Check if input is number or letter javascript

Question

I'm using forms in HTML and javascript. I would like an alert to pop up only if the user inputs a LETTER and clicks submit.

So I have the HTML code:

<form name="myForm" action="" onsubmit="return checkInp()" method="post">
    First name: <input type="text" name="age">
<input type="submit" value="Submit">

And the javascript code:

function checkInp()
{
var x=document.forms["myForm"]["age"].value;
if (x consists of any letters) // this is the code I need to change
{
alert("Must input numbers");
return false;
}
}

Define “letter”. Is “é” a letter? What about “α”? If you mean Ascii letters from A to Z and from a to z, please say so. — Jukka K. Korpela, Aug 04 '13 at 11:43
You may also want to change the HTML. Switch `` to ``. In modern browsers this will check for an numeric input without any JS. — fboes, Sep 26 '14 at 10:53

Bat_Programmer · Accepted Answer · 2015-01-10T22:48:46.577

138

You can use the isNaN function to determine if a value does not convert to a number. Example as below:

function checkInp()
{
  var x=document.forms["myForm"]["age"].value;
  if (isNaN(x)) 
  {
    alert("Must input numbers");
    return false;
  }
}

edited Jan 10 '15 at 22:48

answered Aug 04 '13 at 10:56

Bat_Programmer

6,717
10
56
67

25

Beware the empty string: `isNaN('') === false;` #jsrage – KFunk Oct 16 '14 at 22:01
16

Don't do this, strings like '0xFF' and '2e32' will pass through this validation. – Matt McClellan Dec 17 '14 at 21:16
5

You cannot use this to check for numbers. It fails if an alphabet follows the number. Try this "3a". – Nidhin David Oct 23 '15 at 09:43
6

And the craziest fact is `NAN` is also a number :( – Abhijith A C Sep 05 '16 at 16:58
@StormcrowSX what's about http://stackoverflow.com/questions/14636536/how-to-check-if-a-variable-is-an-integer-in-javascript ? – Kiquenet Sep 30 '16 at 09:16
1

this method is not reliable – Pablo Pazos Oct 18 '19 at 02:05

score 56 · Answer 2 · edited Jun 22 '18 at 15:53

56

Use Regular Expression to match for only letters. It's also good to have knowledge about, if you ever need to do something more complicated, like make sure it's a certain count of numbers.

function checkInp()
{
    var x=document.forms["myForm"]["age"].value;
    var regex=/^[a-zA-Z]+$/;
    if (!x.match(regex))
    {
        alert("Must input string");
        return false;
    }
}

Even better would be to deny anything but numbers:

function checkInp()
{
    var x=document.forms["myForm"]["age"].value;
    var regex=/^[0-9]+$/;
    if (x.match(regex))
    {
        alert("Must input numbers");
        return false;
    }
}

edited Jun 22 '18 at 15:53

Anil Suwal Qode

110
5

answered Aug 04 '13 at 11:02

Kim Kling

751
4
5

19

Are you missing a `!` before `x.match(regex)` in your second code snippet? – Submits Apr 08 '16 at 11:57
3

This is the cleanest solution – Faris Rayhan Dec 08 '16 at 04:31
1

Or you mean `regex.test(x)` ? – Sonson Ixon May 03 '20 at 10:36

mael · Answer 3 · 2013-08-04T11:41:41.940

14

You could use the isNaN Function. It returns true if the data is not a number. That would be something like that:

function checkInp()
{
    var x=document.forms["myForm"]["age"].value;
    if (isNaN(x)) // this is the code I need to change
    {
        alert("Must input numbers");
        return false;
    }
}

Note: isNan considers 10.2 as a valid number.

edited Aug 04 '13 at 11:41

answered Aug 04 '13 at 10:54

mael

2,214
1
19
20

1

*isNaN -yes, that may be problem, is there any possible ways to input whole numbers only? Thanks for the answer. – Greg Peckory Aug 04 '13 at 11:01
1

For whole numbers only, use regex and take a look at my answer. – Kim Kling Aug 04 '13 at 11:15

score 8 · Answer 4 · edited Feb 24 '23 at 23:13

8

You can use isNaN(). It returns true when passing a value that is not a number.

var data = 'hello there';

if (isNaN(data)) {
  alert("it is not a valid number");
} else {
  alert("it is a valid number");
}

edited Feb 24 '23 at 23:13

Michael M.

