2016-12-13 JavaScript 中几种不同的基于 prototype 继承方式的区别

JavaScript 中几种不同的基于 prototype 继承方式的区别
普通属性的继承
第一种方式
来自于 MDN 对象模型的细节​
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function Employee1 (name, dept) {
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    console.log('Employee constructor')
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    this.name = name || "";
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    this.dept = dept || "general";
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    this.work = function() {console.log("Employee.work")}
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    this.workAsEmployee = function() { this.work() }
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}
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​
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function WorkerBee1 (projs) {
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    console.log('WorkerBee constructor')
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    this.projects = projs || [];
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    this.work = function() {console.log("WorkerBee.work")}
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    this.workAsBee = function() { this.work() }
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}
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WorkerBee1.prototype = new Employee1;
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​
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function Engineer1 (mach) {
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    console.log('Engineer constructor')
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    this.dept = "engineering";
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    this.machine = mach || "";
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    this.work = function() {console.log("Engineer.work")}
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    this.workAsEngineer = function() { this.work() }
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}
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​
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Engineer1.prototype = new WorkerBee1;
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​
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e1 = new Engineer1()
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e1.work()
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console.log(Employee1.prototype.isPrototypeOf(e1))
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console.log(WorkerBee1.prototype.isPrototypeOf(e1))
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console.log(e1)
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第二种方式
来自于 YUI 的实现，利用中间对象传递 prototype
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function extend(Child, Parent) {
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    var F = function () { };
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    F.prototype = Parent.prototype;
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    Child.prototype = new F();
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    Child.prototype.constructor = Child;
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    Object.defineProperty(Child, "super", { "value": Parent.prototype })
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}
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​
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function Employee2 (name, dept) {
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    console.log('Employee constructor')
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    this.name = name || "";
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    this.dept = dept || "general";
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    this.work = function() {console.log("Employee.work")}
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    this.workAsEmployee = function() { this.work() }
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}
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​
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function WorkerBee2 (projs) {
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    console.log('WorkerBee constructor')
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    this.projects = projs || [];
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    this.work = function() {console.log("WorkerBee.work")}
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    this.workAsBee = function() { this.work() }
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}
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extend(WorkerBee2, Employee2);
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​
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function Engineer2 (mach) {
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    console.log('Engineer constructor')
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    this.dept = "engineering";
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    this.machine = mach || "";
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    this.work = function() {console.log("Engineer.work")}
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    this.workAsEngineer = function() { this.work() }
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}
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extend(Engineer2, WorkerBee2);
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​
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e2 = new Engineer2()
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e2.work()
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​
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console.log(Employee2.prototype.isPrototypeOf(e2))
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console.log(WorkerBee2.prototype.isPrototypeOf(e2))
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console.log(e2)
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可以看到，区别主要在于，直接 Child.prototype = new Parent() 会把定义在 Parent 里面的方法也带到 prototype 里面去。另外，这种方式并没有执行父类的构造函数。
对于定义在 prototype 里面的方法呢
下面对上面的方法定义进行一点改进，把方法定义在 prototype 里，类似正常的 OO 编程中在类里面定义方法。
第一种方式改进
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function Employee3 (name, dept) {
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    console.log('Employee constructor')
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    this.name = name || "";
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    this.dept = dept || "general";
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}
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Employee3.prototype.work = function() {console.log("Employee.work")}
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Employee3.prototype.workAsEmployee = function() { this.work() }
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​
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function WorkerBee3 (projs) {
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    console.log('WorkerBee constructor')
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    this.projects = projs || [];
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}
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WorkerBee3.prototype = new Employee3;
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WorkerBee3.prototype.work = function() {console.log("WorkerBee.work")}
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WorkerBee3.prototype.workAsBee = function() { this.work() }
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​
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function Engineer3 (mach) {
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    console.log('Engineer constructor')
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    this.dept = "engineering";
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    this.machine = mach || "";
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}
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Engineer3.prototype = new WorkerBee3;
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Engineer3.prototype.work = function() {console.log("Engineer.work")}
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Engineer3.prototype.workAsEngineer = function() { this.work() }
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​
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e3 = new Engineer3()
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e3.work()
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​
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console.log(Employee3.prototype.isPrototypeOf(e3))
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console.log(WorkerBee3.prototype.isPrototypeOf(e3))
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console.log(e3)
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第二种方式的改进
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function extend(Child, Parent) {
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    var F = function () { };
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    F.prototype = Parent.prototype;
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    Child.prototype = new F();
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    Child.prototype.constructor = Child;
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    Object.defineProperty(Child, "super", { "value": Parent.prototype })
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}
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​
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function Employee4 (name, dept) {
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    console.log('Employee constructor')
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    this.name = name || "";
