Closures

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other words, a closure gives you access to an outer function's scope from an inner function. In JavaScript, closures are created every time a function is created, at function creation time.

Closure means functions binded together with the lexical environment.

function x() {
  var a = 7;
  function y() {
    console.log(a);
  }
  y();
}
x();

function x() {
  var a = 7;
  function y() {
    console.log(a);
  }
  return y;
}

var z = x();
z(); // 7

when the function returns its function, it still maintain its lexically scope and closures with it.

function z() {
    var b = 900;
    function x() {
        var a = 7;
        function y() {
            console.log(a,b);
        }
        y();
    }
    x();
}
z();

uses of closures

• Module Design Pattern
• Currying
• functions like once
• memoize
• maintaining state in async world
• setTimeouts
• Iterators
• and many more.