I\'m new to Javascript, and notice that you don\'t need to specify an array\'s size and often see people dynamically creating arrays one element at time. This would be a hug
It most likely depends on what JavaScript engine you use.
Internet Explorer uses a mix of sparse arrays and dense arrays to make that work. Some of the more gory details are explained here: http://blogs.msdn.com/b/jscript/archive/2008/04/08/performance-optimization-of-arrays-part-ii.aspx.