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This article mainly introduces the introduction of ES6 Class inheritance and super. It has certain reference value. Now I share it with everyone. Friends in need can refer to it
class can extend from another class. It's a nice syntax, technically based on prototypal inheritance.
To inherit an object, you need to specify extends and the parent object before {..}.
This Rabbit inherits from Animal:
class Animal {
constructor(name) {
this.speed = 0;
this.name = name;
}
run(speed) {
this.speed += speed;
alert(`${this.name} runs with speed ${this.speed}.`);
}
stop() {
this.speed = 0;
alert(`${this.name} stopped.`);
}
}
// Inherit from Animal
class Rabbit extends Animal {
hide() {
alert(`${this.name} hides!`);
}
}
let rabbit = new Rabbit("White Rabbit");
rabbit.run(5); // White Rabbit runs with speed 5.
rabbit.hide(); // White Rabbit hides!
As you can see, as you think, extend keywords In fact, [Prototype]] is added to Rabbit.prototype and referenced to Animal.prototype.
So now rabbit can access both its own methods and Animal's methods.
extends can be followed by an expression Class syntax `extends' is not limited to specifying a class, but can also be an expression.
For example, a function that generates a parent class:
function f(phrase) {
return class {
sayHi() { alert(phrase) }
}
}
class User extends f("Hello") {}
new User().sayHi(); // Hello
In the example, class User inherits the result returned by f('Hello').
For advanced programming patterns, this is useful when we use classes that are generated using functions based on many conditions.
Now let’s move to the next step and override a method. So far, Rabbit inherits the stop method from Animal, this.speed = 0.
If we specify our own stop in Rabbit, it will be used first:
class Rabbit extends Animal {
stop() {
// ...this will be used for rabbit.stop()
}
}
...but usually We don't want to completely replace the parent method, but rather adapt or extend its functionality based on the parent method. We do something so that it calls the parent method before/after or within the procedure.
Class provides the super keyword for this purpose.
Use super.method(...) to call the parent method.
Use super(...) to call the parent constructor (only in the constructor function).
For example, to have the rabbit automatically hide when stop:
class Animal {
constructor(name) {
this.speed = 0;
this.name = name;
}
run(speed) {
this.speed += speed;
alert(`${this.name} runs with speed ${this.speed}.`);
}
stop() {
this.speed = 0;
alert(`${this.name} stopped.`);
}
}
class Rabbit extends Animal {
hide() {
alert(`${this.name} hides!`);
}
stop() {
super.stop(); // call parent stop
this.hide(); // and then hide
}
}
let rabbit = new Rabbit("White Rabbit");
rabbit.run(5); // White Rabbit runs with speed 5.
rabbit.stop(); // White Rabbit stopped. White rabbit hides!
Now, Rabbit’s stop Method calls the method of the parent class through super.stop().
super
As mentioned in the arrow-functions chapter, arrow functions have no super.
It will get super from the external function. For example:
class Rabbit extends Animal {
stop() {
setTimeout(() => super.stop(), 1000); // call parent stop after 1sec
}
}
The super in the arrow function is the same as the stop(), so it works as expected. If we use a normal function here, we will get an error:
// Unexpected super
setTimeout(function() { super.stop() }, 1000);
For constructors, this is a bit tricky.
Until now, Rabbit has not had its own constructor.
Till now, Rabbit did not have its own constructor.
According to the specification, if a class extends another class and does not have a constructor, then the following constructor will be automatically generated:
class Rabbit extends Animal {
// generated for extending classes without own constructors
constructor(...args) {
super(...args);
}
}
We can see that it calls the parent constructor passing all parameters. This will happen if we don't write the constructor ourselves.
Now we will add a custom constructor to Rabbit. In addition to name, we will also set earLength:
class Animal {
constructor(name) {
this.speed = 0;
this.name = name;
}
// ...
}
class Rabbit extends Animal {
constructor(name, earLength) {
this.speed = 0;
this.name = name;
this.earLength = earLength;
}
// ...
}
// Doesn't work!
let rabbit = new Rabbit("White Rabbit", 10); // Error: this is not defined.
Oops, something went wrong! Now we can't spawn rabbits, why?
In simple words: the constructor in the inherited class must call super(...),(!) and execute it before using this.
...but why? What's happening here? Hmm...this request does seem strange.
Now let’s discuss the details so that you can truly understand why -
In JavaScript, constructors that inherit other classes are special. In inherited classes, the corresponding constructor is marked with the special internal attribute [[ConstructorKind]]: "derived" .
The difference is:
When a normal constructor runs, it creates an empty object as this and then continues running.
