Monday, July 7, 2008

Back from PageForest Haitus

That was a long time between posts. But is now up and running, and in the capable hands of Zach who's dealing with the day-to-day organization, so I'm getting to spend more time on PageForest.

I've also switched my application platform to Google AppEngine - I'm having a grand time learning Python and doing a bunch more JavaScript development.

I've spent quite a bit of time building on my JavaScript unit testing framework. I'm open-sourcing that code, and I've made some significant improvements during PageForest development, so I'll update the core project soon (hosted on Google Code).

I've been working so much in Firefox, I hadn't noticed that IE testing was getting stale. In general, the different browsers are very compatible when it comes to JavaScript - most of the platform differences are in HTML, CSS, and eventing models. The core language is very standardized. But I have hit one major gotcha when it comes to IE JavaScript (aka JScript): handling of empty elements at the end of an array definition.

As a (now) Python programmer, it's very common to define an (open ended) array as:

a = [1,2,3, ];
Note the dangling comma with no trailing element. In Firefox (and Python), this creates a 3 element array - the final comma is ignored. But, in IE, this creates a 4 element array with undefined as the 4th element.

I had used this paradigm extensively, w/o really realizing it. So when I switched to testing on IE, it took me quite a while to track down all the subtle bugs based on extra undefined elements added to my arrays.

JavaScript SHOULD be well enough defined that one of Firefox or IE is INCORRECT in their implementation of arrays (and I hope it's IE - since it's nice to be able to leave placeholders for future array elements in array definitions). Others have experience this problem too; the mozilla JavaScript documentation explicitly allows for it:

If you include a trailing comma at the end of the list of elements, the comma is ignored.
According to my reading of the ECMA-262 spec IE is non-conforming. Section 11.1.4 states:

Whenever a comma in the element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined.

so, this definition:

a = [1,,2,];

should be equivalent to:

a = [1,undefined,2]; // Firefox - correct

but NOT:

a = [1,undefined,2,undefined]; // IE - incorrect

Even worse, a trailing comma at the end of an Object initializer is fatal in IE. This statement:

o = {a:1, b:2,};

produces a Runtime Error ("Error: Expected identifier, string or number") and your code will not execute! Here, it may be that IE is actually behaving correctly (according to spec), but it's still rather annoying.

You can see how your browser performs by visiting dangle.htm.