Why not just use (or make a shortcut to) the Javascript built-in Boolean? https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean Or if you are going for speed: bool = _ => !(!_ | (_ === "false"))

Boolean("false") returns true, so that won't work, and even though you implementation is potentially faster, it is also less clear. I might change bool to return false for empty arrays as well, like in Python.