Lady, EVOLVED 
browser engines
webkit is still the only one which lets you do proper functional programming, huh
re: browser engines
; { const sum = (() => {"use strict"; return (x,i=0) => x === 0 ? i+x : sum(x-1,i+x)})(); sum(5000000) }
Works On My Machine
re: browser engines
@Lady oh I did not know that. Thanks
re: browser engines
@wallhackio proper tail calls are only a thing in strict mode, which can't be set in functions which take default parameters, so i threw an IIFE around it