But when the derived constructor runs, unlike the above, it looks to the parent constructor to do the work.
So if we are constructing our own constructor then we must call super otherwise the object with this will not be created , and report an error.
For Rabbit, we need to call super() before using this, as follows:
class Animal {
constructor(name) {
this.speed = 0;
this.name = name;
}
// ...
}
class Rabbit extends Animal {
constructor(name, earLength) {
super(name);
this.earLength = earLength;
}
// ...
}
// now fine
let rabbit = new Rabbit("White Rabbit", 10);
alert(rabbit.name); // White Rabbit
alert(rabbit.earLength); // 10
Let's further understand the underlying implementation of super, we will see some interesting things.
The first thing to say is that with what we have learned so far, implementing super is impossible.
那么思考一下，这是什么原理？当一个对象方法运行时，它将当前对象作为 this。如果我们调用 super.method()，那么如何检索 method？很容易想到，我们需要从当前对象的原型中取出 method。从技术上讲，我们（或JavaScript引擎）可以做到这一点吗？
也许我们可以从 this 的 [[Prototype]] 中获得方法，就像 this .__ proto __.method 一样？不幸的是，这是行不通的。
让我们试一试，简单起见，我们不使用 class 了，直接使用普通对象。
在这里，rabbit.eat() 调用父对象的 animal.eat() 方法：
let animal = {
name: "Animal",
eat() {
alert(`${this.name} eats.`);
}
};
let rabbit = {
__proto__: animal,
name: "Rabbit",
eat() {
// that's how super.eat() could presumably work
this.__proto__.eat.call(this); // (*)
}
};
rabbit.eat(); // Rabbit eats.
在 (*) 这一行，我们从原型（animal）中取出 eat，并以当前对象的上下文中调用它。请注意，.call（this） 在这里很重要，因为只写 this .__ proto __.eat() 的话 eat 的调用对象将会是 animal，而不是当前对象。
以上代码的 alert 是正确的。
但是现在让我们再添加一个对象到原型链中，就要出事了：
let animal = {
name: "Animal",
eat() {
alert(`${this.name} eats.`);
}
};
let rabbit = {
__proto__: animal,
eat() {
// ...bounce around rabbit-style and call parent (animal) method
this.__proto__.eat.call(this); // (*)
}
};
let longEar = {
__proto__: rabbit,
eat() {
// ...do something with long ears and call parent (rabbit) method
this.__proto__.eat.call(this); // (**)
}
};
longEar.eat(); // Error: Maximum call stack size exceeded
噢，完蛋！调用 longEar.eat() 报错了！
这原因一眼可能看不透，但如果我们跟踪 longEar.eat() 调用，大概就知道为什么了。在 (*) 和 (**) 两行中， this 的值是当前对象(longEar)。重点来了：所有方法都将当前对象作为 this，而不是原型或其他东西。
因此，在两行 (*) 和 (**) 中，this.__ proto__ 的值都是 rabbit。他们都调用了 rabbit.eat，于是就这么无限循环下去。
情况如图：
1.在 longEar.eat() 里面，(**) 行中调用了 rabbit.eat，并且this = longEar。
// inside longEar.eat() we have this = longEar this.__proto__.eat.call(this) // (**) // becomes longEar.__proto__.eat.call(this) // that is rabbit.eat.call(this);
2.然后在rabbit.eat的 (*) 行中，我们希望传到原型链的下一层，但是 this = longEar，所以 this .__ proto __.eat又是 rabbit.eat！
// inside rabbit.eat() we also have this = longEar this.__proto__.eat.call(this) // (*) // becomes longEar.__proto__.eat.call(this) // or (again) rabbit.eat.call(this);
...因此 rabbit.eat 在无尽循环调动，无法进入下一层。
这个问题不能简单使用 this 解决。
[[HomeObject]]为了提供解决方案，JavaScript 为函数添加了一个特殊的内部属性：[[HomeObject]]。
当函数被指定为类或对象方法时，其 [[HomeObject]] 属性为该对象。
这实际上违反了 unbind 函数的思想，因为方法记住了它们的对象。并且 [[HomeObject]] 不能被改变，所以这是永久 bind（绑定）。所以在 JavaScript 这是一个很大的变化。
但是这种改变是安全的。 [[HomeObject]] 仅用于在 super 中获取下一层原型。所以它不会破坏兼容性。
让我们来看看它是如何在 super 中运作的：
let animal = {
name: "Animal",
eat() { // [[HomeObject]] == animal
alert(`${this.name} eats.`);
}
};
let rabbit = {
__proto__: animal,
name: "Rabbit",
eat() { // [[HomeObject]] == rabbit
super.eat();
}
};
let longEar = {
__proto__: rabbit,
name: "Long Ear",
eat() { // [[HomeObject]] == longEar
super.eat();
}
};
longEar.eat(); // Long Ear eats.
