The JavaScript language

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).

In practice, we often need to create many objects of the same kind, like users, or goods or whatever.

As we already know from the chapter info:constructor-new, new function can help with that.

But in the modern JavaScript, there's a more advanced "class" construct, that introduces great new features which are useful for object-oriented programming.

The "class" syntax

The basic syntax is:

class MyClass {
  // class methods
  constructor() { ... }
  method1() { ... }
  method2() { ... }
  method3() { ... }
  ...
}

Then use new MyClass() to create a new object with all the listed methods.

The constructor() method is called automatically by new, so we can initialize the object there.

For example:

class User {

  constructor(name) {
    this.name = name;
  }

  sayHi() {
    alert(this.name);
  }

}

// Usage:
let user = new User("John");
user.sayHi();

When new User("John") is called:

A new object is created.
The constructor runs with the given argument and assigns it to this.name.

...Then we can call object methods, such as user.sayHi().

A common pitfall for novice developers is to put a comma between class methods, which would result in a syntax error.

The notation here is not to be confused with object literals. Within the class, no commas are required.

What is a class?

So, what exactly is a class? That's not an entirely new language-level entity, as one might think.

Let's unveil any magic and see what a class really is. That'll help in understanding many complex aspects.

In JavaScript, a class is a kind of function.

Here, take a look:

class User {
  constructor(name) { this.name = name; }
  sayHi() { alert(this.name); }
}

// proof: User is a function
*!*
alert(typeof User); // function
*/!*

What class User {...} construct really does is:

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).
Stores class methods, such as sayHi, in User.prototype.

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.

We can illustrate the result of class User declaration as:

Here's the code to introspect it:

class User {
  constructor(name) { this.name = name; }
  sayHi() { alert(this.name); }
}

// class is a function
alert(typeof User); // function

// ...or, more precisely, the constructor method
alert(User === User.prototype.constructor); // true

// The methods are in User.prototype, e.g:
alert(User.prototype.sayHi); // the code of the sayHi method

// there are exactly two methods in the prototype
alert(Object.getOwnPropertyNames(User.prototype)); // constructor, sayHi

Not just a syntactic sugar

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:

// rewriting class User in pure functions

// 1. Create constructor function
function User(name) {
  this.name = name;
}
// a function prototype has "constructor" property by default,
// so we don't need to create it

// 2. Add the method to prototype
User.prototype.sayHi = function() {
  alert(this.name);
};

// Usage:
let user = new User("John");
user.sayHi();

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.

Still, there are important differences.

First, a function created by class is labelled by a special internal property [[IsClassConstructor]]: true. So it's not entirely the same as creating it manually.

The language checks for that property in a variety of places. For example, unlike a regular function, it must be called with new:
```
class User {
  constructor() {}
}

alert(typeof User); // function
User(); // Error: Class constructor User cannot be invoked without 'new'
```
Also, a string representation of a class constructor in most JavaScript engines starts with the "class..."
```
class User {
  constructor() {}
}

alert(User); // class User { ... }
```
There are other differences, we'll see them soon.
Class methods are non-enumerable.
A class definition sets enumerable flag to false for all methods in the "prototype".

That's good, because if we for..in over an object, we usually don't want its class methods.
Classes always use strict.
All code inside the class construct is automatically in strict mode.

Besides, class syntax brings many other features that we'll explore later.

Class Expression

Just like functions, classes can be defined inside another expression, passed around, returned, assigned, etc.

Here's an example of a class expression:

let User = class {
  sayHi() {
    alert("Hello");
  }
};

Similar to Named Function Expressions, class expressions may have a name.

If a class expression has a name, it's visible inside the class only:

// "Named Class Expression"
// (no such term in the spec, but that's similar to Named Function Expression)
let User = class *!*MyClass*/!* {
  sayHi() {
    alert(MyClass); // MyClass name is visible only inside the class
  }
};

new User().sayHi(); // works, shows MyClass definition

alert(MyClass); // error, MyClass name isn't visible outside of the class

We can even make classes dynamically "on-demand", like this:

function makeClass(phrase) {
  // declare a class and return it
  return class {
    sayHi() {
      alert(phrase);
    }
  };
}

// Create a new class
let User = makeClass("Hello");

new User().sayHi(); // Hello

Getters/setters

Just like literal objects, classes may include getters/setters, computed properties etc.

Here's an example for user.name implemented using get/set:

class User {

  constructor(name) {
    // invokes the setter
    this.name = name;
  }

*!*
  get name() {
*/!*
    return this._name;
  }

*!*
  set name(value) {
*/!*
    if (value.length < 4) {
      alert("Name is too short.");
      return;
    }
    this._name = value;
  }

}

let user = new User("John");
alert(user.name); // John

user = new User(""); // Name is too short.

Technically, such class declaration works by creating getters and setters in User.prototype.

Computed names [...]

Here's an example with a computed method name using brackets [...]:

class User {

*!*
  ['say' + 'Hi']() {
*/!*
    alert("Hello");
  }

}

new User().sayHi();

Such features are easy to remember, as they resemble that of literal objects.

Class fields

Class fields are a recent addition to the language.

Previously, our classes only had methods.

"Class fields" is a syntax that allows to add any properties.

For instance, let's add name property to class User:

class User {
*!*
  name = "John";
*/!*

  sayHi() {
    alert(`Hello, ${this.name}!`);
  }
}

new User().sayHi(); // Hello, John!

So, we just write " = " in the declaration, and that's it.

The important difference of class fields is that they are set on individual objects, not User.prototype:

class User {
*!*
  name = "John";
*/!*
}

let user = new User();
alert(user.name); // John
alert(User.prototype.name); // undefined

We can also assign values using more complex expressions and function calls:

class User {
*!*
  name = prompt("Name, please?", "John");
*/!*
}

let user = new User();
alert(user.name); // John

Making bound methods with class fields

As demonstrated in the chapter info:bind functions in JavaScript have a dynamic this. It depends on the context of the call.

So if an object method is passed around and called in another context, this won't be a reference to its object any more.

For instance, this code will show undefined:

class Button {
  constructor(value) {
    this.value = value;
  }

  click() {
    alert(this.value);
  }
}

let button = new Button("hello");

*!*
setTimeout(button.click, 1000); // undefined
*/!*

The problem is called "losing this".

There are two approaches to fixing it, as discussed in the chapter info:bind:

Pass a wrapper-function, such as setTimeout(() => button.click(), 1000).
Bind the method to object, e.g. in the constructor.

Class fields provide another, quite elegant syntax:

class Button {
  constructor(value) {
    this.value = value;
  }
*!*
  click = () => {
    alert(this.value);
  }
*/!*
}

let button = new Button("hello");

setTimeout(button.click, 1000); // hello

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.

That's especially useful in browser environment, for event listeners.

Summary

The basic class syntax looks like this:

class MyClass {
  prop = value; // property

  constructor(...) { // constructor
    // ...
  }

  method(...) {} // method

  get something(...) {} // getter method
  set something(...) {} // setter method

  [Symbol.iterator]() {} // method with computed name (symbol here)
  // ...
}

MyClass is technically a function (the one that we provide as constructor), while methods, getters and setters are written to MyClass.prototype.

In the next chapters we'll learn more about classes, including inheritance and other features.