Question

我正在玩JavaScript类继承（我正在使用node.js）。我得到子类实例的“未定义”值。这是我的例子：

我定义了一个Squirrel类，我想在KillerSquirrel子类中专门化这个类。我想创建Squirrel和KillerSquirrel类的实例。

function Squirrel(name, color) {
        this.name = name;
        this.color = color;
        console.log("A new " + this.color + " squirrel named " + this.name + " is born!!!");
    };

    // add a new method called speak()
    Squirrel.prototype.speak = function(text) {
        console.log(this.name + " squirrel says: '" + text + "'");
    };

    // specialize the Squirrel class by creating a new KillerSquirrel constructor 
    function KillerSquirrel(name, color) {    
      this.soul = 'very bad';
      console.log("Bhrrr ... a new " + this.color + " killer squirrel named " + this.name + " is born!!!");
    }  
    KillerSquirrel.prototype.__proto__ = Squirrel.prototype;

    // add kill method  
    KillerSquirrel.prototype.kill = function(obj){  
      console.log(this.name + " squirrel killed " + obj.name + " squirrel"); 
    }

    // replace the speak() method  
    KillerSquirrel.prototype.speak = function(text) {
                       console.log(this.name + " squirrel says: 'Grhhh! " + text + "' Grhhh!");
    };

    var squirrel = new Squirrel("Gummy", "brown");
    squirrel.speak("My name is " + squirrel.name);
    var killer = new KillerSquirrel("Mr.Hide", "black");
    killer.speak("My name is " + killer.name);

我使用它的构造函数创建一个Squirrel的Squirrel实例，并将一些值传递给构造函数，这可以按预期工作。当我尝试使用其构造函数创建子类KillerSquirrel的实例并将一些值传递给它时，杀手松鼠实例具有“未定义的属性”。

请参阅：

$ node killersquirrel.js
A new brown squirrel named Gummy is born!!!
Gummy squirrel says: 'My name is Gummy'
Bhrrr ... a new undefined killer squirrel named undefined is born!!!
undefined squirrel says: 'Grhhh! My name is undefined' Grhhh!

Answer 1

子类构造函数应该通过特殊构造（apply或call）手动调用超类构造函数，如下所示：

function KillerSquirrel(name, color) {
  Squirrel.apply(this, arguments);    
  this.soul = 'very bad';
  console.log("Bhrrr ... a new " + this.color + " killer squirrel named " + this.name + " is born!!!");
}

或

function KillerSquirrel(name, color) {
  Squirrel.call(this, name, color);    
  this.soul = 'very bad';
  console.log("Bhrrr ... a new " + this.color + " killer squirrel named " + this.name + " is born!!!");
}

虽然对于这种情况（当参数相同时），第一种形式是首选。

如何专门化Javascript类？

1 个答案: