Closure in Javascript

 An inner function has always access to the outer function's  variable and parameters even after the outer function has returned . Lets understood with a simple example,

function retirement(retirementAge){

    var a="years left until retirement";

    return function(yearOfBirth){//inner function

        var age = 2016-yearOfBirth;

        console.log((retirementAge-age)+a);

    }

}

var retirementUS=retirement(66);

retirementUS(1990);

we already know about lexical scoping , an inner function has access to the variable of the outer function . When a function gets called it is invoked into execution context .Then VO(variable  objects) stores the variable, and the scope chain keep track of the variables to use by the function . When a function returned it will popped up from execution context . So the execution context gets cleared .When the inner function invoked then it is gets called into execution context ,then how inner function can access to the outer functions variable . It is possible only for closures. 

The scope chain always stays intact .We no need to define closure manually . It is built into javascript. Javascript is always do this for us automatically .