What would be a good mini-project to get intimate with JavaScript, as an advanced 'introduction' to the language? I want to actually code an application in JS, not hook up bits of it to enhance a web application.
I would implement a simple game like sokoban first.
The second application would be an AJAX-based multiuser chat application, first fetching other people's responses by polling, later with AJAX push.
try making an advanced AJAX application like for example try to recreate the google calander.
The Holy Grail - a WYSIWYG editor. They wouldn't need to complete it, but just seeing their plan of attack would be interesting. Plays right into patterns and OO.
Interesting question.
Really you could do any sort of application. In order to make sure you're using the latest and greatest stuff, I'd try making a simple CRUD style application using DHTML and AJAX. Perhaps a contacts list or calendar. If you're feeling really energetic, you could write the back-end in JavaScript as well.
Unless you want to get really friendly with the DOM and browser compatibility, I'd learn Javascript through the mask of one of the nice frameworks like Jquery or Prototype.
A lot of stuff you could learn by doing an RSS reader on a page. Google shows what can be done. The whole lection concentrates on javascript, network access, security restrictions and medium data mangeling. If you have the ability to do any sort of backend programming than AJAX is really neat to do. You can get a lot of good effects with less efforts. A good thing to build on up.
Although not a project, watch the Douglas Crockford videos at YUI theater.
The biggest web based Javascript projects are going to deal with the DOM. Do some nifty stuff with JQuery. Make a table with rows that highlight when you hover. Make them update themselves through AJAX and JSON when you click on them.
If you're really looking for something magical and usefull write a scrollable table with fixed headers and footers for IE8.
If you want to stay away from the WEB use the JDK 1.6 and run Javascript code in that. You could do TONS with that.
I would argue that if you're really an advanced programmer then the exercises above would not really give you any insight into the language as they are just variations on things you probably have already done. Javascript's strongest suit is it's LISP style ability to grow. Write something AI(ish) that creates new functions. Most people don't utilize the language in this way, but, its ability to augment its own classes on the fly is, I would argue, it's most unusual and most powerful feature.
Whenever I'm trying to get familiar with a language, I will work on Project Euler problems with it.
I suggest you create a Google Gadget. You can create one for free and perhaps make something useful out of it. If you don't have a Google account, sign up for one. Then add the Google Gadget Editor to begin writing your code.
With the gadget, you'll be able to mess with JavaScript, JSON, CSS, etc. Furthermore, you'll be able to store the file on Google's server so you can work on it from any computer.
I created a simple RSS reader and wrote JavaScript to get the feed (using Google's API) then dealing with that JavaScript object because it came back as JSON. I then developed some JavaScript to hide/show div tags.
It was a good starter project for me to learn JavaScript.
How about a firefox plugin to monitor StackOverflow? It could use RSS to monitor feeds and let you know when new questions are asked with your tags.
It could also be grown as your js skills progress.
Write yet another javascript framework, but focused specially in something, ie game programming.
Get JavaScript the Good Parts by Douglas Crockford. Also check out his web site: http://www.crockford.com
Key reason: just because JavaScript looks like C/C++/Java/C# doesn't mean it actually is like them. Things are significantly different. I suggest reading his book to get a grasp of those differences.
Otherwise, I would look at the JQuery web site. JavaScript is cool and all, but a good framework will save you from a lot of the pitfalls and make you much more productive faster.