使用javascript super方法设置属性 [英] set attribute with javascript super method

问题描述

可能重复:
为什么我的JS对象属性被其他对象覆盖实例

在调用setT之后，为什么属性"t"没有更改?我希望输出为"4"，但它会显示默认".

Why isn't the attribute "t" changed after setT was called? I would expect "4" as output, but it prints "default".

function Car(i) {
  var id = i;
  var t = "default";

  this.getT = function() { return t; }
  this.setT = function(p) {
    t = p;  // attribute t isn't changed ..
  }
}

function ECar(id) {  
  Car.call(this, id);  // super constructor call

  this.setT = function(p) {  // override
    ECar.prototype.setT.call(this, p); // super call
  }
}

ECar.prototype = new Car();

ecar = new ECar(3);
ecar.setT(4);
alert(ecar.getT()); // prints default, not 4

推荐答案

ECar.prototype = new Car();

在此行， ECar 的原型获得一个上下文，所有 ECar 的实例将在其中共享.

At this line ECar's prototype get a context, in which all ECar's instance will be shared.

ECar.prototype.setT.call(this，p);

此行将在那个上下文中调用，而不是在 Car.call(this，id); 上调用 super 时创建的内容

This line will call at that context, not what has been created while calling super at Car.call(this, id);.

您可以使用以下方式修复代码

You can fix your code with

function ECar(id) {  
  Car.call(this, id);  // super constructor call
  var carSetT = this.setT;
  this.setT = function(p) {
    carSetT.call(this, p);
  }
}

，但是使用真实的原型(例如

but it would be better (and more readable) to use real prototypes, such as

function Car() {}

Car.prototype.getT = function () { /* ... */ };
Car.prototype.setT = function () { /* ... */ };

function ECar() {}

ECar.prototype = new Car();
ECar.prototype.setT = function () { /* ... */ };

编辑: note (如@Bergi所建议)

Edit: note (as @Bergi suggested)

如果您必须支持旧版浏览器&，则应仅将 Child.prototype = new Parent()用作继承.那么您应该只使用空的构造函数.

You should only use Child.prototype = new Parent() as inheritance if you must support legacy browsers & then you should only use empty constructors.

JavaScript中用于继承的(其他语言)最兼容的方式是

The most (other-language) compatible way in JavaScript for inheritance is

Child.prototype = Object.create(Parent.prototype)

( MDN 说，它已被支持来自IE 9)

(MDN says it is supprted from IE 9)

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