Big compatibility break in jQuery 1.6

jQuery 1.6 is out, and unusually for jQuery, there's a pretty big API compatibility break with regard to the attr function. Details in the release notes, but basically, attr really is just for attributes now, whereas before it sort of conflated attributes and properties. There's a new prop function for interacting with properties.

Update 2011/05/11: jQuery 1.6.1 changes attr again to make it more backward-compatible for older code, details in the jQuery 1.6.1 blog post.

It's a seemingly-small change, but I've seen a lot of code that will be affected, especially around the "checked" property. If you've been checking whether a checkbox is checked using code along the lines of

// In an event handler, etc.:
if ($(this).attr('checked')) {
    // It's checked
}
// Or if you already have a jQuery object for the checkbox:
if (foo.attr('checked')) {
    // It's checked
}

...you'll need to update your code. Personally, I prefer going straight to the DOM element (that's what the jQuery team recommends as well):

// In an event handler, etc.:
if (this.checked)) {
    // It's checked
}
// Or if you already have a jQuery object for the checkbox:
if (foo[0] && foo[0].checked) {
    // It's checked
}

...and the above has the advantage of working regardless of what version of jQuery you're using. But you could use prop instead if you like:

// In an event handler, etc.:
if ($(this).prop('checked')) {
    // It's checked
}
// Or if you already have a jQuery object for the checkbox:
if (foo.prop('checked')) {
    // It's checked
}

(There's also foo.is(':checked'), but that's awfully round-about.)

The big properties I see people stumbling over are "checked" (hence the above) and "value" (but hopefully you were already using val anyway).