243 lines
7.6 KiB
Markdown
243 lines
7.6 KiB
Markdown
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# Class patterns
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```quote author="Wikipedia"
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In object-oriented programming, a *class* is an extensible program-code-template for creating objects, providing initial values for state (member variables) and implementations of behavior (member functions or methods).
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```
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There's a special syntax construct and a keyword `class` in JavaScript. But before studying it, we should consider that the term "class" comes the theory of object-oriented programming. The definition is cited above, and it's language-independant.
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In JavaScript there are several well-known programming patterns to make classes even without using the `class` keyword. And here we'll talk about them first.
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The `class` construct will be described in the next chapter, but in JavaScript it's a "syntax sugar" and an extension of one of the patterns that we'll study here.
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[cut]
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## Functional class pattern
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The constructor function below can be considered a "class" according to the definition:
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```js run
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function User(name) {
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this.sayHi = function() {
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alert(name);
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};
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}
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let user = new User("John");
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user.sayHi(); // John
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```
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It follows all parts of the definition:
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1. It is a "program-code-template" for creating objects (callable with `new`).
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2. It provides initial values for the state (`name` from parameters).
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3. It provides methods (`sayHi`).
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This is called *functional class pattern*.
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In the functional class pattern, local variables and nested functions inside `User`, that are not assigned to `this`, are visible from inside, but not accessible by the outer code.
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So we can easily add internal functions and variables, like `calcAge()` here:
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```js run
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function User(name, birthday) {
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*!*
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// only visible from other methods inside User
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function calcAge() {
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new Date().getFullYear() - birthday.getFullYear();
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}
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*/!*
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this.sayHi = function() {
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alert(name + ', age:' + calcAge());
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};
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}
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let user = new User("John", new Date(2000,0,1));
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user.sayHi(); // John
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```
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In this code variables `name`, `birthday` and the function `calcAge()` are internal, *private* to the object. They are only visible from inside of it.
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From the other hand, `sayHi` is the external, *public* method. The external code that creates `user` can access it.
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This way we can hide internal implementation details and helper methods from the outer code. Only what's assigned to `this` becomes visible outside.
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## Factory class pattern
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We can create a class without using `new` at all.
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Like this:
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```js run
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function User(name, birthday) {
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// only visible from other methods inside User
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function calcAge() {
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new Date().getFullYear() - birthday.getFullYear();
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}
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return {
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sayHi() {
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alert(name + ', age:' + calcAge());
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}
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};
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}
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*!*
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let user = User("John", new Date(2000,0,1));
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*/!*
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user.sayHi(); // John
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```
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As we can see, the function `User` returns an object with public properties and methods. The only benefit of this method is that we can omit `new`: write `let user = User(...)` instead of `let user = new User(...)`. In other aspects it's almost the same as the functional pattern.
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## Prototype-based classes
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Prototype-based classes is the most important and generally the best. Functional and factory class patterns are rarely used in practice.
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Soon you'll see why.
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Here's the same class rewritten using prototypes:
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```js run
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function User(name, birthday) {
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*!*
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this._name = name;
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this._birthday = birthday;
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*/!*
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}
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*!*
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User.prototype._calcAge = function() {
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*/!*
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return new Date().getFullYear() - this._birthday.getFullYear();
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};
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User.prototype.sayHi = function() {
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alert(this._name + ', age:' + this._calcAge());
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};
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let user = new User("John", new Date(2000,0,1));
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user.sayHi(); // John
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```
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The code structure:
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- The constructor `User` only initializes the current object state.
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- Methods are added to `User.prototype`.
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As we can see, methods are lexically not inside `function User`, they do not share a common lexical environment. If we declare variables inside `function User`, then they won't be visible to methods.
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So, there is a widely known agreement that internal properties and methods are prepended with an underscore `"_"`. Like `_name` or `_calcAge()`. Technically, that's just an agreement, the outer code still can access them. But most developers recognize the meaning of `"_"` and try not to touch prefixed properties and methods in the external code.
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Here are the advantages over the functional pattern:
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- In the functional pattern, each object has its own copy of every method. We assign a separate copy of `this.sayHi = function() {...}` and other methods in the constructor.
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- In the prototypal pattern, all methods are in `User.prototype` that is shared between all user objects. An object itself only stores the data.
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So the prototypal pattern is more memory-efficient.
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...But not only that. Prototypes allow us to setup the inheritance in a really efficient way. Built-in JavaScript objects all use prototypes. Also there's a special syntax construct: "class" that provides nice-looking syntax for them. And there's more, so let's go on with them.
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## Prototype-based inheritance for classes
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Let's say we have two prototype-based classes.
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`Rabbit`:
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```js
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function Rabbit(name) {
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this.name = name;
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}
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Rabbit.prototype.jump = function() {
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alert(this.name + ' jumps!');
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};
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let rabbit = new Rabbit("My rabbit");
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```
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...And `Animal`:
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```js
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function Animal(name) {
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this.name = name;
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}
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Animal.prototype.eat = function() {
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alert(this.name + ' eats.');
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};
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let animal = new Animal("My animal");
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```
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Right now they are fully independent.
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But we'd want `Rabbit` to extend `Animal`. In other words, rabbits should be based on animals, have access to methods of `Animal` and extend them with its own methods.
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What does it mean in the language on prototypes?
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Right now methods for `rabbit` objects are in `Rabbit.prototype`. We'd like `rabbit` to use `Animal.prototype` as a "fallback", if the method is not found in `Rabbit.prototype`.
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So the prototype chain should be `rabbit` -> `Rabbit.prototype` -> `Animal.prototype`.
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Like this:
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The code to implement that:
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```js run
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// Same Animal as before
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function Animal(name) {
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this.name = name;
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}
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// All animals can eat, right?
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Animal.prototype.eat = function() {
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alert(this.name + ' eats.');
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};
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// Same Rabbit as before
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function Rabbit(name) {
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this.name = name;
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}
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Rabbit.prototype.jump = function() {
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alert(this.name + ' jumps!');
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};
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*!*
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// setup the inheritance chain
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Rabbit.prototype.__proto__ = Animal.prototype; // (*)
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*/!*
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let rabbit = new Rabbit("White Rabbit");
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*!*
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rabbit.eat(); // rabbits can eat too
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*/!*
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rabbit.jump();
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```
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The line `(*)` sets up the prototype chain. So that `rabbit` first searches methods in `Rabbit.prototype`, then `Animal.prototype`. And then, just for completeness, let's mention that if the method is not found in `Animal.prototype`, then the search continues in `Object.prototype`, because `Animal.prototype` is a regular plain object, so it inherits from it.
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So here's the full picture:
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## Summary
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The term "class" comes from the object-oriented programming. In JavaScript it usually means the functional class pattern or the prototypal pattern. The prototypal pattern is more powerful and memory-efficient, so it's recommended to stick to it.
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According to the prototypal pattern:
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1. Methods are stored in `Class.prototype`.
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2. Prototypes inherit from each other.
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In the next chapter we'll study `class` keyword and construct. It allows to write prototypal classes shorter and provides some additional benefits.
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