Best way to validate an argument that is being passed into a function is a number itself?

Question

What is the most reliable way to check if a string is a number or not?

 var checkForNumber = (arg) => {
   if (typeof arg === 'number') {
     return arg + ' is a number'
   } else if (Number(arg)) {
     return arg + ' is a number'
   }
  return 'Not a number';
 };

 checkForNumber('a')

Well, if the arg is a **string**, as your intro line says, the first if will always be false. — Taplar, Feb 05 '20 at 18:50
You don't need `typeof` since `Number(25)` returns `25` (truthy) anyways. If `arg` is in fact always a string, then `Number(arg)` is sufficient. Otherwise (e.g. `arg = {val: 25}`), you'll need more checks. — junvar, Feb 05 '20 at 18:51
```if (isNaN(parseFloat(number)) && isFinite(number)) return '';``` ===> will this be the right way to check whether the number that is passed in is a number or string. If its not then return an empty string — Jeena, Feb 05 '20 at 19:17

Jeremy Harris · Answer 1 · 2020-02-05T19:38:45.090

0

Given that the constructor of Number() will return NaN if invalid, I think this might be a good way to get a boolean value as to whether a number is valid or not:

function isNum(num) {
   return (
      !isNaN(num) && 
      (parseInt(num) == num || parseFloat(num) == num)
   );
}

console.log('"1"', isNum('1'));
console.log('1', isNum(1));
console.log('"a"', isNum('a'));
console.log('empty string', isNum(''));
console.log('object', isNum({}));
console.log('undefined', isNum(undefined));
console.log('false', isNum(false));
console.log('true', isNum(true));
console.log('[]', isNum([]));
console.log('PI short string', isNum('3.14'));
console.log('PI short float', isNum(3.14));

console.log('PI long', isNum(3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679));

You have to also check if boolean or array though, because they will come back as a number. I am doing that by comparing the output of parseInt.

edited Feb 05 '20 at 19:38

answered Feb 05 '20 at 18:57

Jeremy Harris

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By using !isNaN in combination with parseFloat() to check if the argument is a number will work right? – Jeena Feb 05 '20 at 19:02
I'm not sure if parseFloat() would modify the float value any. Do your needs require float values as well? – Jeremy Harris Feb 05 '20 at 19:03
Yes. it does. The number that is being passed in is an amount. – Jeena Feb 05 '20 at 19:05
1

I edited it to support floating point numbers too. – Jeremy Harris Feb 05 '20 at 19:38
```if (!isNaN(parseFloat(number)) & isFinite(number)) return ''; ``` ===> will this be the right way to check whether the number that is passed in is a number or string. If it's not a valid number then return an empty string? I tried using it in my code in Vscode but it is yelling at me sayin **Unexpected use of 'isNaN'..** and also on **Unexpected use of 'isFinite.** – Jeena Feb 05 '20 at 19:46
I didn't use `isFinite`... – Jeremy Harris Feb 05 '20 at 19:54
I know. I was just asking whether you have come across this way. – Jeena Feb 05 '20 at 19:56
I'm not sure, I have never used that. – Jeremy Harris Feb 05 '20 at 19:57

score 0 · Answer 2 · answered Feb 05 '20 at 19:04

The best way that I consider to check numbers is the function isNaN(), "is Not a Number" the opposite of "Is a Number"

>isNaN(1)
 false
>isNaN('1')
 false
>isNaN('1e')
 true
>isNaN('1.1')
 false
>isNaN('1,1')
 true
>isNaN('0000001')
 false
>isNaN('f12f')
 true

You should use this function as negated: !isNaN()

Best way to validate an argument that is being passed into a function is a number itself?

2 Answers2