JavaScript function closure

In JavaScript you see the function reside in another function. Well, the inner function has access to variables in the outer function.

A closure is a way to access and manipulate external variables from within a function.

In other words, the function defined in the closure knows the environment in which it was created.

Example:

function f1(x) {
  var a = 3;
  function f2(y) {
    alert(y + (a)); 
  }
  f2(0);
}
f1(2);