I got tripped up today by something that took me a few minutes to figure out. I wrote this:

if (a == b == 0) {
    // only execute if both a and b are zero
}

But this was wrong. In fact, you can write this:

alert(3 == 4 == 0); // alerts "true"

Why is that? Because of the order things are evaluated. I made the mistake of thinking == has the same result as doing =:

var x, y;

x = y = 10;

alert(x); // 10
alert(y); // 10

But when you use == like that, it actually compares the firsts two values, then compares the result (true or false) against the 3rd value. It's the same as writing:

alert(3 == 4 == 0); // true
alert((3 == 4) == 0); // true

because 3 == 4 is false, and false == 0 is true!

If you use Ajax with jQuery, you may have noticed that you don't get any error messages when things go wrong. Even if you have major bugs in your callback functions, jQuery just silently fails, sweeping any errors under the rug, and leaving you clueless as to what just happened.

For example, you can do this and you won't get any errors:

$.get('page.html', function(){
    this_function_does_not_exist();
});

There are two ways to fix this. You can use $.ajax with the error parameter, passing an error handling function for that particular Ajax call. Or, you can define a global Ajax error handler. If you do a lot of Ajax, and you use Firebug, the latter is a great option. Try this:

$(document).ajaxError(function(){
    if (window.console && window.console.error) {
        console.error(arguments);
    }
});

After running this code, you'll start getting error messages in your Firebug console (if anything breaks with your Ajax calls or callbacks). The error messages aren't the greatest, but at least you don't have to stay in the dark any longer.

Peter-Paul Koch has put together yet another masterful overview, this time covering Drag and drop, something that has been on my to-do list for over a year. His script even works with just the keyboard! Not only does he offer a great Drag-and-drop script, he explains how it was written so you can better understand how Drag-and-drop works in JavaScript (and take it apart or put it together yourself).

Good news for all Windows-based web developers: Safari for Windows is now available (via).

Hopefully this means that we can make our web sites even more cross-browser (and hopefully doesn't just add another browser with its own quirky differences).

Who’s Got Style? - Nicholas C. Zakas writes a good analysis of dynamically creating and attaching <style> elements to a page using JavaScript. He ends up with a function that works in Internet Explorer, Firefox, Opera and Safari.