if (typeof Object.assign2 != 'function') {
  // Attention 1
  Object.defineProperty(Object, "assign2", {
    value: function (target) {
      'use strict';
      if (target == null) { // Attention 2
        throw new TypeError('Cannot convert undefined or null to object');
      }

      // Attention 3
      var to = Object(target);
        
      for (var index = 1; index < arguments.length; index++) {
        var nextSource = arguments[index];

        if (nextSource != null) {  // Attention 2
          // Attention 4
          for (var nextKey in nextSource) {
            if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {
              to[nextKey] = nextSource[nextKey];
            }
          }
        }
      }
      return to;
    },
    writable: true,
    configurable: true
  });
}

参考文章链接

【进阶4-2期】Object.assign 原理及其实现 (opens new window)