209 lines
6.5 KiB
Markdown
209 lines
6.5 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. The external code that creates the `user` only can see a *public* method `sayHi`.
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In works, because functional classes provide a shared lexical environment (of `User`) for private variables and methods.
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## Prototype-based classes
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Functional class pattern is rarely used, because prototypes are generally better.
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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 constructor `User` only initializes the current object state.
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- Methods reside in `User.prototype`.
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Here methods are technically not inside `function User`, so they do not share a common lexical environment.
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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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We already can see benefits over the functional pattern:
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- In the functional pattern, each object has its own copy of methods like `this.sayHi = function() {...}`.
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- In the prototypal pattern, there's a common `User.prototype` shared between all user objects.
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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, precisely the same way as built-in JavaScript constructors do. Functional pattern allows to wrap a function into another function, and kind-of emulate inheritance this way, but that's far less effective, so here we won't go into details to save our time.
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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 naturally `Rabbit` is a "subtype" of `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 `rabbit` objects have access to `Rabbit.prototype`. We should add `Animal.prototype` to it. So the chain would be `rabbit -> Rabbit.prototype -> Animal.prototype`.
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Like this:
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The code example:
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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, the search may continue in `Object.prototype`, because `Animal.prototype` is a regular plain object, so it inherits from it. But that's not painted for brevity.
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Here's what the code does:
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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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