(Built-in) way in JavaScript to check if a string is a valid number

Question

I\'m hoping there\'s something in the same conceptual space as the old VB6 IsNumeric() function?

Answer 1

Maybe there are one or two people coming across this question who need a much stricter check than usual (like I did). In that case, this might be useful:

if(str === String(Number(str))) {
  // it's a "perfectly formatted" number
}

Beware! This will reject strings like .1, 40.000, 080, 00.1. It's very picky - the string must match the "most minimal perfect form" of the number for this test to pass.

It uses the String and Number constructor to cast the string to a number and back again and thus checks if the JavaScript engine's "perfect minimal form" (the one it got converted to with the initial Number constructor) matches the original string.