Option1 : multiple var without assignment
function MyFunction() {
var a = null;
var b = null;
....
var z = null;
a = SomeValue;
"premature optimization is the root of all evil"
I don't think there will be any significant performance change with any of this options.
(IMO) The third option is the most readable option and closest to dynamic memory allocation like C#
etc'. But this is my humble opinion, Choose what you like the most.
If it really bothers you and you can't sleep without an answer, test it with jsPerf.
@Chad made a jsPerf so you can sleep well tonight...