray(obj2)) { return obj1.concat(obj2); }
for (var p in obj2) {
try {
// Property in destination object set; update its value.
if ( obj2[p].constructor==Object ) {
obj1[p] = mergeRecursive(obj1[p], obj2[p]);
} else if (Array.isArray(obj2[p])) {
obj1[p] = obj1[p].concat(obj2[p]);
} else {
obj1[p] = obj2[p];
}
} catch(e) {
// Property in destination object not set; create it and set its value.
obj1[p] = obj2[p];
}
}
return obj1;
}

It seems like this should be all you need:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
var obj3 = { ...obj1, ...obj2 }
After that obj3 should now have the following value:
{food: "pizza", car: "ford", animal: "dog"}
Try it out here:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
var obj3 = { ...obj1, ...obj2 }
console.log(obj3);

You should use lodash's defaultsDeep
_.defaultsDeep({ 'user': { 'name': 'barney' } }, { 'user': { 'name': 'fred', 'age': 36 } });
// → { 'user': { 'name': 'barney', 'age': 36 } }

Producing a customized object in Javascript [duplicate]

I need to be able to merge two (very simple) JavaScript objects at runtime. For example I'd like to:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
obj1.merge(obj2);
//obj1 now has three properties: food, car, and animal
Is there a built in way to do this? I do not need recursion, and I do not need to merge functions, just methods on flat objects.

ECMAScript 2018 Standard Method
You would use object spread:
let merged = {...obj1, ...obj2};
merged is now the union of obj1 and obj2. Properties in obj2 will overwrite those in obj1.
/** There's no limit to the number of objects you can merge.
* Later properties overwrite earlier properties with the same name. */
const allRules = {...obj1, ...obj2, ...obj3};
Here is also the MDN documentation for this syntax. If you're using babel you'll need the #babel/plugin-proposal-object-rest-spread plugin for it to work (This plugin is included in #babel/preset-env, in ES2018).
ECMAScript 2015 (ES6) Standard Method
/* For the case in question, you would do: */
Object.assign(obj1, obj2);
/** There's no limit to the number of objects you can merge.
* All objects get merged into the first object.
* Only the object in the first argument is mutated and returned.
* Later properties overwrite earlier properties with the same name. */
const allRules = Object.assign({}, obj1, obj2, obj3, etc);
(see MDN JavaScript Reference)
Method for ES5 and Earlier
for (var attrname in obj2) { obj1[attrname] = obj2[attrname]; }
Note that this will simply add all attributes of obj2 to obj1 which might not be what you want if you still want to use the unmodified obj1.
If you're using a framework that craps all over your prototypes then you have to get fancier with checks like hasOwnProperty, but that code will work for 99% of cases.
Example function:
/**
* Overwrites obj1's values with obj2's and adds obj2's if non existent in obj1
* #param obj1
* #param obj2
* #returns obj3 a new object based on obj1 and obj2
*/
function merge_options(obj1,obj2){
var obj3 = {};
for (var attrname in obj1) { obj3[attrname] = obj1[attrname]; }
for (var attrname in obj2) { obj3[attrname] = obj2[attrname]; }
return obj3;
}

jQuery also has a utility for this: http://api.jquery.com/jQuery.extend/.
Taken from the jQuery documentation:
// Merge options object into settings object
var settings = { validate: false, limit: 5, name: "foo" };
var options = { validate: true, name: "bar" };
jQuery.extend(settings, options);
// Now the content of settings object is the following:
// { validate: true, limit: 5, name: "bar" }
The above code will mutate the existing object named settings.
If you want to create a new object without modifying either argument, use this:
var defaults = { validate: false, limit: 5, name: "foo" };
var options = { validate: true, name: "bar" };
/* Merge defaults and options, without modifying defaults */
var settings = $.extend({}, defaults, options);
// The content of settings variable is now the following:
// {validate: true, limit: 5, name: "bar"}
// The 'defaults' and 'options' variables remained the same.

The Harmony ECMAScript 2015 (ES6) specifies Object.assign which will do this.
Object.assign(obj1, obj2);
Current browser support is getting better, but if you're developing for browsers that don't have support, you can use a polyfill.

I googled for code to merge object properties and ended up here. However since there wasn't any code for recursive merge I wrote it myself. (Maybe jQuery extend is recursive BTW?) Anyhow, hopefully someone else will find it useful as well.
(Now the code does not use Object.prototype :)
Code
/*
* Recursively merge properties of two objects
*/
function MergeRecursive(obj1, obj2) {
for (var p in obj2) {
try {
// Property in destination object set; update its value.
if ( obj2[p].constructor==Object ) {
obj1[p] = MergeRecursive(obj1[p], obj2[p]);
} else {
obj1[p] = obj2[p];
}
} catch(e) {
// Property in destination object not set; create it and set its value.
obj1[p] = obj2[p];
}
}
return obj1;
}
An example
o1 = { a : 1,
b : 2,
c : {
ca : 1,
cb : 2,
cc : {
cca : 100,
ccb : 200 } } };
o2 = { a : 10,
c : {
ca : 10,
cb : 20,
cc : {
cca : 101,
ccb : 202 } } };
o3 = MergeRecursive(o1, o2);
Produces object o3 like
o3 = { a : 10,
b : 2,
c : {
ca : 10,
cb : 20,
cc : {
cca : 101,
ccb : 202 } } };

Note that underscore.js's extend-method does this in a one-liner:
_.extend({name : 'moe'}, {age : 50});
=> {name : 'moe', age : 50}

Similar to jQuery extend(), you have the same function in AngularJS:
// Merge the 'options' object into the 'settings' object
var settings = {validate: false, limit: 5, name: "foo"};
var options = {validate: true, name: "bar"};
angular.extend(settings, options);

I need to merge objects today, and this question (and answers) helped me a lot. I tried some of the answers, but none of them fit my needs, so I combined some of the answers, added something myself and came up with a new merge function. Here it is:
var merge = function() {
var obj = {},
i = 0,
il = arguments.length,
key;
for (; i < il; i++) {
for (key in arguments[i]) {
if (arguments[i].hasOwnProperty(key)) {
obj[key] = arguments[i][key];
}
}
}
return obj;
};
Some example usages:
var t1 = {
key1: 1,
key2: "test",
key3: [5, 2, 76, 21]
};
var t2 = {
key1: {
ik1: "hello",
ik2: "world",
ik3: 3
}
};
var t3 = {
key2: 3,
key3: {
t1: 1,
t2: 2,
t3: {
a1: 1,
a2: 3,
a4: [21, 3, 42, "asd"]
}
}
};
console.log(merge(t1, t2));
console.log(merge(t1, t3));
console.log(merge(t2, t3));
console.log(merge(t1, t2, t3));
console.log(merge({}, t1, { key1: 1 }));

You can use the object spread syntax to achieve this. It's a part of ES2018 and beyond.
const obj1 = { food: 'pizza', car: 'ford' };
const obj2 = { animal: 'dog' };
const obj3 = { ...obj1, ...obj2 };
console.log(obj3);

The given solutions should be modified to check source.hasOwnProperty(property) in the for..in loops before assigning - otherwise, you end up copying the properties of the whole prototype chain, which is rarely desired...

