為什么我的子類沒有從 Javascript 中的父類繼承屬性？

Question

我正在嘗試了解 Javascript 的原型繼承模型，但似乎缺少某些東西。 我創建了以下對象集：

 function Dog() { this.legs = 4; this.arms = 0; } Dog.prototype.describe = function() { console.log("Has " + this.legs + " legs and " + this.arms + " arms"); } function Schnauzer(name) { Dog.call(this); this.name = name; } Schnauzer.prototype.describe = function() { console.log(this.name + " is a Schnauzer with a shaggy beard"); Dog.prototype.describe(this); } var my_dog = new Schnauzer("Rupert"); my_dog.describe();

我的期望是這將輸出：

Rupert is a Schnauzer with a shaggy beard
Has 4 legs and 0 arms

然而，這是我實際得到的：

Rupert is a Schnauzer with a shaggy beard
Has undefined legs and undefined arms

Answer 1

和

Dog.prototype.describe(this);

您正在使用Dog.prototype的調用上下文（ this值）調用describe方法，並將 dog 實例作為第一個參數傳遞。 但是describe不接受任何參數，它試圖從this獲取屬性。

做Dog.prototype.describe.call(this); 相反，要使用 dog 實例的this值調用該方法：

 function Dog() { this.legs = 4; this.arms = 0; } Dog.prototype.describe = function() { console.log("Has " + this.legs + " legs and " + this.arms + " arms"); } function Schnauzer(name) { Dog.call(this); this.name = name; } Schnauzer.prototype.describe = function() { console.log(this.name + " is a Schnauzer with a shaggy beard"); Dog.prototype.describe.call(this); } var my_dog = new Schnauzer("Rupert"); my_dog.describe();

盡管如此，這還是有點難看，因為您必須使用.call來獲取一個方法，在正常情況下，該方法在當前對象的原型鏈上會更高。 如果可能，您可以考慮重新命名您的Schnauzer.prototype.describe方法，這樣它就不會掩蓋Dog.prototype方法：

 function Dog() { this.legs = 4; this.arms = 0; } Dog.prototype.describe = function() { console.log("Has " + this.legs + " legs and " + this.arms + " arms"); } function Schnauzer(name) { Dog.call(this); this.name = name; } Schnauzer.prototype = Object.create(Dog.prototype); Schnauzer.prototype.describeSchnauzer = function() { console.log(this.name + " is a Schnauzer with a shaggy beard"); this.describe(); } var my_dog = new Schnauzer("Rupert"); my_dog.describeSchnauzer();

或者使用class語法和super ：

 class Dog { legs = 4; arms = 0; describe() { console.log("Has " + this.legs + " legs and " + this.arms + " arms"); } } class Schnauzer extends Dog { constructor(name) { super(); this.name = name; } describe() { console.log(this.name + " is a Schnauzer with a shaggy beard"); super.describe(); } } var my_dog = new Schnauzer("Rupert"); my_dog.describe();