Object.assign

Object.assign(target, ...sources)

var obj = { a: 1 };
var copy = Object.assign{}, obj
console.log(copy // { a: 1 }

function test() {
  'use strict';

  let obj1 = { a: 0 , b: { c: 0}};
  let obj2 = Object.assign{}, obj1
  console.log(JSON.stringify(obj2) // { a: 0, b: { c: 0}}
  
  obj1.a = 1;
  console.log(JSON.stringify(obj1) // { a: 1, b: { c: 0}}
  console.log(JSON.stringify(obj2) // { a: 0, b: { c: 0}}
  
  obj2.a = 2;
  console.log(JSON.stringify(obj1) // { a: 1, b: { c: 0}}
  console.log(JSON.stringify(obj2) // { a: 2, b: { c: 0}}
  
  obj2.b.c = 3;
  console.log(JSON.stringify(obj1) // { a: 1, b: { c: 3}}
  console.log(JSON.stringify(obj2) // { a: 2, b: { c: 3}}
  
  // Deep Clone
  obj1 = { a: 0 , b: { c: 0}};
  let obj3 = JSON.parse(JSON.stringify(obj1)
  obj1.a = 4;
  obj1.b.c = 4;
  console.log(JSON.stringify(obj3) // { a: 0, b: { c: 0}}
}

test(

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.

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

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

var o1 = { a: 1 };
var o2 = { [Symbol('foo')]: 2 };

var obj = Object.assign{}, o1, o2
console.log(obj // { a : 1, [Symbol("foo")]: 2 } (cf. bug 1207182 on Firefox)
Object.getOwnPropertySymbols(obj // [Symbol(foo)]

var obj = Object.create{ foo: 1 }, { // foo is on obj's prototype chain.
  bar: {
    value: 2  // bar is a non-enumerable property.
  },
  baz: {
    value: 3,
    enumerable: true  // baz is an own enumerable property.
  }
}

var copy = Object.assign{}, obj
console.log(copy // { baz: 3 }

var v1 = 'abc';
var v2 = true;
var v3 = 10;
var v4 = Symbol('foo'

var obj = Object.assign{}, v1, null, v2, undefined, v3, v4 
// Primitives will be wrapped, null and undefined will be ignored.
// Note, only string wrappers can have own enumerable properties.
console.log(obj // { "0": "a", "1": "b", "2": "c" }

var target = Object.defineProperty{}, 'foo', {
  value: 1,
  writable: false
} // target.foo is a read-only property

Object.assign(target, { bar: 2 }, { foo2: 3, foo: 3, foo3: 3 }, { baz: 4 }
// TypeError: "foo" is read-only
// The Exception is thrown when assigning target.foo

console.log(target.bar  // 2, the first source was copied successfully.
console.log(target.foo2 // 3, the first property of the second source was copied successfully.
console.log(target.foo  // 1, exception is thrown here.
console.log(target.foo3 // undefined, assign method has finished, foo3 will not be copied.
console.log(target.baz  // undefined, the third source will not be copied either.

var obj = {
  foo: 1,
  get bar() {
    return 2;
  }
};

var copy = Object.assign{}, obj 
console.log(copy 
// { foo: 1, bar: 2 }, the value of copy.bar is obj.bar's getter's return value.

// This is an assign function that copies full descriptors
function completeAssign(target, ...sources) {
  sources.forEach(source => {
    let descriptors = Object.keys(source).reduce((descriptors, key) => {
      descriptors[key] = Object.getOwnPropertyDescriptor(source, key
      return descriptors;
    }, {}
    // by default, Object.assign copies enumerable Symbols too
    Object.getOwnPropertySymbols(source).forEach(sym => {
      let descriptor = Object.getOwnPropertyDescriptor(source, sym
      if (descriptor.enumerable) {
        descriptors[sym] = descriptor;
      }
    }
    Object.defineProperties(target, descriptors
  }
  return target;
}

var copy = completeAssign{}, obj
console.log(copy
// { foo:1, get bar() { return 2 } }

if (typeof Object.assign != 'function') {
  // Must be writable: true, enumerable: false, configurable: true
  Object.defineProperty(Object, "assign", {
    value: function assign(target, varArgs) { // .length of function is 2
      'use strict';
      if (target == null) { // TypeError if undefined or null
        throw new TypeError('Cannot convert undefined or null to object'
      }

      var to = Object(target

      for (var index = 1; index < arguments.length; index++) {
        var nextSource = arguments[index];

        if (nextSource != null) { // Skip over if undefined or null
          for (var nextKey in nextSource) {
            // Avoid bugs when hasOwnProperty is shadowed
            if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {
              to[nextKey] = nextSource[nextKey];
            }
          }
        }
      }
      return to;
    },
    writable: true,
    configurable: true
  }
}