10,486
9
18
34

answered Aug 04 '13 at 10:58

Deepak Gangore

343
1
13

1

isNaN(true) returns false – Memet Olsen Dec 14 '15 at 15:27

str8up7od · Answer 5 · 2020-12-09T15:41:36.673

I know this post is old but it was the first one that popped up when I did a search. I tried @Kim Kling RegExp but it failed miserably. Also prior to finding this forum I had tried almost all the other variations listed here. In the end, none of them worked except this one I created; it works fine, plus it is es6:

    const regex = new RegExp(/[^0-9]/, 'g');
    const val = document.forms["myForm"]["age"].value;

    if (val.match(regex)) {
       alert("Must be a valid number");
    }

score 6 · Answer 6 · answered Dec 13 '15 at 17:01

6

A better(error-free) code would be like:

function isReallyNumber(data) {
    return typeof data === 'number' && !isNaN(data);
}

This will handle empty strings as well. Another reason, isNaN("12") equals to false but "12" is a string and not a number, so it should result to true. Lastly, a bonus link which might interest you.

answered Dec 13 '15 at 17:01

Ram Patra

16,266
13
66
81

Another reference: http://stackoverflow.com/questions/14636536/how-to-check-if-a-variable-is-an-integer-in-javascript – Kiquenet Sep 30 '16 at 09:16

score 6 · Answer 7 · answered Jan 28 '16 at 18:08

Just find the remainder by dividing by 1, that is x%1. If the remainder is 0, it means that x is a whole number. Otherwise, you have to display the message "Must input numbers". This will work even in the case of strings, decimal numbers etc.

function checkInp()
{
    var x = document.forms["myForm"]["age"].value;
    if ((x%1) != 0) 
    {
        alert("Must input numbers");
        return false;
    }
}

score 4 · Answer 8 · answered Aug 04 '13 at 10:55

4

Try this:

if(parseInt("0"+x, 10) > 0){/* x is integer */}

answered Aug 04 '13 at 10:55

Masoud Nazari

476
2
6
17

2

or simply `if ( (x >> 0) > 0 )` – Prasanth Aug 04 '13 at 10:57
Worked grand also, except there is still the issue with the decimal numbers. Thanks a million for the answer though. – Greg Peckory Aug 04 '13 at 11:04
3

Could you guys explain in detail how does it work? What does `>>`operator means? Thanks in advance – sospedra Sep 17 '14 at 08:44
Unfortunately, it gives 32 for "32!" instead of an error. – Martin Jambon Mar 13 '15 at 21:52

Vikas Bansal · Answer 9 · 2017-10-23T11:59:23.307

3

I think the easiest would be to create a Number object with the string and check if with the help of isInteger function provided by Number class itself.

Number.isInteger(Number('1')) //true
Number.isInteger(Number('1 mango')) //false
Number.isInteger(Number(1)) //true
Number.isInteger(Number(1.9)) //false

edited Oct 23 '17 at 11:59

answered Oct 23 '17 at 11:52

Vikas Bansal

10,662
14
58
100

beware Number(''); //0 – Sarath Aug 08 '20 at 18:32
`Number.isInteger('1')` wont return true. it returns false. It will still read as string. – aRtoo May 30 '21 at 01:16
pass ```Number('1') ``` – Vikas Bansal Jun 01 '21 at 07:27

score 2 · Answer 10 · answered Jul 08 '20 at 03:26

Thanks, I used @str8up7od answer to create a function today which also checks if the input is empty:

    function is_number(input) {
        if(input === '')
            return false;
        let regex = new RegExp(/[^0-9]/, 'g');
        return (input.match(regex) === null);
    }

score 0 · Answer 11 · answered May 16 '18 at 02:16

0

The best and modern way is typeof (variable) if you care about real number not number from string. For example:

var a = 1;
var b = '1';

typeof a: // Output: "number"
typeof b: // Output: "string

answered May 16 '18 at 02:16

Check if input is number or letter javascript

11 Answers11

Linked

Related