JavaScript and its love for zeroes
This post is part of a series called JavaScript Demystified.

Answer quick. Do you know what date is being created here?

var year = '2009', month = '09', day = '01';
var date = new Date( 

At first glance, it wouldn't surprising that someone guesseed September 1st 2009. However, I'd not be writing this post if that was the correct answer, right?

There's an interesting and tricky thing with the JavaScript parseInt function: it can parse strings with a numeric value in the decimal radix, but also in other radices. See the following examples.

//passing the radix explicitly
parseInt('1011', 10); // ==> 1011
parseInt('1011',  2); // ==> 11
parseInt('1011',  8); // ==> 521
parseInt('1011', 16); // ==> 4113

Maybe you thought that if you didn't pass the radix, then it would default to 10 because it's the obvious behavior. Well, no. In JavaScript the default behavior is to try to identify one of the literal formats and interpret that. So here's that in action:

//leaving JavaScript on its own
parseInt('1011'); // ==> 1011 (decimal literal)
parseInt('0x12'); // ==> 18   (hexadecimal literal)
parseInt('0511'); // ==> 329  (octal literal)
parseInt('0182'); // ==> 1    (whaaaa?!?!)

If you are familiar with the literal notation for integer numbers in JavaScript, and after I explained the default behavior of parseInt, then you probaly understood the results shown above. Well, maybe the last one deserves some comments.

When JavaScript is parsing the string, if it finds a digit (number or alpha) that is invalid in the chosen radix, it stops right there and parses only the portion of the string that comes before that digit. So, since we started '0182' with a leading zero, the octal radix is assumed. Then, because 8 is not a valid octal digit, only '01' will be parsed, which becomes 1.

Tip #1: If there's any chance the string value you plan to parse into an integer number has a leading zero (or a less likely 0x,) then be safe and pass the radix parameter to your parseInt call. If you're extra paranoid, then always pass the radix.

Back to our original question

Armed with the clarification made above, we can expand our example like this:

var year = '2009', month = '09', day = '01';
// then the following statement:
var date = new Date( 
// equivalent to:
var date = new Date( 
         0,  // ===> oopsie

Hmmm, a zero in the month parameter. Will we have an error here? No, here comes the second potential surprise of this post.

Tip #2: When creating a new date using new Date(year, month, day), the month parameter, and only the month parameter is zero-based (0 to 11).

So, in case the tips and the picture in this text were not enough to help you guessing the date being created, here goes another completely gratuitous one with the answer.

Posted 09-19-2009 8:54 AM by sergiopereira



ITRushn wrote re: JavaScript and its love for zeroes
on 09-19-2009 4:49 PM

Run in to this parsing issue two month ago and since then always use second parameter of parseInt.

someone wrote re: JavaScript and its love for zeroes
on 09-21-2009 11:37 AM

Very nice, well written post - thanks!

Sanjeev Agarwal wrote Daily tech links for .net and related technologies - September 21-23, 2009
on 09-22-2009 2:35 AM

Daily tech links for .net and related technologies - September 21-23, 2009 Web Development Simplicity

Code Monkey Labs wrote Weekly Web Nuggets #78
on 10-12-2009 10:59 AM

Pick of the week: An Engineer's Guide to Bandwidth General Prefer Dependency Injection to Service Location : Steven Harman explains the difference between two common practices for using Inversion of Control containers and shares his opinion why one is

Steve Strong's Blog wrote Sporadic Digest of Interesting Stuff
on 10-20-2009 11:13 AM

Sporadic Digest of Interesting Stuff

Luca wrote re: JavaScript and its love for zeroes
on 01-24-2011 7:05 PM

great post, it will save me from madness :)

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