JavaScript - Test for an integer

Question

I have a text field that allows a user to enter their age. I am trying to do some client-side validation on this field with JavaScript. I have server-side validation already in place. However, I cannot seem to verify that the user enters an actual integer. I am currently trying the following code:

    function IsValidAge(value) {
        if (value.length == 0) {
            return false;
        }

        var intValue = parseInt(value);
        if (intValue == Number.NaN) {
            return false;
        }

        if (intValue <= 0)
        {
            return false;
        }
        return true;
    }

The odd thing is, I have entered individual characters into the textbox like "b" and this method returns true. How do I ensure that the user is only entering an integer?

Thank you

user_70393 · Answer

var intRegex = /^\d+$/;
if(intRegex.test(someNumber)) {
   alert('I am an int');
   ...
}

That will absolutely, positively fail if the user enters anything other than an nonnegative integer.

user_42847 · Answer

For real int checking, use this:

function isInt(value) { 
    return !isNaN(parseInt(value,10)) && (parseFloat(value,10) == parseInt(value,10)); 
}

The problem with many int checks is that they return 'false' for 1.0, which is a valid integer. This method checks to make sure that the value of float and int parsing are equal, so for #.00 it will return true.

UPDATE:

Two issues have been discussed in the comments I'll add to the answer for future readers:

First, when parsing string values that use a comma to indicate the decimal place, this method doesn't work. (Not surprising, how could it? Given "1,001" for example in the US it's an integer while in Germany it isn't.)
Second, the behavior of parseFloat and parseInt has changed in certain browsers since this answer was written and vary by browser. ParseInt is more aggressive and will discard letters appearing in a string. This is great for getting a number but not so good for validation.

My recommendation and practice to use a library like Globalize.js to parse numeric values for/from the UI rather than the browser implementation and to use the native calls only for known "programmatically" provided values, such as a string parsed from an XML document.

user_60797 · Answer

use isNaN(n)

i.e.

if(isNaN(intValue))

in place of

if (intValue == Number.NaN)

user_2281999 · Answer

I did this to check for number and integer value

if(isNaN(field_value * 1) || (field_value % 1) != 0 ) not integer;
else integer;

Modular Divison

Example
1. 25.5 % 1 != 0 and ,
2. 25 % 1 == 0

And if(field_value * 1) NaN if string eg: 25,34 or abcd etc ... else integer or number

JavaScript - Test for an integer

11 Answers