Javascript function to determine if a value is an integer

Question

I came across a problem where I needed to determine if the field being entered by the user was an integer or float. The answer would then preselect a drop down further along a form. After much digging I found lots of framework code but none that actually did the job properly. The test data I used was;

Blank answer, Non Numeric (Alpha), 1.0, 10, 1.10, 2.4, 3.0, 0.30, 0.00

A lot of other posts were also tested with the above data and I could not find one that passed ALL of the data correctly.

So I have written the following so that it may be reviewed by your good selves and hopefully it will help someone else out should they come across the same situation.

    function isInteger(value) 
        {
        //if(isNaN(value))return Nan;//optional check
        //test for decimal point
        if(!( /^-?\d+$/.test(String(value))))
            {
            //decimal point found
            //if parseInt changes value it must be a float
            if(parseInt(value) / 1 != value)return false; 
            }
        //no decimal point so must be integer
        return true;
        }

Answer 1

Testing for integer values

ECMAScript 6 standard introduces a Number.isInteger().

This function is not yet supported by all major browsers, but a polyfill is listed on the site:

Number.isInteger = Number.isInteger || function isInteger (value) {
  return typeof value === 'number' && 
    isFinite(value) && 
    Math.floor(value) === value
}

In case of user input (which is a string, not an integer), we can use the Number function to perform a type conversion into a number:

var input = '123' // Imagine this came from user

if (Number.isInteger(Number(input)) {
  // User entered a valid integer value
}

Note, however, that the type conversion returns a valid integer-like value even for hexadecimal or octal strings. If this is not desired, you would need to further validate the original string. For detailed information about how the type conversion works, see MDN.

If such strict validation is desired, MDN also provides a good implementation using Regex (see the link for example output):

function filterInt (value) {
  if(/^(\-|\+)?([0-9]+|Infinity)$/.test(value))
    return Number(value)

  return NaN
}

Testing for floating point numbers

isFinite() in combination with Number.isInteger() can help achieve our goal.

In case of user input (which is a string, not a float), we must use the Number function to perform a type conversion into a number:

var input = '123.5'
// Perform type conversion to a number
input = Number(input)

if (Number.isFinite(input) && ! Number.isInteger(input)) {
  // A finite number that is not an integer can only be a float
}

Alternatively, a stricter variant of the parseFloat() implementation may be used instead, as listed on MDN (see the link for example output):

function filterFloat (value) {
  if(/^(\-|\+)?([0-9]+(\.[0-9]+)?|Infinity)$/
    .test(value))
    return Number(value)
  return NaN
}