How to write isNumber() in JavaScript?

Question

My guess would be:

function isNumber(val) {
    return val === +val;
}

Is there a better way?

Previous Reference

Validate decimal numbers in JavaScript - IsNumeric()

What is your expectation for the following values: `NaN`, `Infinity`, `'1'`, `1`? — zzzzBov, Nov 23 '13 at 23:32
You can use the implementation from jQuery for this: [jQuery.isNumeric](http://jsapi.info/jquery/1.8.3/jQuery.isNumeric) — Andreas, Nov 23 '13 at 23:34
I just have to ask, couldn't you find an answer in the exact duplicate question you yourself posted in the question ? — adeneo, Nov 23 '13 at 23:36
The accepted answer is properly tested, and it's the same solution I always tend to use without any issues, so what's the problem with that ? — adeneo, Nov 23 '13 at 23:39
What do you mean by "this would not handle negative numbers"? `-1` is equal to `+-1`. — Qantas 94 Heavy, Nov 23 '13 at 23:54

score 39 · Answer 1 · edited Mar 17 '21 at 14:04

39

var isNumber = function isNumber(value) 
{
   return typeof value === 'number' && isFinite(value);
}

This code above is taken from the book "JavaScript - the good parts".

/* ANSWER UPDATE BEGIN - in reply to some comments
below from the person who asked the question and others */

var isNumber = function isNumber(value) {
  return typeof value === 'number' && isFinite(value);
}

var isNumberObject = function isNumberObject(n) {
  return (Object.prototype.toString.apply(n) === '[object Number]');
}

var isCustomNumber = function isCustomNumber(n){
  return isNumber(n) || isNumberObject(n);
}

console.log(isCustomNumber(new Number(5)));
console.log(isCustomNumber(new Number(5.2)));
console.log(isCustomNumber(new Number(5.5)));
console.log(isCustomNumber(new Number(-1)));
console.log(isCustomNumber(new Number(-1.5)));
console.log(isCustomNumber(new Number(-0.0)));
console.log(isCustomNumber(new Number(0.0)));
console.log(isCustomNumber(new Number(0)));
console.log(isCustomNumber(new Number(1e5)));

console.log(isCustomNumber(5));
console.log(isCustomNumber(5.2));
console.log(isCustomNumber(5.5));
console.log(isCustomNumber(-1));
console.log(isCustomNumber(-1.5));
console.log(isCustomNumber(-0.0));
console.log(isCustomNumber(0.0));
console.log(isCustomNumber(0));
console.log(isCustomNumber(1e5));

/* ANSWER UPDATE END - in reply to some comments
below from the person who asked the question and others */

edited Mar 17 '21 at 14:04

Martin

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answered Nov 23 '13 at 23:47

peter.petrov

38,363
16
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1

I like typeof but what is the isFinite for? – nativist.bill.cutting Nov 23 '13 at 23:47
"The isFinite function is the best way of determining whether a value can be used as a number because it rejects NaN and Infinity." This is also a quote from the same book. – peter.petrov Nov 23 '13 at 23:48
I see, JavaScript considers these values number because they are of type number but used to describe Not a Number. – nativist.bill.cutting Nov 23 '13 at 23:49
2

Why you use named function expression? – Givi Nov 23 '13 at 23:53
1

@Givi: most likely so that similar to native functions, the name is shown in the `toString` representation of it. – Qantas 94 Heavy Nov 23 '13 at 23:55
I don't particularly like this one because `isNumber(new Number(5))` would return `false`. – zzzzBov Nov 24 '13 at 00:02
what in the holy heck does new Number(5) do? `var test = 5 vs var test = new Number(5)` – nativist.bill.cutting Nov 24 '13 at 22:40
how does that created something that is not a number?? – nativist.bill.cutting Nov 24 '13 at 22:41
@nativist.bill.cutting, `new Number(5)` creates a `Number` object, rather than a value. It's pretty much never used, so it's not really an issue, but I prefer to allow for objects to be treated as numbers, such as `Date`, `Boolean`, and any custom objects that have `valueOf` overridden. – zzzzBov Nov 25 '13 at 00:01
if the language allow it, it must have some purpose? – nativist.bill.cutting Nov 25 '13 at 23:22
1

This check I quoted in my answer is as strict as possible. It is a book quote. The object new Number(5) is not a number in JS, that's obvious. So if you want to customize this check for special purposes - feel free. And my best guess of implementing that custom definition is there - see the update to my answer (this extension I think follows the philosophy of the book, and I like the book a lot, as the author really really knows what he's talking about). – peter.petrov Nov 26 '13 at 13:12
2

Just use `Number.isFinite` instead. It returns a `false` for non-`number` values. – Константин Ван Dec 30 '20 at 10:48

score 13 · Answer 2 · answered Nov 24 '13 at 01:14

If you do not want to include strings, and you do want to include Infinity, you can compare the Number coercion of the argument to the argument:

function isNumber(n){
    return Number(n)=== n;
}

//test

[0, 1, 2, -1, 1.345e+17, Infinity, false, true, NaN, '1', '0'].map(function(itm){
    return itm+'= '+isNumber(itm);
});

// returned values
0= true
1= true
2= true
-1= true
134500000000000000= true
Infinity= true
false= false
true= false
NaN= false
'1'= false
'0'= false

Do you consider `01` Numeric? If so this fails that case – icc97 Sep 27 '20 at 22:17 — icc97, Sep 27 '20 at 22:17

score 9 · Answer 3 · answered Nov 23 '13 at 23:30

9

function isNumber(val){
    return typeof val==='number';
}

answered Nov 23 '13 at 23:30

markasoftware

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3

and of course `&& !isNaN(val)` because `typeof NaN` is number. – Farzad Yousefzadeh May 22 '20 at 14:35
3

@FarzadYousefzadeh TIL that 'Not a Number' is of type 'number'. Gotta love javascript. – Alvaro Carvalho Nov 22 '21 at 18:10

score 7 · Answer 4 · answered Nov 23 '13 at 23:32

7

If you want "23" to be a number, then

function isNumber(val) {
    return !isNaN(val);
}

answered Nov 23 '13 at 23:32

Ari Porad

2,834
3
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above function will given wrong with null and empty string , use follwing for right one function isNumber(n) { return !n && isFinite(n) } – user8640104 May 17 '18 at 08:14
it return true when val is empty string. but nice and simple answer. – JaeIL Ryu Aug 25 '21 at 05:56

score 3 · Answer 5 · answered Nov 23 '13 at 23:32

3

function isNumber(val) {
        // negative or positive
        return /^[-]?\d+$/.test(val);
    }

answered Nov 23 '13 at 23:32

davs

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1

`1.1` will fail, is that intentional? – zzzzBov Nov 23 '13 at 23:35
...can you break Joren's answser? – nativist.bill.cutting Nov 23 '13 at 23:39
I think his solution is ok – davs Nov 23 '13 at 23:48

score -4 · Answer 6 · answered Nov 23 '13 at 23:33

-4

I like this solution:

function isNumber(val) {
    return (val >=0 || val < 0);
}

answered Nov 23 '13 at 23:33

Joren

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detects true an false. – nativist.bill.cutting Nov 23 '13 at 23:47
Well since true and false can be seen as 1 or 0 I guess that it is a desired functionality. – Joren Nov 24 '13 at 00:16
3

"can be seen as" and "is" are two very different things. What happens when your function returns `true` for a boolean, and so you try to invoke a `Number.prototype` method on the `val`? Oops! – Blue Skies Nov 26 '13 at 16:31

How to write isNumber() in JavaScript?

6 Answers6