Question

My Code:

I tried the following code

<SCRIPT type="text/javascript"> 

var num = "10";
var expRegex = /^\d+$/;

if(expRegex.test(num)) 
{
   alert('Integer');
}
else
{
   alert('Not an Integer');
}

</SCRIPT>

I am getting the result as Integer. Actually I declared the num varibale with double quotes. Obviously it is considered as a string. Actually I need to get the result as Not an Integer. How to change the RegEx so that I can get the expected result.

In this case, it should give the result as Not an Integer. But I am getting as Integer.

Answer 1

if(typeof num === "number" &&
   Math.floor(num) === num)
    alert('Integer');
else
    alert('Not an Integer');

Regular expressions are there to work on strings. So if you tried it with something else than a string the string would either be converted or you would get an error. And yours returns true, because obviously the string only contains digit characters (and that is what you are checking for).

Use the typeof operator instead. But JavaScript doesn’t have dedicated types for int and float. So you have to do the integer check yourself. If floor doesn’t change the value, then you have an integer.

There is one more caveat. Infinity is a number and calling Math.floor() on it will result in Infinity again, so you get a false positive there. You can change that like this:

if(typeof num === "number" &&
   isFinite(num) &&
   Math.floor(num) === num)
    ...

Seeing your regex you might want to accept only positive integers:

if(typeof num === "number" &&
   isFinite(num) &&
   Math.floor(Math.abs(num)) === num)
    ...