use strict 27 in javascript

1//It's a new feature of ECMAScript 5. 
2//With strict mode, you can not, for example, use undeclared variables.
3
4Insert a 'use strict'; statement on top of your script:
5/***START ANY JS FILE***/
6'use strict';
7var a = 2;
8....
9/***END***/ 
10Or, insert a 'use strict'; statement on top of your function body:
11
12function doSomething() {
13    'use strict';
14    ...
15}    

1// Whole-Script Strict Mode Syntax
2'use strict';
3var v = "Hi! I'm a strict mode script!";

1// File: myscript.js
2
3'use strict';
4var a = 2;
5....

1function doSomething() {
2    'use strict';
3    ...
4}

1// Non-strict code...
2
3(function(){
4  "use strict";
5
6  // Define your library strictly...
7})();
8
9// Non-strict code...