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

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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 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?

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1 Answer

493

If people are worried about using use strict it might be worth checking out this article:

ECMAScript 5 'Strict mode' support in browsers. What does this mean?
NovoGeek.com - Krishna's weblog

It talks about browser support, but more importantly how to deal with it safely:

function isStrictMode(){
    return !this;
} 
/*
   returns false, since 'this' refers to global object and 
   '!this' becomes false
*/

function isStrictMode(){   
    "use strict";
    return !this;
} 
/* 
   returns true, since in strict mode the keyword 'this'
   does not refer to global object, unlike traditional JS. 
   So here, 'this' is 'undefined' and '!this' becomes true.
*/

answered 2012-07-15T23:25:53.957

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

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