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

"Use Strict"; is an insurance that programmer will not use the loose or the bad properties of JavaScript. It is a guide, just like a ruler will help you make straight lines. "Use Strict" will help you do "Straight coding".

Those that prefer not to use rulers to do their lines straight usually end up in those pages asking for others to debug their code.

Believe me. The overhead is negligible compared to poorly designed code. Doug Crockford, who has been a senior JavaScript developer for several years, has a very interesting post here. Personally, I like to return to his site all the time to make sure I don't forget my good practice.

Modern JavaScript practice should always evoke the "Use Strict"; pragma. The only reason that the ECMA Group has made the "Strict" mode optional is to permit less experienced coders access to JavaScript and give then time to adapt to the new and safer coding practices.