Merge properties of N objects in one line of code
An Object.assign method is part of the ECMAScript 2015 (ES6) standard and does exactly what you need. (IE not supported)
var clone = Object.assign({}, obj);
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.
Read more...
The polyfill to support older browsers:
if (!Object.assign) {
Object.defineProperty(Object, 'assign', {
enumerable: false,
configurable: true,
writable: true,
value: function(target) {
'use strict';
if (target === undefined || target === null) {
throw new TypeError('Cannot convert first argument to object');
}
var to = Object(target);
for (var i = 1; i < arguments.length; i++) {
var nextSource = arguments[i];
if (nextSource === undefined || nextSource === null) {
continue;
}
nextSource = Object(nextSource);
var keysArray = Object.keys(nextSource);
for (var nextIndex = 0, len = keysArray.length; nextIndex < len; nextIndex++) {
var nextKey = keysArray[nextIndex];
var desc = Object.getOwnPropertyDescriptor(nextSource, nextKey);
if (desc !== undefined && desc.enumerable) {
to[nextKey] = nextSource[nextKey];
}
}
}
return to;
}
});
}

The following two are probably a good starting point. lodash also has a customizer function for those special needs!
_.extend (http://underscorejs.org/#extend)
_.merge (https://lodash.com/docs#merge)

Here's my stab which
Supports deep merge
Does not mutate arguments
Takes any number of arguments
Does not extend the object prototype
Does not depend on another library (jQuery, MooTools, Underscore.js, etc.)
Includes check for hasOwnProperty
Is short :)
/*
Recursively merge properties and return new object
obj1 <- obj2 [ <- ... ]
*/
function merge () {
var dst = {}
,src
,p
,args = [].splice.call(arguments, 0)
;
while (args.length > 0) {
src = args.splice(0, 1)[0];
if (toString.call(src) == '[object Object]') {
for (p in src) {
if (src.hasOwnProperty(p)) {
if (toString.call(src[p]) == '[object Object]') {
dst[p] = merge(dst[p] || {}, src[p]);
} else {
dst[p] = src[p];
}
}
}
}
}
return dst;
}
Example:
a = {
"p1": "p1a",
"p2": [
"a",
"b",
"c"
],
"p3": true,
"p5": null,
"p6": {
"p61": "p61a",
"p62": "p62a",
"p63": [
"aa",
"bb",
"cc"
],
"p64": {
"p641": "p641a"
}
}
};
b = {
"p1": "p1b",
"p2": [
"d",
"e",
"f"
],
"p3": false,
"p4": true,
"p6": {
"p61": "p61b",
"p64": {
"p642": "p642b"
}
}
};
c = {
"p1": "p1c",
"p3": null,
"p6": {
"p62": "p62c",
"p64": {
"p643": "p641c"
}
}
};
d = merge(a, b, c);
/*
d = {
"p1": "p1c",
"p2": [
"d",
"e",
"f"
],
"p3": null,
"p5": null,
"p6": {
"p61": "p61b",
"p62": "p62c",
"p63": [
"aa",
"bb",
"cc"
],
"p64": {
"p641": "p641a",
"p642": "p642b",
"p643": "p641c"
}
},
"p4": true
};
*/

Just by the way, what you're all doing is overwriting properties, not merging...
This is how JavaScript objects area really merged: Only keys in the to object which are not objects themselves will be overwritten by from. Everything else will be really merged. Of course you can change this behaviour to not overwrite anything which exists like only if to[n] is undefined, etc...:
var realMerge = function (to, from) {
for (n in from) {
if (typeof to[n] != 'object') {
to[n] = from[n];
} else if (typeof from[n] == 'object') {
to[n] = realMerge(to[n], from[n]);
}
}
return to;
};
Usage:
var merged = realMerge(obj1, obj2);

Object.assign()
ECMAScript 2015 (ES6)
This is a new technology, part of the ECMAScript 2015 (ES6) standard.
This technology's specification has been finalized, but check the compatibility table for usage and implementation status in various browsers.
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.
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 }
console.log(o1); // { a: 1, b: 2, c: 3 }, target object itself is changed.

For not-too-complicated objects you could use JSON:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog', car: 'chevy'}
var objMerge;
objMerge = JSON.stringify(obj1) + JSON.stringify(obj2);
// {"food": "pizza","car":"ford"}{"animal":"dog","car":"chevy"}
objMerge = objMerge.replace(/\}\{/, ","); // \_ replace with comma for valid JSON
objMerge = JSON.parse(objMerge); // { food: 'pizza', animal: 'dog', car: 'chevy'}
// Of same keys in both objects, the last object's value is retained_/
Mind you that in this example "}{" must not occur within a string!

There's a library called deepmerge on GitHub: That seems to be getting some traction. It's a standalone, available through both the npm and bower package managers.
I would be inclined to use or improve on this instead of copy-pasting code from answers.

The best way for you to do this is to add a proper property that is non-enumerable using Object.defineProperty.
This way you will still be able to iterate over your objects properties without having the newly created "extend" that you would get if you were to create the property with Object.prototype.extend.
Hopefully this helps:
Object.defineProperty(Object.prototype, "extend", {
enumerable: false,
value: function(from) {
var props = Object.getOwnPropertyNames(from);
var dest = this;
props.forEach(function(name) {
if (name in dest) {
var destination = Object.getOwnPropertyDescriptor(from, name);
Object.defineProperty(dest, name, destination);
}
});
return this;
}
});
Once you have that working, you can do:
var obj = {
name: 'stack',
finish: 'overflow'
}
var replacement = {
name: 'stock'
};
obj.extend(replacement);
I just wrote a blog post about it here: http://onemoredigit.com/post/1527191998/extending-objects-in-node-js

You can simply use jQuery extend
var obj1 = { val1: false, limit: 5, name: "foo" };
var obj2 = { val2: true, name: "bar" };
jQuery.extend(obj1, obj2);
Now obj1 contains all the values of obj1 and obj2

Prototype has this:
Object.extend = function(destination,source) {
for (var property in source)
destination[property] = source[property];
return destination;
}
obj1.extend(obj2) will do what you want.

