Using private members and methods in javascript

It's not being displayed because this.age is undefined. You want age.

Because of the scope of variables in js

A variable is visible inside a function not outside

var a = "foo" function fooBar () { var b = "bar" console.log(a) // foo console.log(b) // bar } console.log(a) // foo console.log(b) // undefined 

You want this:

var test = function(id, company){ //public members this.id = id; this.company = company; //private member var age = 24; //private method this.getAge = function(){ return age; }; //public method this.displayAge = function(){ console.log(this.getAge()); } } //invoking var t = new test(1, 'XYZ Corp'); t.displayAge(); //undefined 

Note that both "getAge" and "displayAge" need to be attached to this but your private variable "age" should not be.