Unusual if (a in b) JavaScript syntax?

I have just stumbled upon the following webpage and am somewhat intrigued by the use of the in keyword.

http://diveintohtml5.ep.io/examples/input-autofocus-with-fallback.html

• Is this valid JavaScript?
• Do all browsers accept this?
• How does it actually work?

They are using this syntax for fallback when web browser doesn't support the autofocu开发者_JAVA技巧s attribute. So this would lead me to believe that this syntax is valid.

The in operator checks if a property is defined on an object. So, this is valid Javascript and is accepted in almost all browsers.
In this case, the code is checking if "autofocus" is a property of a new element. If it is, then most likely the browser supports autofocus and will not need .focus() (or someone may be extending prototypes).

You can think of it like this:

"localStorage" in window; // true
!!window["localStorage"]; // true
window["localStorage"] !== undefined; // true

These statements are basically the same.

"autofocus" is an arbitrary boolean attibute that the script looks for. It's really no different than <input id="q" class="autofocus">, except in this case, the script would need to look for the class name (which a lot of validation scripts usually use) versus the attribute.

The browser doesn't "support" it, the script makes it work.