(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

Old question, but there are several points missing in the given answers.

Scientific notation.

!isNaN('1e+30') is true, however in most of the cases when people ask for numbers, they do not want to match things like 1e+30.

Large floating numbers may behave weird

Observe (using Node.js):

> var s = Array(16 + 1).join('9')
undefined
> s.length
16
> s
'9999999999999999'
> !isNaN(s)
true
> Number(s)
10000000000000000
> String(Number(s)) === s
false
>

On the other hand:

> var s = Array(16 + 1).join('1')
undefined
> String(Number(s)) === s
true
> var s = Array(15 + 1).join('9')
undefined
> String(Number(s)) === s
true
>

So, if one expects String(Number(s)) === s, then better limit your strings to 15 digits at most (after omitting leading zeros).

Infinity

> typeof Infinity
'number'
> !isNaN('Infinity')
true
> isFinite('Infinity')
false
>

Given all that, checking that the given string is a number satisfying all of the following:

• non scientific notation
• predictable conversion to Number and back to String
• finite

is not such an easy task. Here is a simple version:

  function isNonScientificNumberString(o) {
    if (!o || typeof o !== 'string') {
      // Should not be given anything but strings.
      return false;
    }
    return o.length <= 15 && o.indexOf('e+') < 0 && o.indexOf('E+') < 0 && !isNaN(o) && isFinite(o);
  }

However, even this one is far from complete. Leading zeros are not handled here, but they do screw the length test.