Wow.. this is the first StackOverflow post I've seen with multiple pages. Apologies for adding another "answer"
ES5 & Earlier
This method is for ES5 & Earlier - there are plenty of other answers addressing ES6.
I did not see any "deep" object merging utilizing the arguments property. Here is my answer - compact & recursive, allowing unlimited object arguments to be passed:
function extend() {
for (var o = {}, i = 0; i < arguments.length; i++) {
// Uncomment to skip arguments that are not objects (to prevent errors)
// if (arguments[i].constructor !== Object) continue;
for (var k in arguments[i]) {
if (arguments[i].hasOwnProperty(k)) {
o[k] = arguments[i][k].constructor === Object
? extend(o[k] || {}, arguments[i][k])
: arguments[i][k];
}
}
}
return o;
}
Example
/**
* Extend objects
*/
function extend() {
for (var o = {}, i = 0; i < arguments.length; i++) {
for (var k in arguments[i]) {
if (arguments[i].hasOwnProperty(k)) {
o[k] = arguments[i][k].constructor === Object
? extend(o[k] || {}, arguments[i][k])
: arguments[i][k];
}
}
}
return o;
}
/**
* Example
*/
document.write(JSON.stringify(extend({
api: 1,
params: {
query: 'hello'
}
}, {
params: {
query: 'there'
}
})));
// outputs {"api": 1, "params": {"query": "there"}}
This answer is now but a drop in the ocean ...

Just if anyone is using Google Closure Library:
goog.require('goog.object');
var a = {'a': 1, 'b': 2};
var b = {'b': 3, 'c': 4};
goog.object.extend(a, b);
// Now object a == {'a': 1, 'b': 3, 'c': 4};
Similar helper function exists for array:
var a = [1, 2];
var b = [3, 4];
goog.array.extend(a, b); // Extends array 'a'
goog.array.concat(a, b); // Returns concatenation of array 'a' and 'b'

**Merging objects is simple using Object.assign or the spread ... operator **
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog', car: 'BMW' }
var obj3 = {a: "A"}
var mergedObj = Object.assign(obj1,obj2,obj3)
// or using the Spread operator (...)
var mergedObj = {...obj1,...obj2,...obj3}
console.log(mergedObj);
The objects are merged from right to left, this means that objects which have identical properties as the objects to their right will be overriden.
In this example obj2.car overrides obj1.car

I extended David Coallier's method:
Added the possibility to merge multiple objects
Supports deep objects
override parameter (that's detected if the last parameter is a boolean)
If override is false, no property gets overridden but new properties will be added.
Usage:
obj.merge(merges... [, override]);
Here is my code:
Object.defineProperty(Object.prototype, "merge", {
enumerable: false,
value: function () {
var override = true,
dest = this,
len = arguments.length,
props, merge, i, from;
if (typeof(arguments[arguments.length - 1]) === "boolean") {
override = arguments[arguments.length - 1];
len = arguments.length - 1;
}
for (i = 0; i < len; i++) {
from = arguments[i];
if (from != null) {
Object.getOwnPropertyNames(from).forEach(function (name) {
var descriptor;
// nesting
if ((typeof(dest[name]) === "object" || typeof(dest[name]) === "undefined")
&& typeof(from[name]) === "object") {
// ensure proper types (Array rsp Object)
if (typeof(dest[name]) === "undefined") {
dest[name] = Array.isArray(from[name]) ? [] : {};
}
if (override) {
if (!Array.isArray(dest[name]) && Array.isArray(from[name])) {
dest[name] = [];
}
else if (Array.isArray(dest[name]) && !Array.isArray(from[name])) {
dest[name] = {};
}
}
dest[name].merge(from[name], override);
}
// flat properties
else if ((name in dest && override) || !(name in dest)) {
descriptor = Object.getOwnPropertyDescriptor(from, name);
if (descriptor.configurable) {
Object.defineProperty(dest, name, descriptor);
}
}
});
}
}
return this;
}
});
Examples and TestCases:
function clone (obj) {
return JSON.parse(JSON.stringify(obj));
}
var obj = {
name : "trick",
value : "value"
};
var mergeObj = {
name : "truck",
value2 : "value2"
};
var mergeObj2 = {
name : "track",
value : "mergeObj2",
value2 : "value2-mergeObj2",
value3 : "value3"
};
assertTrue("Standard", clone(obj).merge(mergeObj).equals({
name : "truck",
value : "value",
value2 : "value2"
}));
assertTrue("Standard no Override", clone(obj).merge(mergeObj, false).equals({
name : "trick",
value : "value",
value2 : "value2"
}));
assertTrue("Multiple", clone(obj).merge(mergeObj, mergeObj2).equals({
name : "track",
value : "mergeObj2",
value2 : "value2-mergeObj2",
value3 : "value3"
}));
assertTrue("Multiple no Override", clone(obj).merge(mergeObj, mergeObj2, false).equals({
name : "trick",
value : "value",
value2 : "value2",
value3 : "value3"
}));
var deep = {
first : {
name : "trick",
val : "value"
},
second : {
foo : "bar"
}
};
var deepMerge = {
first : {
name : "track",
anotherVal : "wohoo"
},
second : {
foo : "baz",
bar : "bam"
},
v : "on first layer"
};
assertTrue("Deep merges", clone(deep).merge(deepMerge).equals({
first : {
name : "track",
val : "value",
anotherVal : "wohoo"
},
second : {
foo : "baz",
bar : "bam"
},
v : "on first layer"
}));
assertTrue("Deep merges no override", clone(deep).merge(deepMerge, false).equals({
first : {
name : "trick",
val : "value",
anotherVal : "wohoo"
},
second : {
foo : "bar",
bar : "bam"
},
v : "on first layer"
}));
var obj1 = {a: 1, b: "hello"};
obj1.merge({c: 3});
assertTrue(obj1.equals({a: 1, b: "hello", c: 3}));
obj1.merge({a: 2, b: "mom", d: "new property"}, false);
assertTrue(obj1.equals({a: 1, b: "hello", c: 3, d: "new property"}));
var obj2 = {};
obj2.merge({a: 1}, {b: 2}, {a: 3});
assertTrue(obj2.equals({a: 3, b: 2}));
var a = [];
var b = [1, [2, 3], 4];
a.merge(b);
assertEquals(1, a[0]);
assertEquals([2, 3], a[1]);
assertEquals(4, a[2]);
var o1 = {};
var o2 = {a: 1, b: {c: 2}};
var o3 = {d: 3};
o1.merge(o2, o3);
assertTrue(o1.equals({a: 1, b: {c: 2}, d: 3}));
o1.b.c = 99;
assertTrue(o2.equals({a: 1, b: {c: 2}}));
// checking types with arrays and objects
var bo;
a = [];
bo = [1, {0:2, 1:3}, 4];
b = [1, [2, 3], 4];
a.merge(b);
assertTrue("Array stays Array?", Array.isArray(a[1]));
a = [];
a.merge(bo);
assertTrue("Object stays Object?", !Array.isArray(a[1]));
a = [];
a.merge(b);
a.merge(bo);
assertTrue("Object overrides Array", !Array.isArray(a[1]));
a = [];
a.merge(b);
a.merge(bo, false);
assertTrue("Object does not override Array", Array.isArray(a[1]));
a = [];
a.merge(bo);
a.merge(b);
assertTrue("Array overrides Object", Array.isArray(a[1]));
a = [];
a.merge(bo);
a.merge(b, false);
assertTrue("Array does not override Object", !Array.isArray(a[1]));
My equals method can be found here: Object comparison in JavaScript

