Testing:
If you're planning on writing a significant amount of either PHP or JavaScript code then all of the standard PHP and JavaScript development tools will be useful, regardless of the fact that your code will be running within a Joomla environment rather than stand-alone.
On the PHP side:
- XDebug: A PHP extension for developers that allows you to do things like step through your code line by line to help you find bugs. Invaluable.
- phpUnit: A testing tool. Write phpUnit tests to make sure your code works
- PHPMD: PHP "Mess detector". Points out poor quality code. (A lot of well-known projects could really have benefited from using this kind of tool from the start).
- CodeSniffer and the Joomla coding standards: enforces clean code practice and makes your code easier to read for new developers.
On the JavaScript side:
- The various browser DevTools features. All the major browsers have good quality dev tools now. Firefox also has Firebug available as a plugin in addition to the built-in dev tools, which is also worth having.
- JSHint: Points out poor quality code and potential bugs.
There are tons of tools like these, but obviously it depends how serious your coding is going to get -- if you're just knocking up a few dozen lines of code, there's not going to be much point messing around with the most of the above. But on the other hand if you're planning on writing a reasonably sized Joomla plugin and supporting it for some time, then the above tools will be invaluable to you for helping you write it well.
