JavaScript Puzzler – Let’s Print Some ZIP-Codes!

Let’s take a look at a JavaScript Puzzler. If you don’t know what a Puzzler is, it is essentially a short problem used to demonstrate a quirk or edge-case with the language that you might encounter. Here is the format:

  1. Code – I introduce the code
  2. Question – I pose a multiple-choice question and you guess what the outcome is…think hard!
  3. Walkthrough – I walk through a reasonable explanation
  4. Answer – I tell you the real outcome (it might surprise you), and explain why
  5. Moral – How can we avoid making mistakes like this in our own code

Now that we all know what a Puzzler is, here is a simple JavaScript Puzzler:

Code:

<script type="text/javascript">
 
	// array of 6 valid zip-codes
	var zipCodes = new Array("93021", "16284", "02392", "20341", "08163", "32959");
 
	// let's do something with each zip-code - for now, print them out
	for (i = 0; i < zipCodes.length; i++) {
 
		// sanity check
		if (!isNaN(parseInt(zipCodes[i])) && parseInt(zipCodes[i]) > 0) {
			document.write(parseInt(zipCodes[i]) + " ");
		}
 
	}
 
</script>

Question:

What does this print?

  1. 93021 16284 2392 20341 32959
  2. 93021 16284 2392 20341 8163 32959
  3. 93021 16284 20341 32959
  4. none of the above

Walkthrough:

So, we have an array of 6 ZIP-codes in string format. We iterate through each one and print it, but first we do a sanity check. Let’s walk through each number and see if it passes.

  • 93021 – This will return a number and it will be greater than 0, so “93021″ will be written.
  • 16284 – This will return a number and it will be greater than 0, so “16284″ will be written.
  • 02392- This number has a leading 0, but parseInt() will just trim it. So, this will also return a number greater than 0, and “2392″ will also be printed (Note: no leading 0).
  • 20341 – This will return a number and it will be greater than 0, so “20341″ will be written.
  • 08163 – The same as the other number with a leading 0, parseInt() will trim this and return valid number, so “8163″ will be printed (Note again: no leading 0).
  • 32959 – This will return a number and it will be greater than 0, so “32959″ will be written.

Therefore, we should see all six numbers printed, with the numbers that have trailing 0′s being trimmed. So, my answer is b, this code will print the string “93021 16284 2392 20341 8163 32959″.

 

*SPOILER ALERT – ANSWER BELOW*


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Answer:

If you chose a, you were close. The answer is actually d – none of the above. This is because the ECMAScript standard does not define the interpretation of certain number literals, in particular number literals with a leading 0. Some JavaScript implementations will interpret these as integers with the leading 0 trimmed. Others will treat them as octal literals. It should be noted, though, that MOST JavaScript interpreters will perform the latter and interpret number literals with leading 0′s as octal literals. This is different from other languages, like Java’s Integer.parseInt(), which will conversely trim the leading 0.

So what actually gets printed? In most browsers, including IE 8, Firefox 3 and Chrome, the string “93021 16284 19 20341 32959″ gets printed (Notice the 19!). Let’s examine this step-by-step one more time…

  • 93021 – There is no leading 0, and so this number will get interpreted just as we expect, as the integer 93021, and so “93021″ will be written.
  • 16284 – Again, no leading 0, so no surprises here. “16284″ will be written.
  • 02392- This number has a leading 0, and so it will be interpreted as an octal literal. Notice, though, that the second least-significant-digit is a 9, which is not allowed in an octal number! And so, by the specification of parseInt(), it will read all digits until an illegal character is read. Therefore, parseInt() will parse the octal value 23, which equals ((2 x 8) + (3 * 1)) = (16 + 3) = 19 in decimal.
  • 20341 – This will return a number and it will be greater than 0, so “20341″ will be written.
  • 08163 – Again, this number has a leading 0. But, this time, the most-significant-digit is an 8, which is also not allowed in an octal number. So, parseInt() will interpret the octal value 0, and return 0. This fails the second part of our sanity check (that our ZIP-code must be non-zero), and so this doesn’t get printed!
  • 32959 – This will get printed as is. No surprise here either.

Moral:

Whenever possible, do not write an integer with a leading zero. Doing so can cause unexpected behaviour across browsers with different JavaScript implementations. If you can’t control the input values (as in our example above), then make use of parseInt()’s second optional parameter, radix – e.g. use parseInt(“08163″, 10) to parse “08163″ to a base-10 integer. Passing in the radix explicitly eliminates the variability between browser implementations.

That’s it for this JavaScript Puzzler! Until next time, happy coding!

Charles

javascript-puzzler-lets-print-some-zip-codes.zip (source code)

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