Learning JavaScript

Null or undefined
There are more than a dozen ways of comparing or looking at a variable to determine wether it is null or undefined (or either of them). Look at the following for actual performance numbers and decide for yourself what gives the best performance for your scenario:
 * http://jsperf.com/testing-undefined
 * http://jsperf.com/testing-null
 * http://jsperf.com/testing-null-or-undefined

Array
http://jsperf.com/testing-array-type-speed-performance-comparison

WikiEditor
Some stuff Catrope discovered when optimizing EditToolbar. These may not be universally true, so when in doubt run both alternatives through the Firebug profiler or use http://jsperf.com


 * Building simple HTML that doesn't need stuff like .data or .click yourself and feeding it to .html or .append is literally about 100 times as fast (YMMV) as building it with jQuery
 * Even if you do need .data or .click, something like this is still faster (at least for 20-200 children, YMMV):
 * Avoid setting .data on hundreds of elements that have a common parent or near ancestor. Instead, add an empty object to the parent's .data and fill that through a reference using the child's rel property as keys. This reduces the number of .data calls a lot.
 * .text is faster than .html if you're only adding text
 * .addClass('foo') is faster than .attr('class', 'foo')
 * but in complex cases involving more attributes and multiple classes, .attr({ ... }); is faster