What's the difference between window.location and document.location in JavaScript?

According to the W3C, they are the same. In reality, for cross browser safety, you should use window.location rather than document.location.

See: http://www.w3.org/TR/html/browsers.html#dom-location

The canonical way to get the current location object is window.location (see this MSDN page from 1996 and the W3C draft from 2006).

Compare this to document.location, which originally only returned the current URL as a string (see this page on MSDN). Probably to avoid confusion, document.location was replaced with document.URL (see here on MSDN), which is also part of DOM Level 1.

As far as I know, all modern browsers map document.location to window.location, but I still prefer window.location as that's what I've used since I wrote my first DHTML.

window.location is read/write on all compliant browsers.

document.location is read-only in Internet Explorer (at least), but read/write in Gecko-based browsers (Firefox, SeaMonkey).