In MooTools, there's Object.merge():
Object.merge(obj1, obj2);

var firstObject = {
key1 : 'value1',
key2 : 'value2'
};
var secondObject={
...firstObject,
key3 : 'value3',
key4 : 'value4',
key5 : 'value5'
}
console.log(firstObject);
console.log(secondObject);

In Ext JS 4 it can be done as follows:
var mergedObject = Ext.Object.merge(object1, object2)
// Or shorter:
var mergedObject2 = Ext.merge(object1, object2)
See merge( object ) : Object.

var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
// result
result: {food: "pizza", car: "ford", animal: "dog"}
Using jQuery.extend() - Link
// Merge obj1 & obj2 to result
var result1 = $.extend( {}, obj1, obj2 );
Using _.merge() - Link
// Merge obj1 & obj2 to result
var result2 = _.merge( {}, obj1, obj2 );
Using _.extend() - Link
// Merge obj1 & obj2 to result
var result3 = _.extend( {}, obj1, obj2 );
Using Object.assign() ECMAScript 2015 (ES6) - Link
// Merge obj1 & obj2 to result
var result4 = Object.assign( {}, obj1, obj2 );
Output of all
obj1: { animal: 'dog' }
obj2: { food: 'pizza', car: 'ford' }
result1: {food: "pizza", car: "ford", animal: "dog"}
result2: {food: "pizza", car: "ford", animal: "dog"}
result3: {food: "pizza", car: "ford", animal: "dog"}
result4: {food: "pizza", car: "ford", animal: "dog"}

Based on Markus' and vsync' answer, this is an expanded version. The function takes any number of arguments. It can be used to set properties on DOM nodes and makes deep copies of values. However, the first argument is given by reference.
To detect a DOM node, the isDOMNode() function is used (see Stack Overflow question JavaScript isDOM — How do you check if a JavaScript Object is a DOM Object?)
It was tested in Opera 11, Firefox 6, Internet Explorer 8 and Google Chrome 16.
Code
function mergeRecursive() {
// _mergeRecursive does the actual job with two arguments.
var _mergeRecursive = function (dst, src) {
if (isDOMNode(src) || typeof src !== 'object' || src === null) {
return dst;
}
for (var p in src) {
if (!src.hasOwnProperty(p))
continue;
if (src[p] === undefined)
continue;
if ( typeof src[p] !== 'object' || src[p] === null) {
dst[p] = src[p];
} else if (typeof dst[p]!=='object' || dst[p] === null) {
dst[p] = _mergeRecursive(src[p].constructor===Array ? [] : {}, src[p]);
} else {
_mergeRecursive(dst[p], src[p]);
}
}
return dst;
}
// Loop through arguments and merge them into the first argument.
var out = arguments[0];
if (typeof out !== 'object' || out === null)
return out;
for (var i = 1, il = arguments.length; i < il; i++) {
_mergeRecursive(out, arguments[i]);
}
return out;
}
Some examples
Set innerHTML and style of a HTML Element
mergeRecursive(
document.getElementById('mydiv'),
{style: {border: '5px solid green', color: 'red'}},
{innerHTML: 'Hello world!'});
Merge arrays and objects. Note that undefined can be used to preserv values in the lefthand array/object.
o = mergeRecursive({a:'a'}, [1,2,3], [undefined, null, [30,31]], {a:undefined, b:'b'});
// o = {0:1, 1:null, 2:[30,31], a:'a', b:'b'}
Any argument not beeing a JavaScript object (including null) will be ignored. Except for the first argument, also DOM nodes are discarded. Beware that i.e. strings, created like new String() are in fact objects.
o = mergeRecursive({a:'a'}, 1, true, null, undefined, [1,2,3], 'bc', new String('de'));
// o = {0:'d', 1:'e', 2:3, a:'a'}
If you want to merge two objects into a new (without affecting any of the two) supply {} as first argument
var a={}, b={b:'abc'}, c={c:'cde'}, o;
o = mergeRecursive(a, b, c);
// o===a is true, o===b is false, o===c is false
Edit (by ReaperSoon):
To also merge arrays
function mergeRecursive(obj1, obj2) {
if (Array.isArray(obj2)) { return obj1.concat(obj2); }
for (var p in obj2) {
try {
// Property in destination object set; update its value.
if ( obj2[p].constructor==Object ) {
obj1[p] = mergeRecursive(obj1[p], obj2[p]);
} else if (Array.isArray(obj2[p])) {
obj1[p] = obj1[p].concat(obj2[p]);
} else {
obj1[p] = obj2[p];
}
} catch(e) {
// Property in destination object not set; create it and set its value.
obj1[p] = obj2[p];
}
}
return obj1;
}

It seems like this should be all you need:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
var obj3 = { ...obj1, ...obj2 }
After that obj3 should now have the following value:
{food: "pizza", car: "ford", animal: "dog"}
Try it out here:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
var obj3 = { ...obj1, ...obj2 }
console.log(obj3);

You should use lodash's defaultsDeep
_.defaultsDeep({ 'user': { 'name': 'barney' } }, { 'user': { 'name': 'fred', 'age': 36 } });
// → { 'user': { 'name': 'barney', 'age': 36 } }

Combine Javascript Objects? [duplicate]

I need to be able to merge two (very simple) JavaScript objects at runtime. For example I'd like to:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
obj1.merge(obj2);
//obj1 now has three properties: food, car, and animal
Is there a built in way to do this? I do not need recursion, and I do not need to merge functions, just methods on flat objects.