每个方法都会在内部 [[HomeObject]] 属性中记住它的对象。然后 super 使用它来解析原型。
在类和普通对象中定义的方法中都定义了 [[HomeObject]]，但是对于对象，必须使用：method() 而不是 "method: function()"。
在下面的例子中，使用非方法语法（non-method syntax）进行比较。这么做没有设置 [[HomeObject]] 属性，继承也不起作用：
let animal = {
eat: function() { // should be the short syntax: eat() {...}
// ...
}
};
let rabbit = {
__proto__: animal,
eat: function() {
super.eat();
}
};
rabbit.eat(); // Error calling super (because there's no [[HomeObject]])
class 语法也支持静态属性的继承。
例如：
class Animal {
constructor(name, speed) {
this.speed = speed;
this.name = name;
}
run(speed = 0) {
this.speed += speed;
alert(`${this.name} runs with speed ${this.speed}.`);
}
static compare(animalA, animalB) {
return animalA.speed - animalB.speed;
}
}
// Inherit from Animal
class Rabbit extends Animal {
hide() {
alert(`${this.name} hides!`);
}
}
let rabbits = [
new Rabbit("White Rabbit", 10),
new Rabbit("Black Rabbit", 5)
];
rabbits.sort(Rabbit.compare);
rabbits[0].run(); // Black Rabbit runs with speed 5.
现在我们可以调用 Rabbit.compare，假设继承的 Animal.compare 将被调用。
它是如何工作的？再次使用原型。正如你猜到的那样，extends 同样给 Rabbit 提供了引用到 Animal的 [Prototype]。
所以，Rabbit 函数现在继承 Animal 函数。Animal 自带引用到 Function.prototype 的 [[Prototype]]（因为它不 extend 其他类）。
看看这里：
class Animal {}
class Rabbit extends Animal {}
// for static propertites and methods
alert(Rabbit.__proto__ === Animal); // true
// and the next step is Function.prototype
alert(Animal.__proto__ === Function.prototype); // true
// that's in addition to the "normal" prototype chain for object methods
alert(Rabbit.prototype.__proto__ === Animal.prototype);
这样 Rabbit 可以访问 Animal 的所有静态方法。
请注意，内置类没有静态 [[Prototype]] 引用。例如，Object 具有 Object.defineProperty，Object.keys等方法，但 Array，Date 不会继承它们。
Date 和 Object 的结构：
Date 和 Object 之间毫无关联，他们独立存在，不过 Date.prototype 继承于 Object.prototype，仅此而已。
造成这个情况是因为 JavaScript 在设计初期没有考虑使用 class 语法和继承静态方法。
Array，Map 等内置类也可以扩展。
举个例子，PowerArray 继承自原生 Array：
// add one more method to it (can do more)
class PowerArray extends Array {
isEmpty() {
return this.length === 0;
}
}
let arr = new PowerArray(1, 2, 5, 10, 50);
alert(arr.isEmpty()); // false
let filteredArr = arr.filter(item => item >= 10);
alert(filteredArr); // 10, 50
alert(filteredArr.isEmpty()); // false
请注意一件非常有趣的事情。像 filter，map 和其他内置方法 - 返回新的继承类型的对象。他们依靠 constructor 属性来做到这一点。
在上面的例子中，
arr.constructor === PowerArray
所以当调用 arr.filter() 时，它自动创建新的结果数组，就像 new PowerArray 一样，于是我们可以继续使用 PowerArray 的方法。
我们甚至可以自定义这种行为。如果存在静态 getter Symbol.species，返回新建对象使用的 constructor。
下面的例子中，由于 Symbol.species 的存在，map，filter等内置方法将返回普通的数组：
class PowerArray extends Array {
isEmpty() {
return this.length === 0;
}
// built-in methods will use this as the constructor
static get [Symbol.species]() {
return Array;
}
}
let arr = new PowerArray(1, 2, 5, 10, 50);
alert(arr.isEmpty()); // false
// filter creates new array using arr.constructor[Symbol.species] as constructor
let filteredArr = arr.filter(item => item >= 10);
// filteredArr is not PowerArray, but Array
alert(filteredArr.isEmpty()); // Error: filteredArr.isEmpty is not a function
我们可以在其他 key 使用 Symbol.species，可以用于剥离结果值中的无用方法，或是增加其他方法。
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