Michael Gaigg: Über UI/UX Design


The importance of the JavaScript parseInt radix

Posted by Michael Gaigg


Just recently I had to implement an HTML form that allows users to enter percentage values. Like every good programmer I added client-side validation to check that the input values are between 0 and 100.

Using the JavaScript function parseInt(txtValue) with txtValue being the value of the input field our tester was able to submit the form with a value of 0137.

My first reaction was to restrict the maxlength attribute of the input field to 3 characters only. Even though this is a good and recommended practise there was clearly something else wrong.


The parseInt() function parses a string and returns an integer. The signature is parseInt(string, radix) with

  • string (required) being the string to be parsed, and
  • radix (optional) a number (from 2 to 36) that represents the numeral system to be used

If the radix parameter is omitted, JavaScript assumes the following:

  • If the string begins with “0x”, the radix is 16 (hexadecimal)
  • If the string begins with “0”, the radix is 8 (octal)
  • If the string begins with any other value, the radix is 10 (decimal)


So, what happened? Because I forgot to specify the radix and our QA tester tried the (however unlikely) case of 0137 JavaScript assumed it was an octal number and returned a value of 95. Lesson learned: Always specify the radix (if it decimal set it to 10 ;)!!!!

PS.: Only the first number in the string is returned!
PPS.: Leading and trailing spaces are allowed.
PPPS.: If the first character cannot be converted to a number, parseInt() returns NaN.