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Is this thing a number?

TL;DR - do not use only isNaN for this and write a lot of tests.

StackOverflow implementation (so far ✅):

function isNumeric(n) {
  return !isNaN(parseFloat(n)) && isFinite(n);
}

Facebook implementation (❌ fails for many values like null, booleans, Date object, empty strings, ...):

function isNumber(number) {
  return !isNaN(number) && isFinite(number);
}

@cookielab implementation (❌ fails for values like 7.2acdgs and it's not multiplatform):

function isNumeric(n) {
  return Number.isFinite(Number.parseFloat(n));
}

Please note that isNaN and Number.isNaN differs significantly (isNaN performs a type conversion). The same for isFinite vs Number.isFinite:

In comparison to the global isFinite() function, this method doesn't forcibly convert the parameter to a number. This means only values of the type number, that are also finite, return true.

See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isFinite

Number.isFinite('0'); // false, would've been true with global isFinite('0')
Number.isFinite(null); // false, would've been true with global isFinite(null)

Polyfill (to understand the difference better):

Number.isFinite =
  Number.isFinite ||
  function (value) {
    return typeof value === 'number' && isFinite(value);
  };