Comparing it against the list of examples given in is-number, this solution does not properly handle numbers represented as strings, and it also recognizes Infinity as a number.
Strings are the safest way to hold big numbers in JavaScript due to how JavaScript represents numbers. Also no, infinity is not a number. It's a concept.
Numbers is a concept. For some purposes it is useful to consider a number that is infinite, which is why concepts like the extended real number line have been invented.