Check if an input string is a positive int/float number

Question

On my webpage there is an input box that should only allow user to enter a positive int/float number (i.e. 1234, 123.4, 12.34, 1.234 should all be allowed).

To make sure the value is valid, I have a function to validate the value before sending it to the server:

function isPositiveFloat(s){
    return String(Number(s)) === s && Math.floor(Number(s)) > 0;
}

The function works great expect for one scenario: isPositiveFloat(1.0) will return false, as it turns out Number(1.0) will convert it to 1, therefore made the validation failed.

Any suggestions on how should I resolve this issue? Is using regex the only way to go?

Thanks in advance for any help!

How about `!isNaN(Number(s))`? Or in fact, just `!isNaN(s)`? — Oliver Charlesworth, Dec 05 '17 at 19:54
Possible duplicate of [Is there a (built-in) way in JavaScript to check if a string is a valid number?](https://stackoverflow.com/questions/175739/is-there-a-built-in-way-in-javascript-to-check-if-a-string-is-a-valid-number) — Oliver Charlesworth, Dec 05 '17 at 19:55

score 2 · Accepted Answer · answered Dec 05 '17 at 20:02

You just need to use !isNaN(s)along with Number(s)>0:

function isPositiveFloat(s) {
  return !isNaN(s) && Number(s) > 0;
}

Demo:

function isPositiveFloat(s) {
  return !isNaN(s) && Number(s) > 0;
}

<input type="text" onchange="console.log(isPositiveFloat(this.value))" />

You can check on the MDN reference of isNaN() examples that:

isNaN('37.37'); // false: "37.37" is converted to the number 37.37 which is not NaN

So as you can see it will be working in your case with float numbers.

Thank you! It works great! And thanks for the explanation. – Eliellel Dec 05 '17 at 20:08 — Eliellel, Dec 05 '17 at 20:08

Check if an input string is a positive int/float number

1 Answers1