JavaScript Closure

What is Closure

A Closure is combination of function and the lexical environment (surrounding state). Using Closure, a nested function ( function inside another function) can access local variables of parent functions.

Closure are created every time a function is created inside another function. The child function can access variables of parent function. But parent function cannot access variables of child function because of scope.

Closure example

function outer(){
    var x=3;
    function inner(){
        var y=4;
        return x+y;
    }
    return inner;
}

console.log( outer()() )

In the above example, the function inner can access variables of outer.

Lexical Environment

Lexical Environment is the scope of function. A variable inside function is accessible anywhere in the function. When a nested function is created, it can also access variable of parent function.

Lexical Environment of function

function outer()
{
    var x=3;
    function inner(){
        var y=5;
        return x+y;
    }
}

Lexical Environment of inner function

function outer()
{
var x=3;
function inner(){
    var y=5;
    return x+y;
}
}

Closure Scope

Every Closure has three scopes.

1. global
2. parent function or block
3. local scope

var x=3; // global

function outer(){
    var y=4;    // function

    function inner(){
        var z=5;    // local
        return x+y+z;
    }

    return inner;
}
console.log( outer()() )