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;
        }

Robert Rossmann · Answer 1 · 2015-05-28T07:42:19.157

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
}

Even for strings, you can try to convert them to float or int a check them with this function. I will update the answer momentarily. — Robert Rossmann, May 27 '15 at 10:05
That's better, but still no cigar. `123foo`, `3.14` and `+000` are hardly "integers". — georg, May 27 '15 at 13:03
Interesting! I never expected `parseInt()` to behave this weird. I amended my answer and added a comment about some of my solution's limitations. — Robert Rossmann, May 27 '15 at 13:49

score 1 · Answer 2 · answered May 27 '15 at 09:49

Since you've mentioned user inputs, your question is about strings, so Number.isInteger is not an option.

To answer this question correctly we have to define more precisely what "integer" means when applied to strings. Is it

a sequence of digits? (example: ১২৪৫)
or a sequence of arabic digits? (example: 0000001)
or any mathematically valid integer representation? (example: 989238402389402394820394802)
or a mathematically valid integer that can be represented exactly in Javascript (i.e. it's less than MAX_SAFE_INTEGER)?

My guess is that you're looking for 4), here's the code for this case:

function isValidIntegerRepresentation(str) {
  return /^-?\d+$/.test(str) && String(Number(str)) === str;
}



test = ['0', '00', '123', '-123', '239482039482309820394820394'];
test.forEach(function(n) {
  document.write(n + "=" + isValidIntegerRepresentation(n) + "<br>");
});

This is very similar to what you already have.

Javascript function to determine if a value is an integer

2 Answers2

Testing for integer values

Testing for floating point numbers