Events

In JavaScript, we often tend to use events all over the place. In MooTools, the custom ‘domready’ event is particularly prevalent. However, events suffer from a few drawbacks:

You can’t attach events to non-existent objects.

Pretty self-explanatory, really. What this means in practice, though, is that you can’t easily let object A know when object B exists.

If an object starts to listen for an event after it’s already fired, it’ll never hear it.

Because content on the web isn’t always delivered in perfect order (especially when you’re loading scripts synchronously), it’s possible for an object to try to listen for an event after it’s already been fired. Obviously, this means that your listener object will never run the code that’s dependent on that event, which could be Bad Thing™.

It’s not particularly easy to know which object is listening for which events.

There are ways around this, but you can’t just dir() the listeners in Firebug.

A messaging system

For all of these reasons (and probably a few more that I’ve forgotten about), a messaging system can be an invaluable addition to your arsenal when writing JavaScript. How does a messaging system work? Well, interested objects ‘register’ themselves as listeners for particular message ‘handles’, and other objects can send messages using those ‘handles’. Below is a very simple MooTools messaging system that I knocked up, which has a few cool features, including:

• When you register() a listener, you can have its callback immediately fire if that message has ever been sent before.
• You can very easily see which callbacks are associated with which messages by simply dir()-ing the ‘listeners’ member.
• You can unregister() a listener at any time (provided you’ve got a reference to the function and the handle).
• Handles can be any valid JavaScript type — Strings, Numbers, even Objects.

Feel free to use and extend this system — as I mentioned, this is a very simple system. If you do extend it, let me know in the comments!

Let me know what you think about this system in the comments.

barryvan.base.Messaging = new Class({
	listeners: $H(),
	sentMessages: [],

	initialize: function() {

	},

	/**
	 * Register a listener for a particular handle.
	 * handle [String]: The message 'handle' to listen for.
	 * callback [Function]: The function to be called when the handle is sent a message. The contents of the messages will be included in the function call.
	 * dontCheck [Boolean]: If falsey and the handle has had a message sent to it, immediately call the callback function (without contents), and continue to add the listener as normal.
	 */
	register: function(handle, callback, dontCheck) {
		if ($type(callback) !== 'function') return;

		if (!dontCheck && this.sentMessages[handle]) {
			callback();
		}

		if (!this.listeners.has(handle)) this.listeners[handle] = [];
		this.listeners[handle].push(callback);
	},

	/**
	 * Unregister a listener for a particular handle.
	 * handle [String]: The message 'handle' to cease listening for.
	 * callback [Function]: The function which was earlier assigned as the callback for the messages.
	 */
	unregister: function(handle, callback) {
		if (this.listeners.has(handle)) {
			this.listeners[handle].erase(callback);
		}
	},

	/**
	 * Send a message to the given handle with the given contents -- send the contents to all the registered listeners for that handle.
	 * handle [String]: The message 'handle' to transmit to.
	 * contents [Mixed]: The contents to be sent to the listeners.
	 */
	send: function(handle, contents) {
		this.sentMessages.include(handle);
		if (this.listeners.has(handle)) {
			this.listeners[handle].each(function(callback) {
				callback(contents);
			});
		}
	}
});

Often, you will need to sort an array of objects in Javascript. The inbuilt sort() function can’t do this, but here is a Quicksort implementation for doing just this.

Parameters

array The array to be sorted. (See below for an implementation on the Array Native itself, which makes this variable unnecessary).

key The key to sort by. Make sure every object in your array has this key.