ECMAScript 2018 Standard Method
You would use object spread:
let merged = {...obj1, ...obj2};
merged is now the union of obj1 and obj2. Properties in obj2 will overwrite those in obj1.
/** There's no limit to the number of objects you can merge.
* Later properties overwrite earlier properties with the same name. */
const allRules = {...obj1, ...obj2, ...obj3};
Here is also the MDN documentation for this syntax. If you're using babel you'll need the #babel/plugin-proposal-object-rest-spread plugin for it to work (This plugin is included in #babel/preset-env, in ES2018).
ECMAScript 2015 (ES6) Standard Method
/* For the case in question, you would do: */
Object.assign(obj1, obj2);
/** There's no limit to the number of objects you can merge.
* All objects get merged into the first object.
* Only the object in the first argument is mutated and returned.
* Later properties overwrite earlier properties with the same name. */
const allRules = Object.assign({}, obj1, obj2, obj3, etc);
(see MDN JavaScript Reference)
Method for ES5 and Earlier
for (var attrname in obj2) { obj1[attrname] = obj2[attrname]; }
Note that this will simply add all attributes of obj2 to obj1 which might not be what you want if you still want to use the unmodified obj1.
If you're using a framework that craps all over your prototypes then you have to get fancier with checks like hasOwnProperty, but that code will work for 99% of cases.
Example function:
/**
* Overwrites obj1's values with obj2's and adds obj2's if non existent in obj1
* #param obj1
* #param obj2
* #returns obj3 a new object based on obj1 and obj2
*/
function merge_options(obj1,obj2){
var obj3 = {};
for (var attrname in obj1) { obj3[attrname] = obj1[attrname]; }
for (var attrname in obj2) { obj3[attrname] = obj2[attrname]; }
return obj3;
}

jQuery also has a utility for this: http://api.jquery.com/jQuery.extend/.
Taken from the jQuery documentation:
// Merge options object into settings object
var settings = { validate: false, limit: 5, name: "foo" };
var options = { validate: true, name: "bar" };
jQuery.extend(settings, options);
// Now the content of settings object is the following:
// { validate: true, limit: 5, name: "bar" }
The above code will mutate the existing object named settings.
If you want to create a new object without modifying either argument, use this:
var defaults = { validate: false, limit: 5, name: "foo" };
var options = { validate: true, name: "bar" };
/* Merge defaults and options, without modifying defaults */
var settings = $.extend({}, defaults, options);
// The content of settings variable is now the following:
// {validate: true, limit: 5, name: "bar"}
// The 'defaults' and 'options' variables remained the same.

The Harmony ECMAScript 2015 (ES6) specifies Object.assign which will do this.
Object.assign(obj1, obj2);
Current browser support is getting better, but if you're developing for browsers that don't have support, you can use a polyfill.

I googled for code to merge object properties and ended up here. However since there wasn't any code for recursive merge I wrote it myself. (Maybe jQuery extend is recursive BTW?) Anyhow, hopefully someone else will find it useful as well.
(Now the code does not use Object.prototype :)
Code
/*
* Recursively merge properties of two objects
*/
function MergeRecursive(obj1, obj2) {
for (var p in obj2) {
try {
// Property in destination object set; update its value.
if ( obj2[p].constructor==Object ) {
obj1[p] = MergeRecursive(obj1[p], obj2[p]);
} else {
obj1[p] = obj2[p];
}
} catch(e) {
// Property in destination object not set; create it and set its value.
obj1[p] = obj2[p];
}
}
return obj1;
}
An example
o1 = { a : 1,
b : 2,
c : {
ca : 1,
cb : 2,
cc : {
cca : 100,
ccb : 200 } } };
o2 = { a : 10,
c : {
ca : 10,
cb : 20,
cc : {
cca : 101,
ccb : 202 } } };
o3 = MergeRecursive(o1, o2);
Produces object o3 like
o3 = { a : 10,
b : 2,
c : {
ca : 10,
cb : 20,
cc : {
cca : 101,
ccb : 202 } } };

Note that underscore.js's extend-method does this in a one-liner:
_.extend({name : 'moe'}, {age : 50});
=> {name : 'moe', age : 50}

Similar to jQuery extend(), you have the same function in AngularJS:
// Merge the 'options' object into the 'settings' object
var settings = {validate: false, limit: 5, name: "foo"};
var options = {validate: true, name: "bar"};
angular.extend(settings, options);

I need to merge objects today, and this question (and answers) helped me a lot. I tried some of the answers, but none of them fit my needs, so I combined some of the answers, added something myself and came up with a new merge function. Here it is:
var merge = function() {
var obj = {},
i = 0,
il = arguments.length,
key;
for (; i < il; i++) {
for (key in arguments[i]) {
if (arguments[i].hasOwnProperty(key)) {
obj[key] = arguments[i][key];
}
}
}
return obj;
};
Some example usages:
var t1 = {
key1: 1,
key2: "test",
key3: [5, 2, 76, 21]
};
var t2 = {
key1: {
ik1: "hello",
ik2: "world",
ik3: 3
}
};
var t3 = {
key2: 3,
key3: {
t1: 1,
t2: 2,
t3: {
a1: 1,
a2: 3,
a4: [21, 3, 42, "asd"]
}
}
};
console.log(merge(t1, t2));
console.log(merge(t1, t3));
console.log(merge(t2, t3));
console.log(merge(t1, t2, t3));
console.log(merge({}, t1, { key1: 1 }));

You can use the object spread syntax to achieve this. It's a part of ES2018 and beyond.
const obj1 = { food: 'pizza', car: 'ford' };
const obj2 = { animal: 'dog' };
const obj3 = { ...obj1, ...obj2 };
console.log(obj3);

The given solutions should be modified to check source.hasOwnProperty(property) in the for..in loops before assigning - otherwise, you end up copying the properties of the whole prototype chain, which is rarely desired...

