why we use strict in javascript 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: use strict javascript

// File: myscript.js

'use strict';
var a = 2;
....

Example 3: use strict javascript

function doSomething() {
    'use strict';
    ...
}