What does “use strict” do in JavaScript, and what is the reasoning behind it?

Question

Recently, I ran some of my JavaScript code through Crockford\'s JSLint, and it gave the following error:

Problem at line 1 character 1: Missing \"use

Answer 1

JavaScript “strict” mode was introduced in ECMAScript 5.

(function() {
  "use strict";
  your code...
})();

Writing "use strict"; at the very top of your JS file turns on strict syntax checking. It does the following tasks for us:

1. shows an error if you try to assign to an undeclared variable

2. stops you from overwriting key JS system libraries

3. forbids some unsafe or error-prone language features

use strict also works inside of individual functions. It is always a better practice to include use strict in your code.

Browser compatibility issue: The "use" directives are meant to be backwards-compatible. Browsers that do not support them will just see a string literal that isn't referenced further. So, they will pass over it and move on.