> it's not too bizarre when you consider no version of IE supports Number.isFinite or Number.isInteger.

If you are extensively using nodejs libraries to target IE, then you have other problems than supporting Number.isFinite or Number.isInteger to deal with first, considering that IE doesn't support modern ES versions in anyway. You'd have to use shims,polyfills and whatnot at first place and transpile all that npm code into something IE can run.

This isn't a very good argument to add an "is-even" package.

but this is an npm package, would a client side application be importing this code? surely at that point you could just include some Number polyfill as well (but you're right I was only thinking of node)

Yes, client side applications use npm as well.