全面理解Object.assign

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
 });
}