ECMAScript新特性【一】--Object.create

Object.create(prototype, descriptors) ：创建一个具有指定原型且可选择性地包含指定属性的对象

参数:
prototype 必需。  要用作原型的对象。 可以为 null。
descriptors 可选。 包含一个或多个属性描述符的 JavaScript 对象。
“数据属性”是可获取且可设置值的属性。 数据属性描述符包含 value 特性，以及 writable、enumerable 和 configurable 特性。

如果未指定最后三个特性，则它们默认为 false。 只要检索或设置该值，“访问器属性”就会调用用户提供的函数。 访问器属性描述符包含 set 特性和/或 get 特性。

var pt = {
        say : function(){
            console.log('saying!');    
        }
    }
    
    var o = Object.create(pt);
    
    console.log('say' in o); // true
    console.log(o.hasOwnProperty('say')); // false

如果prototype传入的是null，创建一个没有原型链的空对象。

var o1 = Object.create(null);
console.dir(o1); // object[ No Properties ]

当然，可以创建没有原型链的但带descriptors的对象；

var o2 = Object.create(null, {
        size: {
            value: "large",
            enumerable: true
        },
        shape: {
            value: "round",
            enumerable: true
        }    
    });
    
    console.log(o2.size);    // large
    console.log(o2.shape);     // round
    console.log(Object.getPrototypeOf(o2));     // null

也可以创建带属性带原型链的对象：

var pt = {
        say : function(){
            console.log('saying!');    
        }
    }

var o3 = Object.create(pt, {
        size: {
            value: "large",
            enumerable: true
        },
        shape: {
            value: "round",
            enumerable: true
        }    
    });
    
    console.log(o3.size);    // large
    console.log(o3.shape);     // round
    console.log(Object.getPrototypeOf(o3));     // {say:...}

最重要的是实现继承，看下面实例：

//Shape - superclass
        function Shape() {
          this.x = 0;
          this.y = 0;
        }
        
        Shape.prototype.move = function(x, y) {
            this.x += x;
            this.y += y;
            console.info("Shape moved.");
        };
        
        // Rectangle - subclass
        function Rectangle() {
          Shape.call(this); //call super constructor.
        }
        
        Rectangle.prototype = Object.create(Shape.prototype);
        
        var rect = new Rectangle();

        console.log(rect instanceof Rectangle); //true.
        console.log(rect instanceof Shape); //true.
        
        rect.move(); //"Shape moved."

不支持浏览器的兼容实现：

1、简单实现，也是最常见的实现方式，没有实现第二个参数的功能：

if (!Object.create) {
    Object.create = function (o) {
        if (arguments.length > 1) {
            throw new Error('Object.create implementation only accepts the first parameter.');
        }
        function F() {}
        F.prototype = o;
        return new F();
    };
}

2、复杂实现，实现第二个参数的大部分功能：

if (!Object.create) {

    // Contributed by Brandon Benvie, October, 2012
    var createEmpty;
    var supportsProto = Object.prototype.__proto__ === null;
    if (supportsProto || typeof document == 'undefined') {
        createEmpty = function () {
            return { "__proto__": null };
        };
    } else {
        createEmpty = function () {
            var iframe = document.createElement('iframe');
            var parent = document.body || document.documentElement;
            iframe.style.display = 'none';
            parent.appendChild(iframe);
            iframe.src = 'javascript:';
            var empty = iframe.contentWindow.Object.prototype;
            parent.removeChild(iframe);
            iframe = null;
            delete empty.constructor;
            delete empty.hasOwnProperty;
            delete empty.propertyIsEnumerable;
            delete empty.isPrototypeOf;
            delete empty.toLocaleString;
            delete empty.toString;
            delete empty.valueOf;
            empty.__proto__ = null;

            function Empty() {}
            Empty.prototype = empty;
            // short-circuit future calls
            createEmpty = function () {
                return new Empty();
            };
            return new Empty();
        };
    }

    Object.create = function create(prototype, properties) {

        var object;
        function Type() {}  // An empty constructor.

        if (prototype === null) {
            object = createEmpty();
        } else {
            if (typeof prototype !== "object" && typeof prototype !== "function") {

                throw new TypeError("Object prototype may only be an Object or null"); // same msg as Chrome
            }
            Type.prototype = prototype;
            object = new Type();

            object.__proto__ = prototype;
        }

        if (properties !== void 0) {
            Object.defineProperties(object, properties);
        }

        return object;
    };
}

参考：

https://developer.mozilla.org/zh-CN/docs/JavaScript/Reference/Global_Objects/Object/create

https://developer.mozilla.org/zh-CN/docs/JavaScript/Reference/Global_Objects/Object/defineProperty

https://developer.mozilla.org/zh-CN/docs/JavaScript/Reference/Global_Objects/Object/defineProperties

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/create

https://github.com/kriskowal/es5-shim/blob/master/es5-sham.js