2021-05-05 10:13:49 -07:00
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---
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id: 587d7db1367417b2b2512b87
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title: 繼承後添加方法
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challengeType: 1
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forumTopicId: 301315
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dashedName: add-methods-after-inheritance
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---
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# --description--
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從超類構造函數繼承其 `prototype` 對象的構造函數,除了繼承的方法外,還可以擁有自己的方法。
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請看舉例:`Bird` 是一個構造函數,它繼承了 `Animal` 的 `prototype`:
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```js
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function Animal() { }
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Animal.prototype.eat = function() {
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console.log("nom nom nom");
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};
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function Bird() { }
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Bird.prototype = Object.create(Animal.prototype);
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Bird.prototype.constructor = Bird;
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```
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除了從 `Animal` 構造函數繼承的行爲之外,還需要給 `Bird` 對象添加它獨有的行爲。 這裏,我們給 `Bird` 對象添加一個 `fly()` 函數。 函數會以一種與其他構造函數相同的方式添加到 `Bird's` 的 `prototype` 中:
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```js
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Bird.prototype.fly = function() {
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console.log("I'm flying!");
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};
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```
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現在 `Bird` 的實例中就有了 `eat()` 和 `fly()` 這兩個方法:
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```js
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let duck = new Bird();
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duck.eat();
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duck.fly();
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```
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`duck.eat()` 將在控制檯中顯示字符串 `nom nom nom`, `duck.fly()` 將顯示字符串 `I'm flying!`。
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# --instructions--
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添加必要的代碼,使得 `Dog` 對象繼承 `Animal`,並且把 `Dog` 的 `prototype`上的 constructor 屬性設置爲 `Dog`。 然後給 `Dog` 對象添加一個 `bark()` 方法,這樣的話,`beagle` 將同時擁有 `eat()` 和 `bark()` 這兩個方法。 `bark()` 方法中應該輸出 `Woof!` 到控制檯。
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# --hints--
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`Animal` 應該沒有 `bark()` 方法。
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```js
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assert(typeof Animal.prototype.bark == 'undefined');
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```
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`Dog` 應該繼承了 `Animal` 的 `eat()` 方法。
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```js
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assert(typeof Dog.prototype.eat == 'function');
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```
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2021-05-17 19:00:24 +05:30
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`Dog` 原型應該有一個 `bark()` 方法。
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2021-05-05 10:13:49 -07:00
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```js
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2021-05-17 19:00:24 +05:30
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assert('bark' in Dog.prototype);
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2021-05-05 10:13:49 -07:00
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```
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`beagle` 應該是 `Animal` 的一個 `instanceof`。
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```js
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assert(beagle instanceof Animal);
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```
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`beagle` 的 constructor 屬性應該被設置爲 `Dog`。
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```js
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assert(beagle.constructor === Dog);
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```
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`beagle.eat()` 應該記錄字符串 `nom nom nom`
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```js
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console.log = function (msg) {
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throw msg;
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};
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assert.throws(() => beagle.eat(), 'nom nom nom');
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```
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`beagle.bark()` 應該將字符串 `Woof!` 打印到控制檯
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```js
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console.log = function (msg) {
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throw msg;
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};
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assert.throws(() => beagle.bark(), 'Woof!');
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```
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# --seed--
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## --seed-contents--
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```js
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function Animal() { }
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Animal.prototype.eat = function() { console.log("nom nom nom"); };
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function Dog() { }
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// Only change code below this line
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// Only change code above this line
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let beagle = new Dog();
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```
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# --solutions--
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```js
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function Animal() { }
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Animal.prototype.eat = function() { console.log("nom nom nom"); };
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function Dog() { }
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Dog.prototype = Object.create(Animal.prototype);
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Dog.prototype.constructor = Dog;
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Dog.prototype.bark = function () {
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console.log('Woof!');
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};
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let beagle = new Dog();
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beagle.eat();
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beagle.bark();
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```
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