428 lines
10 KiB
Markdown
428 lines
10 KiB
Markdown
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# Class basic syntax
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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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In practice, we often need to create many objects of the same kind, like users, or goods or whatever.
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As we already know from the chapter <info:constructor-new>, `new function` can help with that.
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But in the modern JavaScript, there's a more advanced "class" construct, that introduces great new features which are useful for object-oriented programming.
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## The "class" syntax
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The basic syntax is:
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```js
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class MyClass {
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// class methods
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constructor() { ... }
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method1() { ... }
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method2() { ... }
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method3() { ... }
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...
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}
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```
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Then use `new MyClass()` to create a new object with all the listed methods.
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The `constructor()` method is called automatically by `new`, so we can initialize the object there.
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For example:
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```js run
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class User {
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constructor(name) {
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this.name = name;
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}
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sayHi() {
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alert(this.name);
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}
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}
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// Usage:
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let user = new User("John");
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user.sayHi();
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```
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When `new User("John")` is called:
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1. A new object is created.
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2. The `constructor` runs with the given argument and assigns it to `this.name`.
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...Then we can call object methods, such as `user.sayHi()`.
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```warn header="No comma between class methods"
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A common pitfall for novice developers is to put a comma between class methods, which would result in a syntax error.
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The notation here is not to be confused with object literals. Within the class, no commas are required.
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```
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## What is a class?
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So, what exactly is a `class`? That's not an entirely new language-level entity, as one might think.
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Let's unveil any magic and see what a class really is. That'll help in understanding many complex aspects.
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In JavaScript, a class is a kind of function.
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Here, take a look:
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```js run
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class User {
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constructor(name) { this.name = name; }
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sayHi() { alert(this.name); }
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}
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// proof: User is a function
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*!*
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alert(typeof User); // function
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*/!*
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```
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What `class User {...}` construct really does is:
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1. Creates a function named `User`, that becomes the result of the class declaration. The function code is taken from the `constructor` method (assumed empty if we don't write such method).
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2. Stores class methods, such as `sayHi`, in `User.prototype`.
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After `new User` object is created, when we call its method, it's taken from the prototype, just as described in the chapter <info:function-prototype>. So the object has access to class methods.
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We can illustrate the result of `class User` declaration as:
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Here's the code to introspect it:
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```js run
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class User {
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constructor(name) { this.name = name; }
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sayHi() { alert(this.name); }
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}
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// class is a function
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alert(typeof User); // function
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// ...or, more precisely, the constructor method
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alert(User === User.prototype.constructor); // true
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// The methods are in User.prototype, e.g:
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alert(User.prototype.sayHi); // alert(this.name);
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// there are exactly two methods in the prototype
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alert(Object.getOwnPropertyNames(User.prototype)); // constructor, sayHi
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```
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## Not just a syntactic sugar
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Sometimes people say that `class` is a "syntactic sugar" (syntax that is designed to make things easier to read, but doesn't introduce anything new), because we could actually declare the same without `class` keyword at all:
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```js run
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// rewriting class User in pure functions
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// 1. Create constructor function
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function User(name) {
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this.name = name;
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}
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// a function prototype has "constructor" property by default,
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// so we don't need to create it
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// 2. Add the method to prototype
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User.prototype.sayHi = function() {
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alert(this.name);
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};
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// Usage:
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let user = new User("John");
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user.sayHi();
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```
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The result of this definition is about the same. So, there are indeed reasons why `class` can be considered a syntactic sugar to define a constructor together with its prototype methods.
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Still, there are important differences.
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1. First, a function created by `class` is labelled by a special internal property `[[FunctionKind]]:"classConstructor"`. So it's not entirely the same as creating it manually.
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The language checks for that property in a variety of places. For example, unlike a regular function, it must be called with `new`:
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```js run
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class User {
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constructor() {}
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}
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alert(typeof User); // function
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User(); // Error: Class constructor User cannot be invoked without 'new'
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```
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Also, a string representation of a class constructor in most JavaScript engines starts with the "class..."
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```js run
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class User {
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constructor() {}
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}
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alert(User); // class User { ... }
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```
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There are other differences, we'll see them soon.
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2. Class methods are non-enumerable.
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A class definition sets `enumerable` flag to `false` for all methods in the `"prototype"`.
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That's good, because if we `for..in` over an object, we usually don't want its class methods.
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3. Classes always `use strict`.
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All code inside the class construct is automatically in strict mode.
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Besides, `class` syntax brings many other features that we'll explore later.
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## Class Expression
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Just like functions, classes can be defined inside another expression, passed around, returned, assigned, etc.
