再具体讲解之前我们先看一个ES6实现继承的例子
class Point {
static NAME='point';
constructor(x, y) {
this.x = x;
this.y = y;
}
outPoint(){
console.log('point:'+this.x+this.y);
}
}
class ColorPoint extends Point {
static COLOR_NAME='ColorPoint';
constructor(x, y, color) {
super(x, y);
this.color = color;
}
outColorPoint(){
console.log('ColorPoint:'+this.x+this.y+this.color);
}
}
现在将其通过babel转换为es5;
'use strict';
var _createClass = function() {
function defineProperties(target, props) {
for (var i = 0; i < props.length; i++) {
var descriptor = props[i];
descriptor.enumerable = descriptor.enumerable || false;
descriptor.configurable = true;
if ("value" in descriptor) descriptor.writable = true;
Object.defineProperty(target, descriptor.key, descriptor);
}
}
return function(Constructor, protoProps, staticProps) {
if (protoProps) defineProperties(Constructor.prototype, protoProps);
if (staticProps) defineProperties(Constructor, staticProps);
return Constructor;
};
} ();
function _possibleConstructorReturn(self, call) {
if (!self) {
throw new ReferenceError("this hasn't been initialised - super() hasn't been called");
}
return call && (typeof call === "object" || typeof call === "function") ? call: self;
}
function _inherits(subClass, superClass) {
if (typeof superClass !== "function" && superClass !== null) {
throw new TypeError("Super expression must either be null or a function, not " + typeof superClass);
}
subClass.prototype = Object.create(superClass && superClass.prototype, {
constructor: {
value: subClass,
enumerable: false,
writable: true,
configurable: true
}
});
if (superClass) Object.setPrototypeOf ? Object.setPrototypeOf(subClass, superClass) : subClass.__proto__ = superClass;
}
function _classCallCheck(instance, Constructor) {
if (! (instance instanceof Constructor)) {
throw new TypeError("Cannot call a class as a function");
}
}
var Point = function() {
function Point(x, y) {
_classCallCheck(this, Point);
this.x = x;
this.y = y;
}
_createClass(Point, [{
key: 'outPoint',
value: function outPoint() {
console.log('point:' + this.x + this.y);
}
}]);
return Point;
} ();
Point.NAME = 'point';
var ColorPoint = function(_Point) {
_inherits(ColorPoint, _Point);
function ColorPoint(x, y, color) {
_classCallCheck(this, ColorPoint);
var _this = _possibleConstructorReturn(this, (ColorPoint.__proto__ || Object.getPrototypeOf(ColorPoint)).call(this, x, y));
_this.color = color;
return _this;
}
_createClass(ColorPoint, [{
key: 'outColorPoint',
value: function outColorPoint() {
console.log('ColorPoint:' + this.x + this.y + this.color);
}
}]);
return ColorPoint;
} (Point);
ColorPoint.COLOR_NAME = 'ColorPoint';
看着还是比较凌乱,我们将代码简化,去掉验证信息,_createClass使用prototype代替,简化后代码
'use strict';
function _inherits(subClass, superClass) {
subClass.prototype = Object.create(superClass.prototype, {
constructor: {
value: subClass,
enumerable: false,
writable: true,
configurable: true
}
});
subClass.__proto__ = superClass;
}
var Point = function(x, y) {
this.x = x;
this.y = y;
}
Point.prototype.outPoint = function outPoint() {
console.log('point:' + this.x + this.y);
}
Point.NAME = 'point';
var ColorPoint = function(_Point) {
_inherits(ColorPoint, _Point);
function ColorPoint(x, y, color) {
ColorPoint.__proto__.call(this,x,y);
this.color = color;
}
ColorPoint.prototype.outColorPoint = function() {
console.log('ColorPoint:' + this.x + this.y + this.color);
}
return ColorPoint;
} (Point);
ColorPoint.COLOR_NAME = 'ColorPoint';
简化后的代码可以看到_inherits 方法是实现父类原型对象的继承,ColorPoint.__proto__上应用了父类构造函数Point,使用ColorPoint.__proto__.call(this,x,y),使用call继承父类构造器中的属性
下面聊一聊Object.create
function _inherits(subClass, superClass) {
subClass.prototype = Object.create(superClass.prototype, {
constructor: {
value: subClass,
enumerable: false,
writable: true,
configurable: true
}
});
subClass.__proto__ = superClass;
}
使用实例对象,生成另一个实例对象!
上面代码中,Object.create
方法以A
对象为原型,生成了B
对象。B
继承了A
的所有属性和方法。
实际上,Object.create
方法可以用下面的代码代替。
Object.create = function (obj) {
function F() {}
F.prototype = obj;
return new F();
};
上面代码表明,Object.create
方法的实质是新建一个空的构造函数F
,然后让F.prototype
属性指向参数对象obj
,最后返回一个F
的实例,从而实现让该实例继承obj
的属性。
或者
Object.create = function (obj) {
var B={};
Object.setPrototypeOf(B,obj);
return B;
};
更直观点
Object.create = function (obj) {
var B={};
B.__proto__=obj;
return B;
};
如果想要生成一个不继承任何属性(比如没有toString
和valueOf
方法)的对象,可以将Object.create
的参数设为null
。
var obj = Object.create(null);
object.create
方法生成的新对象,动态继承了原型。在原型上添加或修改任何方法,会立刻反映在新对象之上。
var obj1 = { p: 1 };
var obj2 = Object.create(obj1);
obj1.p = 2;
obj2.p // 2
上面代码中,修改对象原型obj1
会影响到实例对象obj2
。
Object.create的第二个参数
除了对象的原型,Object.create
方法还可以接受第二个参数。该参数是一个属性描述对象,它所描述的对象属性,会添加到实例对象,作为该对象自身的属性。
var obj = Object.create({}, {
p1: {
value: 123,
enumerable: true,
configurable: true,
writable: true,
},
p2: {
value: 'abc',
enumerable: true,
configurable: true,
writable: true,
}
});
// 等同于
var obj = Object.create({});
obj.p1 = 123;
obj.p2 = 'abc';
理解了Object.create,就可以轻松去理解ES6转ES5的继承代码了!