The first one takes care of molecules like this: Cl2. What it does is repeat the molecule x times. So Cl2 becomes ClCl.
The second one takes care of molecules with braces with a number after them. E.g. (CCHHHH)2 becomes CCHHHHCCHHHH. The third and fourth do the same but with [] and {} respectively.
The fifth one removes all left over braces as all braces that are left are guaranteed to have no number after them (so no repeating needed anymore).
The sixth one takes each atom and adds a space after it.
Then all the atoms can be split by splitting on a space, after which the reduce counts theatoms.

Its not fast but a funny one liner, though it is not perfect as nested braces don't seem to work (e.g. ((C2H6))2)

This is O(n), most if not all solutions with .indexOf/.lastIndexOf are O(n^2)