What is closure in javascript ?

Closure is nothing but function with preserve data.

//Example 1.

var addTo = function(outer){
     var add=function(inner){
            return outer + inner;
     }
     return add;
}

//console.log(addTo(10)(10));//20

var addOne=addTo(1);
var addTwo=addTo(2);
console.log(addOne(1));//2
console.log(addTwo(2)); //4