Heads up on Javascript loop performance in Prototype

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.

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

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.

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