What if you use 3bits for each instruction

[RGFTheCoder]

You could probably lower it more in base 6 but
001=[
010=]
011=(
100=)
101=+
110=!

[fasttime]

Thanks for your ideas. One limitation is that the generated code needs to be valid JavaScript, meaning that parentheses must be balanced, operators must be in their allowed positions, and so many subsequences like )!, (] o [+] are not usable in any context. Also, certain constructs behave identically: if x is an expression, then (x) and ((x)) are the same. This makes it very hard to distinguish between such expressions with a reasonable overhead.