Meaning of keyword "new" in javascript

Question

I've read the topic about "new" keyword in javascript (What is the 'new' keyword in JavaScript?). But, i'm still in the fog; let's talk about this example :

var foo = function() {
    return {
        setA: function(a) {
            this.a = a;
        },
        readA: function() {
            console.log(this.a);
        }
    };
};

And now what's about these two pieces of code :

One:

var bob1 = foo();
bob1.setA(10);
bob1.readA();

Two:

var bob2 = new foo();
bob2.setA(10);
bob2.readA();

I cannot see any differences at my level. So what is the gain to use the keyword "new" ?

Answer 1

If your function returns object directly, then you do not need an new operator. The new keys does more than that.

Lets say

function Animal(kind, name) {
   this.kind = kind;
   this.name = name;
}

Animal.prototype.walk = function() {
    console.log('Walking');
}

Then you are doing

 var animal = new Animal();

Javascript engine will do following things

  var o = Object.create(Animal.prototype)
  Animal.apply(o, arguments);
  return o;

Object.create will do the prototype inheritance of the prototype object of Animal function. So animal object will have its own properties and its inherited property.