simple explanation how to save code that espruino run on start?

@tve sounds like for you, the best solution would be if the Web IDE issues a load() call rather than a reset() one...

A simple hack is just to do reset=load;, and then next time the IDE tries to reset it'll actually end up loading your saved code.

There was a post a while back about adding something like E.setBootUpCode that'd save code that would always be executed - even on reset. That could be an option - but on the whole the idea of reset is to reset the board to a totally fresh state so you always know where you start from.