JavaScript "classes" in ES5

This document is relevant for ES5, in ES6 there is a class keyword to define classes.

JavaScript doesn’t have classes

In ES5 classes can be only simulated using functions. The approach to achieving this is by defining a JavaScript function where we then create an object using the new keyword. This can be used to define new properties and methods for the object as follows:

// Album "class" example 1
function Album(name, year) {
  this.name = name; 
  this.year = year;
  this.getInfo = function () {
    return this.name + " " + this.year;
  };
}

var album = new Album("Hello", 2016);
console.log( album.getInfo() );

or one another way to do the same, but just a touch different:

// Album "class" example 2
function Album( name , year) {
  this.name = name; 
  this.year = year; 
}

Album.prototype.getInfo = function(){
  return this.name + " " + this.year;
}

var album = new Album("Hello", 2016);
console.log( album.getInfo() );

The different approach is: we use Album.prototype or Object.prototype to define the new function inside of the object. We can refer to the prototype as to the object constructor function. And we can use prototype again and again to add new functions to the object if we wish. In the first approach we defined function getInfo inside of the function Album representing the object.

However, this is not all, the equivalent to the previous would be:

// Album "class" example 3
var album = {
  name : "Hello", 
  year : 2016, 
  getInfo : function () {
    return this.name + " " + this.year;
  };
}

anytime we could say:

album.name="Good bye";
album.getInfo();

We can say it different:

// Album "class" example 4
var album = new function(){
  this.name = "Hello"; 
  this.year = 2016; 
  this.getInfo = function () {
    return this.name + " " + this.year;
  }
}

console.log( album.getInfo() );</pre>

These are small differences to create objects in JavaScript from so called classes.