Merge properties of N objects in one line of code
An Object.assign method is part of the ECMAScript 2015 (ES6) standard and does exactly what you need. (IE not supported)
var clone = Object.assign({}, obj);
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.
Read more...
The polyfill to support older browsers:
if (!Object.assign) {
Object.defineProperty(Object, 'assign', {
enumerable: false,
configurable: true,
writable: true,
value: function(target) {
'use strict';
if (target === undefined || target === null) {
throw new TypeError('Cannot convert first argument to object');
}
var to = Object(target);
for (var i = 1; i < arguments.length; i++) {
var nextSource = arguments[i];
if (nextSource === undefined || nextSource === null) {
continue;
}
nextSource = Object(nextSource);
var keysArray = Object.keys(nextSource);
for (var nextIndex = 0, len = keysArray.length; nextIndex < len; nextIndex++) {
var nextKey = keysArray[nextIndex];
var desc = Object.getOwnPropertyDescriptor(nextSource, nextKey);
if (desc !== undefined && desc.enumerable) {
to[nextKey] = nextSource[nextKey];
}
}
}
return to;
}
});
}

The following two are probably a good starting point. lodash also has a customizer function for those special needs!
_.extend (http://underscorejs.org/#extend)
_.merge (https://lodash.com/docs#merge)

Here's my stab which
Supports deep merge
Does not mutate arguments
Takes any number of arguments
Does not extend the object prototype
Does not depend on another library (jQuery, MooTools, Underscore.js, etc.)
Includes check for hasOwnProperty
Is short :)
/*
Recursively merge properties and return new object
obj1 <- obj2 [ <- ... ]
*/
function merge () {
var dst = {}
,src
,p
,args = [].splice.call(arguments, 0)
;
while (args.length > 0) {
src = args.splice(0, 1)[0];
if (toString.call(src) == '[object Object]') {
for (p in src) {
if (src.hasOwnProperty(p)) {
if (toString.call(src[p]) == '[object Object]') {
dst[p] = merge(dst[p] || {}, src[p]);
} else {
dst[p] = src[p];
}
}
}
}
}
return dst;
}
Example:
a = {
"p1": "p1a",
"p2": [
"a",
"b",
"c"
],
"p3": true,
"p5": null,
"p6": {
"p61": "p61a",
"p62": "p62a",
"p63": [
"aa",
"bb",
"cc"
],
"p64": {
"p641": "p641a"
}
}
};
b = {
"p1": "p1b",
"p2": [
"d",
"e",
"f"
],
"p3": false,
"p4": true,
"p6": {
"p61": "p61b",
"p64": {
"p642": "p642b"
}
}
};
c = {
"p1": "p1c",
"p3": null,
"p6": {
"p62": "p62c",
"p64": {
"p643": "p641c"
}
}
};
d = merge(a, b, c);
/*
d = {
"p1": "p1c",
"p2": [
"d",
"e",
"f"
],
"p3": null,
"p5": null,
"p6": {
"p61": "p61b",
"p62": "p62c",
"p63": [
"aa",
"bb",
"cc"
],
"p64": {
"p641": "p641a",
"p642": "p642b",
"p643": "p641c"
}
},
"p4": true
};
*/

Just by the way, what you're all doing is overwriting properties, not merging...
This is how JavaScript objects area really merged: Only keys in the to object which are not objects themselves will be overwritten by from. Everything else will be really merged. Of course you can change this behaviour to not overwrite anything which exists like only if to[n] is undefined, etc...:
var realMerge = function (to, from) {
for (n in from) {
if (typeof to[n] != 'object') {
to[n] = from[n];
} else if (typeof from[n] == 'object') {
to[n] = realMerge(to[n], from[n]);
}
}
return to;
};
Usage:
var merged = realMerge(obj1, obj2);

Object.assign()
ECMAScript 2015 (ES6)
This is a new technology, part of the ECMAScript 2015 (ES6) standard.
This technology's specification has been finalized, but check the compatibility table for usage and implementation status in various browsers.
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.
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 }
console.log(o1); // { a: 1, b: 2, c: 3 }, target object itself is changed.

For not-too-complicated objects you could use JSON:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog', car: 'chevy'}
var objMerge;
objMerge = JSON.stringify(obj1) + JSON.stringify(obj2);
// {"food": "pizza","car":"ford"}{"animal":"dog","car":"chevy"}
objMerge = objMerge.replace(/\}\{/, ","); // \_ replace with comma for valid JSON
objMerge = JSON.parse(objMerge); // { food: 'pizza', animal: 'dog', car: 'chevy'}
// Of same keys in both objects, the last object's value is retained_/
Mind you that in this example "}{" must not occur within a string!

There's a library called deepmerge on GitHub: That seems to be getting some traction. It's a standalone, available through both the npm and bower package managers.
I would be inclined to use or improve on this instead of copy-pasting code from answers.

The best way for you to do this is to add a proper property that is non-enumerable using Object.defineProperty.
This way you will still be able to iterate over your objects properties without having the newly created "extend" that you would get if you were to create the property with Object.prototype.extend.
Hopefully this helps:
Object.defineProperty(Object.prototype, "extend", {
enumerable: false,
value: function(from) {
var props = Object.getOwnPropertyNames(from);
var dest = this;
props.forEach(function(name) {
if (name in dest) {
var destination = Object.getOwnPropertyDescriptor(from, name);
Object.defineProperty(dest, name, destination);
}
});
return this;
}
});
Once you have that working, you can do:
var obj = {
name: 'stack',
finish: 'overflow'
}
var replacement = {
name: 'stock'
};
obj.extend(replacement);
I just wrote a blog post about it here: http://onemoredigit.com/post/1527191998/extending-objects-in-node-js

You can simply use jQuery extend
var obj1 = { val1: false, limit: 5, name: "foo" };
var obj2 = { val2: true, name: "bar" };
jQuery.extend(obj1, obj2);
Now obj1 contains all the values of obj1 and obj2

Prototype has this:
Object.extend = function(destination,source) {
for (var property in source)
destination[property] = source[property];
return destination;
}
obj1.extend(obj2) will do what you want.

