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

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

Also in JavaScript: