431 lines
12 KiB
Markdown
431 lines
12 KiB
Markdown
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# Class inheritance
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Classes can extend one another. There's a nice syntax, technically based on the prototypal inheritance.
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To inherit from another class, we should specify `"extends"` and the parent class before the brackets `{..}`.
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Here `Rabbit` inherits from `Animal`:
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```js run
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class Animal {
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constructor(name) {
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this.speed = 0;
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this.name = name;
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}
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run(speed) {
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this.speed += speed;
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alert(`${this.name} runs with speed ${this.speed}.`);
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}
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stop() {
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this.speed = 0;
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alert(`${this.name} stopped.`);
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}
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}
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*!*
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// Inherit from Animal
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class Rabbit extends Animal {
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hide() {
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alert(`${this.name} hides!`);
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}
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}
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*/!*
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let rabbit = new Rabbit("White Rabbit");
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rabbit.run(5); // White Rabbit runs with speed 5.
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rabbit.hide(); // White Rabbit hides!
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```
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The `extends` keyword actually adds a `[[Prototype]]` reference from `Rabbit.prototype` to `Animal.prototype`, just as you expect it to be, and as we've seen before.
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So now `rabbit` has access both to its own methods and to methods of `Animal`.
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````smart header="Any expression is allowed after `extends`"
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Class syntax allows to specify not just a class, but any expression after `extends`.
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For instance, a function call that generates the parent class:
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```js run
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function f(phrase) {
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return class {
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sayHi() { alert(phrase) }
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}
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}
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*!*
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class User extends f("Hello") {}
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*/!*
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new User().sayHi(); // Hello
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```
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Here `class User` inherits from the result of `f("Hello")`.
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That may be useful for advanced programming patterns when we use functions to generate classes depending on many conditions and can inherit from them.
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````
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## Overriding a method
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Now let's move forward and override a method. As of now, `Rabbit` inherits the `stop` method that sets `this.speed = 0` from `Animal`.
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If we specify our own `stop` in `Rabbit`, then it will be used instead:
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```js
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class Rabbit extends Animal {
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stop() {
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// ...this will be used for rabbit.stop()
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}
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}
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```
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...But usually we don't want to totally replace a parent method, but rather to build on top of it, tweak or extend its functionality. We do something in our method, but call the parent method before/after it or in the process.
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Classes provide `"super"` keyword for that.
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- `super.method(...)` to call a parent method.
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- `super(...)` to call a parent constructor (inside our constructor only).
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For instance, let our rabbit autohide when stopped:
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```js run
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class Animal {
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constructor(name) {
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this.speed = 0;
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this.name = name;
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}
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run(speed) {
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this.speed += speed;
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alert(`${this.name} runs with speed ${this.speed}.`);
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}
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stop() {
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this.speed = 0;
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alert(`${this.name} stopped.`);
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}
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}
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class Rabbit extends Animal {
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hide() {
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alert(`${this.name} hides!`);
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}
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*!*
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stop() {
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super.stop(); // call parent stop
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this.hide(); // and then hide
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}
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*/!*
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}
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let rabbit = new Rabbit("White Rabbit");
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rabbit.run(5); // White Rabbit runs with speed 5.
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rabbit.stop(); // White Rabbit stopped. White rabbit hides!
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```
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Now `Rabbit` has the `stop` method that calls the parent `super.stop()` in the process.
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````smart header="Arrow functions have no `super`"
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As was mentioned in the chapter <info:arrow-functions>, arrow functions do not have `super`.
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If accessed, it's taken from the outer function. For instance:
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```js
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class Rabbit extends Animal {
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stop() {
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setTimeout(() => super.stop(), 1000); // call parent stop after 1sec
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}
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}
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```
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The `super` in the arrow function is the same as in `stop()`, so it works as intended. If we specified a "regular" function here, there would be an error:
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```js
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// Unexpected super
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setTimeout(function() { super.stop() }, 1000);
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```
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````
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## Overriding constructor
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With constructors it gets a little bit tricky.
