what does use strict mean 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: What is strict mode in Java Script ?

Strict mode changes previously accepted "bad syntax" into real errors.

As an example, in normal JavaScript, mistyping a variable name creates a 
new global variable. In strict mode, this will throw an error, making it 
impossible to accidentally create a global variable.

In normal JavaScript, a developer will not receive any error feedback assigning
values to non-writable properties.

In strict mode, any assignment to a non-writable property, a getter-only 
property, a non-existing property, a non-existing variable, or a non-existing 
object, will throw an error.

Strict mode makes it easier to write "secure" JavaScript.

Example 3: use strict javascript

// File: myscript.js

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

Example 4: use strict javascript

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

Example 5: use strict javascript

// Non-strict code...

(function(){
  "use strict";

  // Define your library strictly...
})();

// Non-strict code...