Finally, there is an automated CSS Sprite Generator (via)! Just upload a zip of graphics and it will spit out a single image and a block of CSS you can use.

What are CSS Sprites? If you use a ton of CSS backgrounds for things like icons, buttons and other non-repeating graphics, then you can combine them into a single graphic and use background-position with width and height to show a slice of the larger graphic. This can make a site load faster since there are less files to download.

This is generally a pain to do by hand (not only combining the graphics, but calculating and remembering pixel positions), but with a tool like this it should be a cinch!

Are you attaching an onload function to an image but finding it doesn't always get called?

The problem is almost always this: you need to set the onload before you set the src. The reason is obvious when you think about it: if the image is in the cache, the image will be loaded immediately after setting the src, even before the onload is set.

Adding fuel to the fire, Firefox and Safari will let you set an onload immediately after (it seems they don't call the onload until the block of JavaScript finishes). This doesn't happen in Internet Explorer or Opera.

Long story short:

// evil:
var image = new Image();
image.src = 'image.jpg';
image.onload = function() {
    // sometimes called
    alert('image loaded');
};

// good:
var image = new Image();
image.onload = function() {
    // always called
    alert('image loaded');
};
image.src = 'image.jpg';

No matter how many times I come across this bug and find the solution, I end up making the same mistake a few weeks later. I'm hoping that blogging about it will get this stuck in my long-term memory.

No, I don't recommend smoking hash before doing your JavaScript debugging :) But I did figure out a technique which you can use to switch a page in and out of debugging mode.

What happens if you need to debug a site that's getting traffic, and you don't want everyone getting your alert() messages or whatever else you need to debug?

Well why not check location.hash and look for a special keyword:

function debug_mode() {
    return location.hash == '#debug';
}

if (debug_mode()) {
    alert('We are in debug mode!');
}

Now you can just add #debug to the URL (and hit enter) to turn on debug mode, or just drop the #debug to turn it off. You can even have more than one special "mode" and use different hash values to switch between these modes.

Update: As some of the comments mention, the technique below doesn't work anymore. It's best to use object detection to accomplish what you need, or use conditional comments. But if you need to detect IE6, this should work: /MSIE 6/i.test(navigator.userAgent)

Sometimes you just have to sniff for Internet Explorer 6 (and under) in JavaScript. Using conditional comments is a decent solution, but I don't want them scattered all over my code.

With a bit of help from Dean Edwards, I worked out the following:

var IE6 = false /*@cc_on || @_jscript_version < 5.7 @*/;

With just a single conditional comment, you can have a JavaScript variable that you can reuse throughout your code.

(IE6 will be true in Internet Explorer 6 and under, but does anyone really care about IE5 anymore? Thought not.)

You could use this technique to sniff for other things:

// exactly Internet Explorer 7
var IE7 = false /*@cc_on || @_jscript_version == 5.7 @*/;

// at least Internet Explorer 7
var gteIE7 = false /*@cc_on || @_jscript_version >= 5.7 @*/;


// any Internet Explorer (thanks to Dean)
var isMSIE = /*@cc_on!@*/false;

Note: browser sniffing is evil but sometimes painfully necessary.

Great news: jQuery 1.2 is out, and the heavily-awaited jQuery UI (an "official" replacement to Interface) will be out on Sunday.

Be sure to have a read of the jQuery 1.2 release notes, but here are a few highlights that I can't wait to use:

• Partial .load()

  If you want to load a whole web page with Ajax, but only want to cut out a portion of the page and stick it in to an element on your page, you can now do this:

  $('#stats').load('same_page.php #stats');
  

  This will make it really easy to refresh an element on the page without needing to change the server-side logic.

• New Traversing Features

  Traversing and selecting remain the most powerful features in jQuery, and it'll be great to be able to use .andSelf() to include the original set when traversing, and .prevAll()/.nextAll() to select all the siblings to one side or the other.

• New Effect Features

  There are a ton of new Effects features, but my favourite is probably .stop(), allowing us to finally stop animations. Together with the new :animated selector, it's trivial to stop all ongoing animations at once:

  $(':animated').stop();
  

  Some of the other Effects features have a lot of potential, like being able to use percents or animating by a relative amount (eg. add 100px).

  Extremely powerful is the new ability to write extensible animations. I assume they did this to ease in the development of jQuery UI, but this allows all of us to harness the power of jQuery animations to do anything we want! Okay I'm starting to drool...

Oh, one more thing. If you're used to creating elements in jQuery like this:

var link = $('<a>');

then you need to stop using invalid HTML and either make it XHTML compliant or add a closing tag, otherwise it will break in Internet Explorer. This is true since jQuery version 1.1.4, and I've had to make a lot of changes in my code as a result.

Now any of the following is okay:

// okay, XHTML-style
var xhtmllink = $('<a/>');

// also okay, valid HTML style
var htmllink = $('<a></a>');