Unfortunately "not a number" is typeof "number"

    "number" === typeof NaN

Returns true.

I mean, it depends on context what “numbers” are acceptable. For an overly general solution:

  const allowNumbers = {
    all: () => true,
    notNaN: n => !Number.isNaN(n),
    finite: Number.isFinite,
    integer: Number.isInteger,
    smol: n => Math.abs(n) < Number.EPSILON,
    lorge: n => n > Number.MAX_SAFE_INTEGER || n < Number.MIN_SAFE_INTEGER,
  }

  function isNumber(n, allow = allowNumbers.all) {
    return ("number" === typeof n) && allow(n);
  }

This isn't bad, but for maintainability, it should be split into at least nine packages.

In the type of things that Javascript considers 'numbers', it's a value used to indicate that it's not of the type of things that math considers 'numbers'.

That's a tricky one that I think would depend on use case. Yes, of course NaN isn't a number, but it's still kind of a number, depending on context.

NaN is a float according to IEEE754. All numbers in Javascript being floats, it makes sense that typeof NaN === "number".

Now, the fact that IEEE754 is insane is something else, but JS does nothing wrong (aside from not having not-floats)

> Now, the fact that IEEE754 is insane is something else, but JS does nothing wrong (aside from not having not-floats)

JS has BigInt - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

BigInt is relatively recent, all things considered. JS only having floats is a conception flaw.