Strict Mode

Strict Mode was introduced in ECMAScript 5 that allows you to place a program, or a function, in a “strict” operating context. This strict context prevents certain actions from being taken and throws more exceptions.

Strict mode makes several changes to normal JavaScript semantics.

• First, strict mode eliminates some JavaScript silent errors by changing them to throw errors.
• Second, strict mode 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.
• Third, strict mode prohibits some syntax likely to be defined in future versions of ECMAScript.

Strict mode code and non-strict mode code can coexist in a same script.

// Non-strict code...

(function(){
  "use strict";

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

// Non-strict code... 

Invoking strict mode

Strict mode applies to entire scripts or to individual functions.

Strict mode for scripts

// Whole-script strict mode syntax

"use strict";
var v = "Hi!  I'm a strict mode script!";

Strict mode for functions

function strict(){
  // Function-level strict mode syntax

  'use strict';
  function nested() { return "And so am I!"; }
  return "Hi!  I'm a strict mode function!  " + nested();
}

function notStrict() { return "I'm not strict."; }

Basically it helps you make fewer errors, by detecting things that could lead to breakage which are not detected normally (non-strict mode).

For more information check out this MDN page.