How to define classes in Javascript

Strictly speaking, there’s no class in Javascript. To create objects of class, there are a few options:

1. Directly define an object as well as its vars/methods.  You cant create its instances in this case.

var foo = {
 x: [1,2,3],
 y: function(name) {
   return 'hello '+name;
 }
}

console.log(foo.x); //OK
console.log(foo.y('john')); //OK

var foo2 = new foo();//wrong: object is not a function

operator new has to be used on a function.

2. Define a function as well as its vars/methods. Create instance through new. For the vars there are two options:
2.1 vars are public members through “this.”.

var foo = function(name) {
  this.x = [1,2,3];
  this.y = function() {
    return 'hello '+name;
  }
}

console.log(foo.x);//undefined
console.log(foo.y());//no such method

var foo2 = new foo('john');

console.log(foo2.x);
console.log(foo2.y());

2.2 vars are private by not using this

var foo = function(name) {
  var x = [1,2,3];
  this.y = function() {
    return 'hello '+name;
  }
  this.getX = function() {
    return x;
  }
}

//console.log(foo.x);//undefined
//console.log(foo.y());//no such method

var foo2 = new foo('john');

console.log(foo2.x);//undefined. x becomes private
console.log(foo2.y());
console.log(foo2.getX());

3. Define a function. Vars/methods are added through prototype.. Create instance through new. This way instances all share the same method space improve runtime efficiency.