[frankincense]

typeof x === 'number' && !Number.isNaN(x)

[newsbinator]

This seems like the best solution.

Any weird edge cases against this one?

[JasonCannon]

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.

[TheCoelacanth]

Because strings aren't numbers and Infinity is a number

[JasonCannon]

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.

[boomlinde]

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.

[mirekrusin]

+/- Infinity, better to use Number.isFinite.

[stagas]

Infinities.