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    this.dept = dept || "general";
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}
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Employee4.prototype.work = function() {console.log("Employee.work")}
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Employee4.prototype.workAsEmployee = function() { this.work() }
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​
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function WorkerBee4 (projs) {
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    console.log('WorkerBee constructor')
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    this.projects = projs || [];
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​
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}
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extend(WorkerBee4, Employee4);
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WorkerBee4.prototype.work = function() {console.log("WorkerBee.work")}
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WorkerBee4.prototype.workAsBee = function() { this.work() }
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​
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function Engineer4 (mach) {
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    console.log('Engineer constructor')
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    this.dept = "engineering";
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    this.machine = mach || "";
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}
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extend(Engineer4, WorkerBee4);
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Engineer4.prototype.work = function() {console.log("Engineer.work")}
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Engineer4.prototype.workAsEngineer = function() { this.work() }
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​
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e4 = new Engineer4()
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e4.work()
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​
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console.log(Employee4.prototype.isPrototypeOf(e4))
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console.log(WorkerBee4.prototype.isPrototypeOf(e4))
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console.log(e4)
Copied!
注意观察 constructor 和 __proto__ 属性。
要执行所有构造函数
上述第二种方法，都没有执行父类的构造函数，也就没有真正的继承父类的初始化数据。为了弥补这一点，如下两种写法都可以达到目的。
利用 super 变量
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function extend(Child, Parent) {
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    var F = function () { };
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    F.prototype = Parent.prototype;
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    Child.prototype = new F();
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    Child.prototype.constructor = Child;
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}
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​
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function Employee5 (name, dept) {
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    console.log('Employee constructor')
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    this.name = name || "";
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    this.dept = dept || "general";
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}
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Employee5.prototype.work = function() {console.log("Employee.work")}
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Employee5.prototype.workAsEmployee = function() { this.work() }
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​
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function WorkerBee5 (projs) {
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    console.log('WorkerBee constructor')
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​
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    this.super = Employee5;
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    this.super();
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​
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    this.projects = projs || [];
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​
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}
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extend(WorkerBee5, Employee5);
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WorkerBee5.prototype.work = function() {console.log("WorkerBee.work")}
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WorkerBee5.prototype.workAsBee = function() { this.work() }
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​
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function Engineer5 (mach) {
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    console.log('Engineer constructor')
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​
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    this.super = WorkerBee5;
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    this.super();
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​
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    this.dept = "engineering";
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    this.machine = mach || "";
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}
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extend(Engineer5, WorkerBee5);
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Engineer5.prototype.work = function() {console.log("Engineer.work")}
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Engineer5.prototype.workAsEngineer = function() { this.work() }
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​
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e5 = new Engineer5()
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e5.work()
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​
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console.log(Employee5.prototype.isPrototypeOf(e5))
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console.log(WorkerBee5.prototype.isPrototypeOf(e5))
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console.log(e5)
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类似，但是用 Parent.apply 方法
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function extend(Child, Parent) {
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    var F = function () { };
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    F.prototype = Parent.prototype;
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    Child.prototype = new F();
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    Child.prototype.constructor = Child;
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}
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​
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function Employee6 (name, dept) {
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    console.log('Employee constructor')
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    this.name = name || "";
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    this.dept = dept || "general";
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}
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Employee6.prototype.work = function() {console.log("Employee.work")}
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Employee6.prototype.workAsEmployee = function() { this.work() }
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​
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function WorkerBee6 (projs) {
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    console.log('WorkerBee constructor')
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    Employee6.apply(this)
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    this.projects = projs || [];
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}
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extend(WorkerBee6, Employee6);
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WorkerBee6.prototype.work = function() {console.log("WorkerBee.work")}
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WorkerBee6.prototype.workAsBee = function() { this.work() }
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​
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function Engineer6 (mach) {
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    console.log('Engineer constructor')
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    WorkerBee6.apply(this)
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    this.dept = "engineering";
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    this.machine = mach || "";
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}
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extend(Engineer6, WorkerBee6);
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Engineer6.prototype.work = function() {console.log("Engineer.work")}
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Engineer6.prototype.workAsEngineer = function() { this.work() }
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​
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e6 = new Engineer6()
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e6.work()
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​
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console.log(Employee6.prototype.isPrototypeOf(e6))
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console.log(WorkerBee6.prototype.isPrototypeOf(e6))
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console.log(e6)
Copied!

以前

JavaScript 中几种不同的基于 prototype 继承方式的区别

普通属性的继承

第一种方式

第二种方式

对于定义在 prototype 里面的方法呢

第一种方式改进

第二种方式的改进

要执行所有构造函数

利用 `super` 变量

类似，但是用 `Parent.apply` 方法