"use strict" and "this" in ES5

"this" is a special variable in JavaScript. If a Constructor function is called without "new", the "this" variable inside the constructor will reference the Global object. We can use "use strict" in ES5 to prevent this happening.

(function () {
  "use strict";
  console.log(this === window); //false
 })();

You can also use the "use strict" in global, but this will affect all module

 "use strict";
 console.log(this === window); //true
 (function () {
  console.log(this === window); //false
 })();