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:

The problem at line 1 character 1: Missing "use strict" statement.

Doing some searching, I realized that some people add "use strict"; into their JavaScript code. Once I added the statement, the error stopped appearing. Unfortunately, Google did not reveal much of the history behind this string statement. Certainly, it must have something to do with how the JavaScript is interpreted by the browser, but I have no idea what the effect would be.

So what is "use strict"; all about, what does it imply, and is it still relevant?

Do any of the current browsers respond to the "use strict"; string or is it for future use?

Answer 1

This feature was introduced in ECMAScript 5. So you put it in your JavaScript files at the top of your file or inside of a function below is the example how to write it:

"use strict";

Once you put it in your code it won’t cause any problems with current browsers.