use strict and this code example

Example 1: javascript use strict

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

Example 2: strict mode

Strict mode makes several changes to normal JavaScript semantics:
-Eliminates some JavaScript silent errors by changing them to throw errors.
-Fixes mistakes that make it difficult for JavaScript engines to perform 
optimizations: strict mode code can sometimes be made to run faster than 
identical code that's not strict mode.
-Prohibits some syntax likely to be defined in future versions of ECMAScript.