Object.assign()

Summary

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.

Syntax

Object.assign(target, ...sources)

Parameters

target

The target object.

sources

The source object(s).

Return value

The target object gets returned.

Description

The Object.assign method only copies enumerable and own properties from a source object to a target object. It uses [[Get]] on the source and [[Put]] on the target, so it will invoke getters and setters. Therefore it assigns properties versus just copying or defining new properties.

Both, String and Symbol properties are copied.

In case of an error, for example if a property is non-writable, a TypeError will be raised, but assignment continues and the error is thrown at the end.

Note that Object.assign does not throw on null or undefined source values.

Examples

Example: Cloning an object

var obj = { a: 1 };
var copy = Object.assign({}, obj);

Example: Merging objects

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 }

Example: Exception cases

Object.assign(window, {undefined: true, foo: "foo"}, {bar: "bar"});
// TypeError: "undefined" is read-only
console.log(window.foo);
// "foo", exception is thrown after completing this source
console.log(window.bar);
// undefined, all later sources will be ignored

Polyfill

This polyfill doesn't support symbol properties, since ES5 doesn't have symbols anyway:

if (!Object.assign) {
  Object.defineProperty(Object, "assign", {
    enumerable: false,
    configurable: true,
    writable: true,
    value: function(target, firstSource) {
      "use strict";
      if (target === undefined || target === null)
        throw new TypeError("Cannot convert first argument to object");

      var to = Object(target);

      var hasPendingException = false;
      var pendingException;

      for (var i = 1; i < arguments.length; i++) {
        var nextSource = arguments[i];
        if (nextSource === undefined || nextSource === null)
          continue;

        var keysArray = Object.keys(Object(nextSource));
        for (var nextIndex = 0, len = keysArray.length; nextIndex < len; nextIndex++) {
          var nextKey = keysArray[nextIndex];
          try {
            var desc = Object.getOwnPropertyDescriptor(nextSource, nextKey);
            if (desc !== undefined && desc.enumerable)
              to[nextKey] = nextSource[nextKey];
          } catch (e) {
            if (!hasPendingException) {
              hasPendingException = true;
              pendingException = e;
            }
          }
        }

        if (hasPendingException)
          throw pendingException;
      }
      return to;
    }
  });
}

Specifications

Browser compatibility

See also

• Object.defineProperties()