for example , parseFloat("1") can be used inside the js script in unity. Where can i find the reference to such js functions ?
Probably easiest is the MSDN documentation here. If you use the System classes from there you shouldn't go too wrong.
The back end that is available to the scripting in Unity is Mono 1.2.? and isn't fully up to date with .NET 3.5, it is around the same as .NET 2.0, so if something isn't working check that it is available in the 2.0 release. Ignore any classes under the Microsoft namespace, as I'm pretty sure they don't find their way into Mono.
Thing is, parseFloat() (for instance) is part of the standard js methods as listed here: http://msdn.microsoft.com/en-us/library/310zkceh%28VS.71%29.aspx
I can't say for sure if all these methods are available for use in Mono or if they omit any that are useable (as Murcho said, Mono isn't entirely in line with .NET), but this should get you on your way.
I found this really helpful too: