Heads up on Javascript loop performance in Prototype

Rails
1

For people out there are using Prototype’s enumerable methods, be aware
they
come at a large performance cost.

Some quick tests in Firefox 2 showed the performance difference for 1000
loops over a 100 item array:

var array = $R(0, 100).toArray();

for (var i = 0, length = array.length; i < length; i++)

~30ms

for (var i = 0; i < array.length; i++)

~60ms

Using Firefox’s built-in forEach array method:

array.forEach(function() {})

~230ms

And finally Prototype’s each method:

array.each(function() {})

500ms

In this case, Prototype’s each method is over 16 times slower than the
fastest for() version. Most of the time this won’t matter, but if you’re
doing anything loop-intensive, be aware!

-Jonathan.

2

Jonathan V. wrote:

For people out there are using Prototype’s enumerable methods, be aware
they come at a large performance cost.

I think this is mainly due to the try-catch block that each iteration is
surrounded by to catch the $continue and $break exceptions to simulate
proper control statements. Also the added overhead of the function call
doesn’t help matters either.

For really sensitive areas that use “each” and don’t use $break or
$continue you could use “_each” which should be a bit faster as it
doesn’t have the try-catch block. This is the internal method that
each() calls on the actual enumerable object to do its iteration. I find
the flexibility of methods like map(), findAll(), reject(), etc. that
Enumerable provides well worth the performance hit.

Eric

3

Very true, most of the time the benefits are worth the performance hit.

I didn’t include _each because it looked like it was supposed to be a
private method because of the _ prefix. Interestingly enough, _each is
actually noticeably faster than Firefox’s built-in forEach method (180ms
compared to 230ms), but still 6 times slower than the fastest for()
loop.

-Jonathan.