Examples

var objs = [
	{fruit:"cherry"},
	{fruit:"apple"},
	{fruit:"banana"}
];

console.log(objs.sortObjects('fruit'));
// Logs [{fruit:"apple"},{fruit:"banana"},{fruit:"cherry"}] to the console

The code

sortObjects: function(array, key) {
	for (var i = 0; i < array.length; i++) {
		var currVal = array[i][key];
		var currElem = array[i];
		var j = i - 1;
		while ((j >= 0) && (array[j][key] > currVal)) {
			array[j + 1] = array[j];
			j--;
		}
		array[j + 1] = currElem;
	}
}

Implemented on the Array native:

Array.implement({
	sortObjects: function(key) {
		for (var i = 0; i < this.length; i++) {
			var currVal = this[i][key];
			var currElem = this[i];
			var j = i - 1;
			while ((j >= 0) && (this[j][key] > currVal)) {
				this[j + 1] = this[j];
				j--;
			}
			this[j + 1] = currElem;
		}
	}
});

Putting ellipses into strings that are too long has been around for a very long time. Unfortunately, Javascript doesn’t offer a native method of doing this, so below is a little function that’ll do it for you.

This function returns a copy of the string it’s called on, ellipsised, and takes three parameters:

toLength (required) The number of characters to truncate the string to (or 0 to disable ellipsising)

where (optional, default ‘end’) A string representing where the ellipsis should be placed — ‘front’, ‘middle’, or ‘end’

ellipsis (option, default ‘\u2026′) A string to be used as the ellipsis.

Examples

// Our clichéd string
var s = 'Jackdaws love my great big sphinx of quartz';

alert(s.ellipsise(10));
// Alerts "Jackdaws l…"

alert(s.ellipsise(10, 'front'));
// Alerts "… of quartz"

alert(s.ellipsise(10, 'middle', 'pony'));
// Alerts "Jackdponyuartz"

The code

String.implement({
	ellipsise: function(toLength, where, ellipsis) { // Where is one of ['front','middle','end'] -- default is 'end'
		if (toLength < 1) return this;
		ellipsis = ellipsis || '\u2026';
		if (this.length < toLength) return this;
		switch (where) {
			case 'front':
				return ellipsis + this.substr(this.length - toLength);
				break;
			case 'middle':
				return this.substr(0, toLength / 2) + ellipsis + this.substr(this.length - toLength / 2)
				break;
			case 'end':
			default:
				return this.substr(0, toLength) + ellipsis;
				break;
		}
	}
});

If you’re not using MooTools, you can use this variant instead:

String.prototype.ellipsise = function(toLength, where, ellipsis) { // Where is one of ['front','middle','end'] -- default is 'end'
	if (toLength < 1) return this;
	ellipsis = ellipsis || '\u2026';
	if (this.length < toLength) return this;
	switch (where) {
		case 'front':
			return ellipsis + this.substr(this.length - toLength);
			break;
		case 'middle':
			return this.substr(0, toLength / 2) + ellipsis + this.substr(this.length - toLength / 2)
			break;
		case 'end':
		default:
			return this.substr(0, toLength) + ellipsis;
			break;
	}
}

At work, I’ve recently been putting together a nice little calendar-like utility using Javascript. Basically, it has to generate a table consisting of cells which may span multiple rows. Surely the solution is simple enough: just set the rowspan on each td as we create it. Unfortunately, that doesn’t work, at least not in Firefox.

It appears that in Firefox, if you create a td and set its rowspan to some value when there are no rows for it to expand into, the attribute will be completely ignored, even if you add rows afterwards! Needless to say, this is very annoying. The solution? Build your table backwards.

The code I have now is something like this (note that I’m developing using the Mootools framework):

var tbl = new Element('table');
var trs = new Array();

for (var i = 0; i < 4; i++) {
  var tr = new Element('tr');
  tr.grab(new Element('td', {
    'html': 'Cell ' + i
  }));
  if (i % 2 == 0) {
    tr.grab(new Element('td', {
      'rowspan': 2,
      'html': 'Span ' + (i / 2)
    }));
  }
  trs.push(tr);
}

for (var i = trs.length - 1; i >= 0; i--) {
  tbl.grab(trs[i], 'top');
}

What does this code do? Well basically, we’re creating a table with ten rows and two columns; the cells in the right-hand column each occupy two rows. The result will be something like this: