I had this bit of JavaScript code (never mind that I am compelled to add a

* * * * *

“undefined” vs. undefined

I had this bit of JavaScript code (never mind that I am compelled to add a
semicolon to the end of each line when it isn't needed):

> var lines = req.responseText.split("\n")
>
> for (var i = 0 ; i < lines.length ; i++)
> {
> var row = table.insertRow(-1)
> var fields = lines[i].split("\t")
> var cell;
>
> cell = row.insertCell(0)
> cell.textContent = fields[0]
>
> cell = row.insertCell(1)
> cell.textContent = fields[1]
>
> cell = row.insertCell(2)
> cell.textContent = fields[2]
> }
>

I have a text file which I've just loaded. I break it into lines, and then
each line into sub-fields, each separated by a tab (this is the format I keep
my IP (Internet Protocol) allocation files in), and then stuff the entire
thing into a large table.

So, from a text file that looks like this (where I'm showing the actual tab
and newline characters as arrows):

> 10.0.0.0→00:00:00:00:00:00→Backup network↵
> 10.0.0.1→DE:CA:FB:AD:00:01→mobybackup↵
> 10.0.0.2→DE:CA:FB:AD:00:02→dnsserver↵
> 10.0.0.3↵
> 10.0.0.4↵
> 10.0.0.5→DE:CA:FB:AD:00:03→mailserver↵
> 10.0.0.6↵
>

and I end up with something that looks like:

Table: Sample output
IP MAC (Media Access Control) Notes
10.0.0.0 00:00:00:00:00:00 Backup network
10.0.0.1 DE:CA:FB:AD:00:01 mobybackup
10.0.0.2 DE:CA:FB:AD:00:02 dnsserver
10.0.0.3 undefined undefined
10.0.0.4 undefined undefined
10.0.0.5 DE:CA:FB:AD:00:03 mailserver
10.0.0.6 undefined undefined

The undefines pop out because there's nothing else to the line except an IP
address, so fields[1] and fields[2] literally have nothing in them, not even
the empty string. They are devoid of any value whatsoever.

Okay, fine, whatever.

Now, I go save what I have in the table back to the server. So I write some
code:

> var table = document.getElementById('tdisplay')
> var textfile = ""
>
> for (var i = 0 ; i < table.rows.length ; i++)
> {
> var ip = table.rows[i].cells[0].textContent
> var mac = table.rows[i].cells[1].textContent
> var note = table.rows[i].cells[2].textContent
>
> textfile = textfile + ip + "\t" + mac + "\t" + note + "\n"
> }
>

And what do I get back?

> 10.0.0.0→00:00:00:00:00:00→Backup network↵
> 10.0.0.1→DE:CA:FB:AD:00:01→mobybackup↵
> 10.0.0.2→DE:CA:FB:AD:00:02→dnsserver↵
> 10.0.0.3→undefined→undefined↵
> 10.0.0.4→undefined→undefined↵
> 10.0.0.5→DE:CA:FB:AD:00:03→mailserver↵
> 10.0.0.6→undefined→undefined↵
>

Oh wait! That's not what I wanted! Okay …

> for (var i = 0 ; i < table.rows.length ; i++)
> {
> var ip = table.rows[i].cells[0].textContent
> var mac = table.rows[i].cells[1].textContent
> var note = table.rows[i].cells[2].textContent
>
> if (!ip) { ip = "" }
> if (!mac) { mac = "" }
> if (!note) { note = "" }
>
> textfile = textfile + ip + "\t" + mac + "\t" + note + "\n"
> }
>

And now let's see what I get:

> 10.0.0.0→00:00:00:00:00:00→Backup network↵
> 10.0.0.1→DE:CA:FB:AD:00:01→mobybackup↵
> 10.0.0.2→DE:CA:FB:AD:00:02→dnsserver↵
> 10.0.0.3→undefined→undefined↵
> 10.0.0.4→undefined→undefined↵
> 10.0.0.5→DE:CA:FB:AD:00:03→mailserver↵
> 10.0.0.6→undefined→undefined↵
>

Whoah! Wait! It looks like that doesn't work. Do some reading, oh, I need to
use the “===” equality operator instead of the “==” equality operator (shades
of Lisp here [1]).

> for (var i = 0 ; i < table.rows.length ; i++)
> {
> var ip = table.rows[i].cells[0].textContent
> var mac = table.rows[i].cells[1].textContent
> var note = table.rows[i].cells[2].textContent
>
> if (ip === undefined) { ip = "" }
> if (mac === undefined) { mac = "" }
> if (note === undefined) { note = "" }
>
> textfile = textfile + ip + "\t" + mac + "\t" + note + "\n"
> }
>

And I get …

> 10.0.0.0→00:00:00:00:00:00→Backup network↵
> 10.0.0.1→DE:CA:FE:BA:D0:01→mobybackup↵
> 10.0.0.2→DE:CA:FE:BA:D0:02→dnsserver↵
> 10.0.0.3→undefined→undefined↵
> 10.0.0.4→undefined→undefined↵
> 10.0.0.5→DE:CA:FE:BA:D0:03→mailserver↵
> 10.0.0.6→undefined→undefined↵
>

That's not working? What's going on here?

Then a lightbulb goes on. The variable note isn't undefined, it's
"undefined", as in, the string consisting of the letters u-n-d-e-f-i-n-e-d. I
changed the code used to construct the table:

> var lines = req.responseText.split("\n")
>
> for (var i = 0 ; i < lines.length ; i++)
> {
> var row = table.insertRow(-1)
> var fields = lines[i].split("\t")
> var cell;
>
> cell = row.insertCell(0)
> cell.textContent = (fields[0] !== undefined) ? fields[0] : ""
>
> cell = row.insertCell(1)
> cell.textContent = (fields[1] !== undefined) ? fields[1] : ""
>
> cell = row.insertCell(2)
> cell.textContent = (fields[2] !== undefined) ? fields[2] : ""
> }
>

And I finally got what I was expecting:

> 10.0.0.0→00:00:00:00:00:00→Backup network↵
> 10.0.0.1→DE:CA:FE:BA:D0:01→mobybackup↵
> 10.0.0.2→DE:CA:FE:BA:D0:02→dnsserver↵
> 10.0.0.3→→↵
> 10.0.0.4→→↵
> 10.0.0.5→DE:CA:FE:BA:D0:03→mailserver↵
> 10.0.0.6→→↵
>

Now, I had originally written this as an indictment against dynamically typed
languages, but really, it's not, now that I think about it. C can also have
undefined variables, but they're usually pointers, and they usually contain
NULL (less rarely, some other illegal address). I think my problem here was a
mental mismatch with what was actually going on, which I'm finding is all too
easy in a dynamically typed language like JavaScript. Was the variable
undefined because it was actually undefined, or was it "undefined"?

Would using a statically typed language helped me here? Possibly. It might
have forced the issue of “undefined” at compile time. Or possibly not (on my
system, doing something like printf("%s\n",NULL) results in “(null)” being
printed, which isn't catching the problem at compile time). But the hiding
the issue (or in this case, not crashing) makes it harder to debug such
things.

[1] gopher://gopher.conman.org/0Phlog:2006/01/26.2

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