See the assertiveness section in the Card and Foldouts.
(a) The assert() function is the core of unit testing, a great way to build in reliability. Each assert function is a little test. It's silent when code works, but reports when it doesn't. A very simple example:
Only if the expression results in is false does anything dramatic happen, maybe a popup warning.assert(2+2 == 4);
Unit testing means religiously writing tests before coding a solution, then rerunning the tests often to find out early if something breaks. For more about the power of unit testing, and its contexts, see the Extreme Programming web site, www.xprogramming.com. I especially recommend Extreme Programming Installed by Ron Jeffries and others.
(b) The assert() function is the only function that is not built in. It will probably come standard one day but not yet. Today you probably have to write or install your own assert() function. The good news is it can be very easy. Here's a very simple example:
This failure message isn't very informative but there are lots of ways to improve that.function assert(fact) { if (!fact) { alert("Assert failure!"); } }
Here are some simple asserts.js functions I've developed. Example usages:
assert("2+2 == 4");
assertEquals("2+2", "4");
assertNull("window.nosuchmember");
assertNotNull("window.document");
Note you pass the code as strings.
For a very useful assert() and companion tools for Unit Testing, see Edward Hiett's JavaScript Unit Testing Framework JsUnit at www.jsunit.net. I used a modified version to test all the code in the Card and Foldouts. You can see that on the VisiBone JavaScript Unit Test page. If you end up doing much JavaScript development, or you practice Extreme Programming principles, you'll find this a great package.
Jeff P. reports that there appears to be some kind of assert() function defined in JavaScript 1.4 (when declaring <script language="JavaScript1.4">).
(c) The JavaScript Card and Foldouts uses assert() for a new and wonderful purpose: it's power to describe.
Look up hammer in the nearest dictionary. Lots of words, maybe a picture. Now imagine the sight of someone using a hammer to drive a nail into wood. Not just a still picture but the motion. Doesn't that describe the essence of hammer quick and thorough? With any tool, showing is so much better than telling. This is what assert() can do: show technology in action.
Example:
assert('abc'.toUpperCase() == 'ABC');
If you pretend you can read JavaScript, this little bit of code says a lot. It shows that if 'abc' is the input then 'ABC' is the output. It shows how input and output are handled syntactically. Most of all, since this is real code you can be certain it works as described.
Get used to this assert() function, I know you're gonna love it.