Wow.. this is the first StackOverflow post I've seen with multiple pages. Apologies for adding another "answer"
ES5 & Earlier
This method is for ES5 & Earlier - there are plenty of other answers addressing ES6.
I did not see any "deep" object merging utilizing the arguments property. Here is my answer - compact & recursive, allowing unlimited object arguments to be passed:
function extend() {
for (var o = {}, i = 0; i < arguments.length; i++) {
// Uncomment to skip arguments that are not objects (to prevent errors)
// if (arguments[i].constructor !== Object) continue;
for (var k in arguments[i]) {
if (arguments[i].hasOwnProperty(k)) {
o[k] = arguments[i][k].constructor === Object
? extend(o[k] || {}, arguments[i][k])
: arguments[i][k];
}
}
}
return o;
}
Example
/**
* Extend objects
*/
function extend() {
for (var o = {}, i = 0; i < arguments.length; i++) {
for (var k in arguments[i]) {
if (arguments[i].hasOwnProperty(k)) {
o[k] = arguments[i][k].constructor === Object
? extend(o[k] || {}, arguments[i][k])
: arguments[i][k];
}
}
}
return o;
}
/**
* Example
*/
document.write(JSON.stringify(extend({
api: 1,
params: {
query: 'hello'
}
}, {
params: {
query: 'there'
}
})));
// outputs {"api": 1, "params": {"query": "there"}}
This answer is now but a drop in the ocean ...

Just if anyone is using Google Closure Library:
goog.require('goog.object');
var a = {'a': 1, 'b': 2};
var b = {'b': 3, 'c': 4};
goog.object.extend(a, b);
// Now object a == {'a': 1, 'b': 3, 'c': 4};
Similar helper function exists for array:
var a = [1, 2];
var b = [3, 4];
goog.array.extend(a, b); // Extends array 'a'
goog.array.concat(a, b); // Returns concatenation of array 'a' and 'b'

**Merging objects is simple using Object.assign or the spread ... operator **
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog', car: 'BMW' }
var obj3 = {a: "A"}
var mergedObj = Object.assign(obj1,obj2,obj3)
// or using the Spread operator (...)
var mergedObj = {...obj1,...obj2,...obj3}
console.log(mergedObj);
The objects are merged from right to left, this means that objects which have identical properties as the objects to their right will be overriden.
In this example obj2.car overrides obj1.car

I extended David Coallier's method:
Added the possibility to merge multiple objects
Supports deep objects
override parameter (that's detected if the last parameter is a boolean)
If override is false, no property gets overridden but new properties will be added.
Usage:
obj.merge(merges... [, override]);
Here is my code:
Object.defineProperty(Object.prototype, "merge", {
enumerable: false,
value: function () {
var override = true,
dest = this,
len = arguments.length,
props, merge, i, from;
if (typeof(arguments[arguments.length - 1]) === "boolean") {
override = arguments[arguments.length - 1];
len = arguments.length - 1;
}
for (i = 0; i < len; i++) {
from = arguments[i];
if (from != null) {
Object.getOwnPropertyNames(from).forEach(function (name) {
var descriptor;
// nesting
if ((typeof(dest[name]) === "object" || typeof(dest[name]) === "undefined")
&& typeof(from[name]) === "object") {
// ensure proper types (Array rsp Object)
if (typeof(dest[name]) === "undefined") {
dest[name] = Array.isArray(from[name]) ? [] : {};
}
if (override) {
if (!Array.isArray(dest[name]) && Array.isArray(from[name])) {
dest[name] = [];
}
else if (Array.isArray(dest[name]) && !Array.isArray(from[name])) {
dest[name] = {};
}
}
dest[name].merge(from[name], override);
}
// flat properties
else if ((name in dest && override) || !(name in dest)) {
descriptor = Object.getOwnPropertyDescriptor(from, name);
if (descriptor.configurable) {
Object.defineProperty(dest, name, descriptor);
}
}
});
}
}
return this;
}
});
Examples and TestCases:
function clone (obj) {
return JSON.parse(JSON.stringify(obj));
}
var obj = {
name : "trick",
value : "value"
};
var mergeObj = {
name : "truck",
value2 : "value2"
};
var mergeObj2 = {
name : "track",
value : "mergeObj2",
value2 : "value2-mergeObj2",
value3 : "value3"
};
assertTrue("Standard", clone(obj).merge(mergeObj).equals({
name : "truck",
value : "value",
value2 : "value2"
}));
assertTrue("Standard no Override", clone(obj).merge(mergeObj, false).equals({
name : "trick",
value : "value",
value2 : "value2"
}));
assertTrue("Multiple", clone(obj).merge(mergeObj, mergeObj2).equals({
name : "track",
value : "mergeObj2",
value2 : "value2-mergeObj2",
value3 : "value3"
}));
assertTrue("Multiple no Override", clone(obj).merge(mergeObj, mergeObj2, false).equals({
name : "trick",
value : "value",
value2 : "value2",
value3 : "value3"
}));
var deep = {
first : {
name : "trick",
val : "value"
},
second : {
foo : "bar"
}
};
var deepMerge = {
first : {
name : "track",
anotherVal : "wohoo"
},
second : {
foo : "baz",
bar : "bam"
},
v : "on first layer"
};
assertTrue("Deep merges", clone(deep).merge(deepMerge).equals({
first : {
name : "track",
val : "value",
anotherVal : "wohoo"
},
second : {
foo : "baz",
bar : "bam"
},
v : "on first layer"
}));
assertTrue("Deep merges no override", clone(deep).merge(deepMerge, false).equals({
first : {
name : "trick",
val : "value",
anotherVal : "wohoo"
},
second : {
foo : "bar",
bar : "bam"
},
v : "on first layer"
}));
var obj1 = {a: 1, b: "hello"};
obj1.merge({c: 3});
assertTrue(obj1.equals({a: 1, b: "hello", c: 3}));
obj1.merge({a: 2, b: "mom", d: "new property"}, false);
assertTrue(obj1.equals({a: 1, b: "hello", c: 3, d: "new property"}));
var obj2 = {};
obj2.merge({a: 1}, {b: 2}, {a: 3});
assertTrue(obj2.equals({a: 3, b: 2}));
var a = [];
var b = [1, [2, 3], 4];
a.merge(b);
assertEquals(1, a[0]);
assertEquals([2, 3], a[1]);
assertEquals(4, a[2]);
var o1 = {};
var o2 = {a: 1, b: {c: 2}};
var o3 = {d: 3};
o1.merge(o2, o3);
assertTrue(o1.equals({a: 1, b: {c: 2}, d: 3}));
o1.b.c = 99;
assertTrue(o2.equals({a: 1, b: {c: 2}}));
// checking types with arrays and objects
var bo;
a = [];
bo = [1, {0:2, 1:3}, 4];
b = [1, [2, 3], 4];
a.merge(b);
assertTrue("Array stays Array?", Array.isArray(a[1]));
a = [];
a.merge(bo);
assertTrue("Object stays Object?", !Array.isArray(a[1]));
a = [];
a.merge(b);
a.merge(bo);
assertTrue("Object overrides Array", !Array.isArray(a[1]));
a = [];
a.merge(b);
a.merge(bo, false);
assertTrue("Object does not override Array", Array.isArray(a[1]));
a = [];
a.merge(bo);
a.merge(b);
assertTrue("Array overrides Object", Array.isArray(a[1]));
a = [];
a.merge(bo);
a.merge(b, false);
assertTrue("Array does not override Object", !Array.isArray(a[1]));
My equals method can be found here: Object comparison in JavaScript