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Here's an example of a class expression:
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```js
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let User = class {
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sayHi() {
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alert("Hello");
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}
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};
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```
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Similar to Named Function Expressions, class expressions may have a name.
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If a class expression has a name, it's visible inside the class only:
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```js run
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// "Named Class Expression"
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// (no such term in the spec, but that's similar to Named Function Expression)
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let User = class *!*MyClass*/!* {
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sayHi() {
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alert(MyClass); // MyClass name is visible only inside the class
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}
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};
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new User().sayHi(); // works, shows MyClass definition
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alert(MyClass); // error, MyClass name isn't visible outside of the class
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```
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We can even make classes dynamically "on-demand", like this:
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```js run
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function makeClass(phrase) {
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// declare a class and return it
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return class {
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sayHi() {
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alert(phrase);
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}
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};
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}
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// Create a new class
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let User = makeClass("Hello");
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new User().sayHi(); // Hello
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```
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## Getters/setters
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Just like literal objects, classes may include getters/setters, computed properties etc.
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Here's an example for `user.name` implemented using `get/set`:
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```js run
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class User {
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constructor(name) {
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// invokes the setter
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this.name = name;
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}
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*!*
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get name() {
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*/!*
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return this._name;
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}
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*!*
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set name(value) {
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*/!*
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if (value.length < 4) {
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alert("Name is too short.");
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return;
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}
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this._name = value;
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}
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}
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let user = new User("John");
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alert(user.name); // John
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user = new User(""); // Name is too short.
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```
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Technically, such class declaration works by creating getters and setters in `User.prototype`.
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## Computed names [...]
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Here's an example with a computed method name using brackets `[...]`:
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```js run
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class User {
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*!*
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['say' + 'Hi']() {
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*/!*
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alert("Hello");
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}
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}
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new User().sayHi();
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```
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Such features are easy to remember, as they resemble that of literal objects.
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## Class fields
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```warn header="Old browsers may need a polyfill"
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Class fields are a recent addition to the language.
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```
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Previously, our classes only had methods.
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"Class fields" is a syntax that allows to add any properties.
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For instance, let's add `name` property to `class User`:
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```js run
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class User {
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*!*
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name = "John";
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*/!*
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sayHi() {
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alert(`Hello, ${this.name}!`);
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}
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}
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new User().sayHi(); // Hello, John!
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```
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So, we just write "<property name> = <value>" in the declaration, and that's it.
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The important difference of class fields is that they are set on individual objects, not `User.prototype`:
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```js run
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class User {
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*!*
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name = "John";
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*/!*
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}
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let user = new User();
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alert(user.name); // John
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alert(User.prototype.name); // undefined
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```
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We can also assign values using more complex expressions and function calls:
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```js run
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class User {
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*!*
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name = prompt("Name, please?", "John");
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*/!*
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}
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let user = new User();
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alert(user.name); // John
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```
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### Making bound methods with class fields
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As demonstrated in the chapter <info:bind> functions in JavaScript have a dynamic `this`. It depends on the context of the call.
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So if an object method is passed around and called in another context, `this` won't be a reference to its object any more.
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For instance, this code will show `undefined`:
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```js run
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class Button {
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constructor(value) {
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this.value = value;
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}
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click() {
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alert(this.value);
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}
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}
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let button = new Button("hello");
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*!*
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setTimeout(button.click, 1000); // undefined
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*/!*
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```
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The problem is called "losing `this`".
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There are two approaches to fixing it, as discussed in the chapter <info:bind>:
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1. Pass a wrapper-function, such as `setTimeout(() => button.click(), 1000)`.
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2. Bind the method to object, e.g. in the constructor.
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Class fields provide another, quite elegant syntax:
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```js run
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class Button {
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constructor(value) {
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this.value = value;
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}
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*!*
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click = () => {
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alert(this.value);
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}
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*/!*
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}
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let button = new Button("hello");
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setTimeout(button.click, 1000); // hello
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```
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The class field `click = () => {...}` is created on a per-object basis, there's a separate function for each `Button` object, with `this` inside it referencing that object. We can pass `button.click` around anywhere, and the value of `this` will always be correct.
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That's especially useful in browser environment, for event listeners.
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## Summary
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The basic class syntax looks like this:
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```js
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class MyClass {
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prop = value; // property
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constructor(...) { // constructor
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// ...
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}
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method(...) {} // method
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get something(...) {} // getter method
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set something(...) {} // setter method
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[Symbol.iterator]() {} // method with computed name (symbol here)
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// ...
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}
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```
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`MyClass` is technically a function (the one that we provide as `constructor`), while methods, getters and setters are written to `MyClass.prototype`.
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In the next chapters we'll learn more about classes, including inheritance and other features.
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