Wrapping code is slow on Firefox

UPDATE: Filed as bug 576630 with Mozilla. It would be great if this slowdown can be removed, because wrapping chunks of code in a function wrapper is a widely useful tool to have available.

I just learned, to my dismay, that adding a single layer of wrapping around a body of JavaScript code can cause Firefox to really slow down. That is, there are cases where the following code takes a second to load:

statement1
statement2
...
statement1000

Yet, the following equivalent code takes 30+ seconds to load:

(function() {
statement1
statement2
...
statement1000
})()

This is disappointing, because wrapping code inside a function is a straightforward way to control name visibility. If this code defines a bunch of new functions and vars, you might not want them all to be globally visible throughout a browser window. Yet, because of this parsing problem on Firefox, simply adding a wrapper function might not be a good idea.

After some investigation, the problem only arises when there are a lot of functions defined directly inside a single other function. Adding another layer of wrapping gets rid of the parse time problem. That is, the following parses very quickly:

(function() {
(function() {
statement1
..
statement10
})()
...
(function() {
statement991
...
statement1000
})()
})()

Of course, to use this approach, you have to make sure that the cross-references between the statements still work. In general this requires modifying the statements to install and read properties on some object that is shared among all the chunks.

Example Code and Timings

I wrote a Scala script named genslow.scala that generates two files: test.html and module.html. Load the first page in firefox, and it will cause a load of the second file into an iframe. An alert will pop up once all the code is loaded saying how long the load took.

There are three variables the top of the script that can be used to modify module.html. On my machine, I get the following timings:

default: 1,015 ms
jslink: 1,135 ms
wrapper: 34,288 ms
wrapper+jslink: 52,078 ms
wrapper+jslink+chunk: 1,188 ms

The timings were on Firefox 3.6.3 on Linux. I only report the first trial in the above table, but the pattern is robust across hitting reload.