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Till now, `Rabbit` did not have its own `constructor`.
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According to the [specification](https://tc39.github.io/ecma262/#sec-runtime-semantics-classdefinitionevaluation), if a class extends another class and has no `constructor`, then the following `constructor` is generated:
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```js
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class Rabbit extends Animal {
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// generated for extending classes without own constructors
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*!*
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constructor(...args) {
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super(...args);
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}
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*/!*
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}
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```
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As we can see, it basically calls the parent `constructor` passing it all the arguments. That happens if we don't write a constructor of our own.
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Now let's add a custom constructor to `Rabbit`. It will specify the `earLength` in addition to `name`:
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```js run
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class Animal {
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constructor(name) {
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this.speed = 0;
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this.name = name;
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}
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// ...
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}
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class Rabbit extends Animal {
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*!*
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constructor(name, earLength) {
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this.speed = 0;
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this.name = name;
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this.earLength = earLength;
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}
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*/!*
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// ...
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}
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*!*
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// Doesn't work!
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let rabbit = new Rabbit("White Rabbit", 10); // Error: this is not defined.
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*/!*
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```
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Whoops! We've got an error. Now we can't create rabbits. What went wrong?
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The short answer is: constructors in inheriting classes must call `super(...)`, and (!) do it before using `this`.
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...But why? What's going on here? Indeed, the requirement seems strange.
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Of course, there's an explanation. Let's get into details, so you'd really understand what's going on.
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In JavaScript, there's a distinction between a "constructor function of an inheriting class" and all others. In an inheriting class, the corresponding constructor function is labelled with a special internal property `[[ConstructorKind]]:"derived"`.
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The difference is:
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- When a normal constructor runs, it creates an empty object as `this` and continues with it.
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- But when a derived constructor runs, it doesn't do it. It expects the parent constructor to do this job.
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So if we're making a constructor of our own, then we must call `super`, because otherwise the object with `this` reference to it won't be created. And we'll get an error.
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For `Rabbit` to work, we need to call `super()` before using `this`, like here:
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```js run
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class Animal {
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constructor(name) {
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this.speed = 0;
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this.name = name;
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}
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// ...
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}
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class Rabbit extends Animal {
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constructor(name, earLength) {
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*!*
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super(name);
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*/!*
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this.earLength = earLength;
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}
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// ...
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}
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*!*
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// now fine
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let rabbit = new Rabbit("White Rabbit", 10);
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alert(rabbit.name); // White Rabbit
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alert(rabbit.earLength); // 10
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*/!*
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```
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## Super: internals, [[HomeObject]]
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Let's get a little deeper under the hood of `super`. We'll see some interesting things by the way.
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First to say, from all that we've learned till now, it's impossible for `super` to work.
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Yeah, indeed, let's ask ourselves, how it could technically work? When an object method runs, it gets the current object as `this`. If we call `super.method()` then, how to retrieve the `method`? Naturally, we need to take the `method` from the prototype of the current object. How, technically, we (or a JavaScript engine) can do it?
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Maybe we can get the method from `[[Prototype]]` of `this`, as `this.__proto__.method`? Unfortunately, that doesn't work.
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Let's try to do it. Without classes, using plain objects for the sake of simplicity.
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Here, `rabbit.eat()` should call `animal.eat()` method of the parent object:
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```js run
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let animal = {
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name: "Animal",
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eat() {
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alert(`${this.name} eats.`);
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}
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};
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let rabbit = {
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__proto__: animal,
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name: "Rabbit",
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eat() {
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*!*
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// that's how super.eat() could presumably work
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this.__proto__.eat.call(this); // (*)
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*/!*
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}
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};
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rabbit.eat(); // Rabbit eats.
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```
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At the line `(*)` we take `eat` from the prototype (`animal`) and call it in the context of the current object. Please note that `.call(this)` is important here, because a simple `this.__proto__.eat()` would execute parent `eat` in the context of the prototype, not the current object.
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And in the code above it actually works as intended: we have the correct `alert`.
