I'm afraid Number.isInteger isn't in Espruino, but you can fake it with this I believe:
Number.isInteger
Number.isInteger = it => isFinite(it) && Math.floor(it) === it;
@Gordon started
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
I'm afraid
Number.isInteger
isn't in Espruino, but you can fake it with this I believe: