Javascript: parseInt() and leading 0’s

Here’s a strange one for you… We all know that parseInt() is a really useful function for converting a string into an integer. It enables us to take a string, convert it to a number, perform mathematic equations and get the result we are looking for.

However today I was converting some strings into numbers and came across some very strange results from parseInt() when passing strings of numbers with leading 0’s… In fact it stumped me so much that after scratching my head for a few minutes and debugging to make sure I was passing the number correctly, I had to resort to a Google search!

So now I know how dumb I am, I thought I would share the weirdness of parseInt() so that anyone else stumped by this unexpected behaviour can also learn from it, and learn that parseInt() isn't just a simple String to Integer convertor.

So, lets start with the problem and the strange results – below is a few test cases of strings with numbers from 1 – 9, all with leading 0’s:

alert(parseInt('01')); //returns 1
alert(parseInt('02')); //returns 2
alert(parseInt('03')); //returns 3
alert(parseInt('04')); //returns 4
alert(parseInt('05')); //returns 5
alert(parseInt('06')); //returns 6
alert(parseInt('07')); //returns 7
alert(parseInt('08')); //returns 0
alert(parseInt('09')); //returns 0

As you can see, everything works as expected until you reach ‘08’ and ‘09’.

So is this a bug? No

Why Does It Occur?

It is really quite simple – parseInt() has a second parameter called the ‘radix’, when this is not set the function usually assumes ‘10’ as the radix. However it will automatically assume the radix is 16 (hexadecimal) if the string begins with ‘0X’ and although not a standard, in some browsers the radix is assumed 8 (octal) if the string begins with a ‘0’.

From the Documentation:

If radix is undefined or 0, JavaScript assumes the following:

  • If the input string begins with "0x" or "0X", radix is 16 (hexadecimal).
  • If the input string begins with "0", radix is eight (octal). This feature is non-standard, and some implementations deliberately do not support it (instead using the radix 10). For this reason always specify a radix when using parseInt.
  • If the input string begins with any other value, the radix is 10 (decimal).

If the first character cannot be converted to a number, parseInt returns NaN.

This brings us on to a final problem with parseInt() – if it cannot convert a string to a number, it will return NaN, I have developed a workaround for those wishing for it to return 0 - /2012/02/javascript-parseint-tip-to-avoid-nan/


Always specify the radix!

So the moral of this story is to always specify the Radix so that this problem NEVER occurs, no matter what!

alert(parseInt('09', 10)); //returns 0
Author: Dean WilliamsI'm a Web Developer, Graphics Designer and Gamer, this is my personal site which provides PHP programming advice, hints and tips

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