In MooTools, there's Object.merge():
Object.merge(obj1, obj2);

var firstObject = {
key1 : 'value1',
key2 : 'value2'
};
var secondObject={
...firstObject,
key3 : 'value3',
key4 : 'value4',
key5 : 'value5'
}
console.log(firstObject);
console.log(secondObject);

In Ext JS 4 it can be done as follows:
var mergedObject = Ext.Object.merge(object1, object2)
// Or shorter:
var mergedObject2 = Ext.merge(object1, object2)
See merge( object ) : Object.

var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
// result
result: {food: "pizza", car: "ford", animal: "dog"}
Using jQuery.extend() - Link
// Merge obj1 & obj2 to result
var result1 = $.extend( {}, obj1, obj2 );
Using _.merge() - Link
// Merge obj1 & obj2 to result
var result2 = _.merge( {}, obj1, obj2 );
Using _.extend() - Link
// Merge obj1 & obj2 to result
var result3 = _.extend( {}, obj1, obj2 );
Using Object.assign() ECMAScript 2015 (ES6) - Link
// Merge obj1 & obj2 to result
var result4 = Object.assign( {}, obj1, obj2 );
Output of all
obj1: { animal: 'dog' }
obj2: { food: 'pizza', car: 'ford' }
result1: {food: "pizza", car: "ford", animal: "dog"}
result2: {food: "pizza", car: "ford", animal: "dog"}
result3: {food: "pizza", car: "ford", animal: "dog"}
result4: {food: "pizza", car: "ford", animal: "dog"}

Based on Markus' and vsync' answer, this is an expanded version. The function takes any number of arguments. It can be used to set properties on DOM nodes and makes deep copies of values. However, the first argument is given by reference.
To detect a DOM node, the isDOMNode() function is used (see Stack Overflow question JavaScript isDOM — How do you check if a JavaScript Object is a DOM Object?)
It was tested in Opera 11, Firefox 6, Internet Explorer 8 and Google Chrome 16.
Code
function mergeRecursive() {
// _mergeRecursive does the actual job with two arguments.
var _mergeRecursive = function (dst, src) {
if (isDOMNode(src) || typeof src !== 'object' || src === null) {
return dst;
}
for (var p in src) {
if (!src.hasOwnProperty(p))
continue;
if (src[p] === undefined)
continue;
if ( typeof src[p] !== 'object' || src[p] === null) {
dst[p] = src[p];
} else if (typeof dst[p]!=='object' || dst[p] === null) {
dst[p] = _mergeRecursive(src[p].constructor===Array ? [] : {}, src[p]);
} else {
_mergeRecursive(dst[p], src[p]);
}
}
return dst;
}
// Loop through arguments and merge them into the first argument.
var out = arguments[0];
if (typeof out !== 'object' || out === null)
return out;
for (var i = 1, il = arguments.length; i < il; i++) {
_mergeRecursive(out, arguments[i]);
}
return out;
}
Some examples
Set innerHTML and style of a HTML Element
mergeRecursive(
document.getElementById('mydiv'),
{style: {border: '5px solid green', color: 'red'}},
{innerHTML: 'Hello world!'});
Merge arrays and objects. Note that undefined can be used to preserv values in the lefthand array/object.
o = mergeRecursive({a:'a'}, [1,2,3], [undefined, null, [30,31]], {a:undefined, b:'b'});
// o = {0:1, 1:null, 2:[30,31], a:'a', b:'b'}
Any argument not beeing a JavaScript object (including null) will be ignored. Except for the first argument, also DOM nodes are discarded. Beware that i.e. strings, created like new String() are in fact objects.
o = mergeRecursive({a:'a'}, 1, true, null, undefined, [1,2,3], 'bc', new String('de'));
// o = {0:'d', 1:'e', 2:3, a:'a'}
If you want to merge two objects into a new (without affecting any of the two) supply {} as first argument
var a={}, b={b:'abc'}, c={c:'cde'}, o;
o = mergeRecursive(a, b, c);
// o===a is true, o===b is false, o===c is false
Edit (by ReaperSoon):
To also merge arrays
function mergeRecursive(obj1, obj2) {
if (Array.isArray(obj2)) { return obj1.concat(obj2); }
for (var p in obj2) {
try {
// Property in destination object set; update its value.
if ( obj2[p].constructor==Object ) {
obj1[p] = mergeRecursive(obj1[p], obj2[p]);
} else if (Array.isArray(obj2[p])) {
obj1[p] = obj1[p].concat(obj2[p]);
} else {
obj1[p] = obj2[p];
}
} catch(e) {
// Property in destination object not set; create it and set its value.
obj1[p] = obj2[p];
}
}
return obj1;
}

It seems like this should be all you need:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
var obj3 = { ...obj1, ...obj2 }
After that obj3 should now have the following value:
{food: "pizza", car: "ford", animal: "dog"}
Try it out here:
var obj1 = { food: 'pizza', car: 'ford' }
var obj2 = { animal: 'dog' }
var obj3 = { ...obj1, ...obj2 }
console.log(obj3);

You should use lodash's defaultsDeep
_.defaultsDeep({ 'user': { 'name': 'barney' } }, { 'user': { 'name': 'fred', 'age': 36 } });
// → { 'user': { 'name': 'barney', 'age': 36 } }

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