Testing whether a number is odd is done in 3 characters:
n % 2
if you don't like the % you can also do it using a bitwise AND, this also handles negative numbers:
n & 1
this is equal to 1 if the number is odd, 0 if it is even, and will be converted to true or false in a if statement. You don't need a library for this.
And you should not need a library to handle string conversions, you should be aware of the types you manipulates. If you do need to handle strings, that's still easy:
Number(n) & 1
And Number(n) is a no op if n is already a number. But wait, no, actually, you don't even need this because % and & will auto cast the string to a number anyway. You can use parseInt(n) if there's a chance that your number is going to be non integer, but meh.
So n & 1 will cover everything.
All this is readable and idiomatic, there's no need to write a function for that, let alone a full fledged library with unit tests, package.json and all this crap that bloats the node_module folders of everybody.
isOdd = require('isOdd') and your entry in the dependencies of your package.json is going to be more verbose than isOdd = n => n & 1. Even "isOdd(n)" is longer than "n & 1". Nothing beats the "& 1".
If people are going to write libraries for all kind of similarly small and trivial things, soon we will need more than the Earth to handle our code.