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Now let's add one more object to the chain. We'll see how things break:
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```js run
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let animal = {
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name: "Animal",
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eat() {
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alert(`${this.name} eats.`);
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}
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};
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let rabbit = {
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__proto__: animal,
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eat() {
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// ...bounce around rabbit-style and call parent (animal) method
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this.__proto__.eat.call(this); // (*)
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}
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};
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let longEar = {
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__proto__: rabbit,
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eat() {
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// ...do something with long ears and call parent (rabbit) method
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this.__proto__.eat.call(this); // (**)
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}
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};
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*!*
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longEar.eat(); // Error: Maximum call stack size exceeded
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*/!*
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```
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The code doesn't work anymore! We can see the error trying to call `longEar.eat()`.
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It may be not that obvious, but if we trace `longEar.eat()` call, then we can see why. In both lines `(*)` and `(**)` the value of `this` is the current object (`longEar`). That's essential: all object methods get the current object as `this`, not a prototype or something.
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So, in both lines `(*)` and `(**)` the value of `this.__proto__` is exactly the same: `rabbit`. They both call `rabbit.eat` without going up the chain in the endless loop.
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Here's the picture of what happens:
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1. Inside `longEar.eat()`, the line `(**)` calls `rabbit.eat` providing it with `this=longEar`.
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```js
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// inside longEar.eat() we have this = longEar
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this.__proto__.eat.call(this) // (**)
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// becomes
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longEar.__proto__.eat.call(this)
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// that is
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rabbit.eat.call(this);
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```
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2. Then in the line `(*)` of `rabbit.eat`, we'd like to pass the call even higher in the chain, but `this=longEar`, so `this.__proto__.eat` is again `rabbit.eat`!
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```js
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// inside rabbit.eat() we also have this = longEar
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this.__proto__.eat.call(this) // (*)
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// becomes
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longEar.__proto__.eat.call(this)
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// or (again)
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rabbit.eat.call(this);
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```
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3. ...So `rabbit.eat` calls itself in the endless loop, because it can't ascend any further.
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The problem can't be solved by using `this` alone.
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### `[[HomeObject]]`
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To provide the solution, JavaScript adds one more special internal property for functions: `[[HomeObject]]`.
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**When a function is specified as a class or object method, its `[[HomeObject]]` property becomes that object.**
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This actually violates the idea of "unbound" functions, because methods remember their objects. And `[[HomeObject]]` can't be changed, so this bound is forever. So that's a very important change in the language.
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But this change is safe. `[[HomeObject]]` is used only for calling parent methods in `super`, to resolve the prototype. So it doesn't break compatibility.
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Let's see how it works for `super` -- again, using plain objects:
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```js run
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let animal = {
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name: "Animal",
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eat() { // [[HomeObject]] == animal
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alert(`${this.name} eats.`);
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}
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};
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let rabbit = {
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__proto__: animal,
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name: "Rabbit",
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eat() { // [[HomeObject]] == rabbit
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super.eat();
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}
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};
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let longEar = {
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__proto__: rabbit,
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name: "Long Ear",
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eat() { // [[HomeObject]] == longEar
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super.eat();
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}
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};
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*!*
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longEar.eat(); // Long Ear eats.
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*/!*
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```
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Every method remembers its object in the internal `[[HomeObject]]` property. Then `super` uses it to resolve the parent prototype.
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`[[HomeObject]]` is defined for methods defined both in classes and in plain objects. But for objects, methods must be specified exactly the given way: as `method()`, not as `"method: function()"`.
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In the example below a non-method syntax is used for comparison. `[[HomeObject]]` property is not set and the inheritance doesn't work:
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```js run
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let animal = {
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eat: function() { // should be the short syntax: eat() {...}
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// ...
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}
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};
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let rabbit = {
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__proto__: animal,
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eat: function() {
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super.eat();
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}
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};
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*!*
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rabbit.eat(); // Error calling super (because there's no [[HomeObject]])
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*/!*
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```
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