On Wednesday, March 9, 2011 5:06:52 PM UTC+1, Stamen Georgiev wrote:
>
> Just wrote a small test at jsperf for testing a for loop to iterate an
> array of objects: http://jsperf.com/if-with-i
>
> Can somebody explain why there's such a big speed difference between
> the two methods in Chrome and Safari?
> The results are almost identical in FF (11,074 vs 12,943 ops).
>
> But in Chrome I get 60,266 vs 9,494 ops and in Safari: 18,343 vs 9,509
> ops.
>
> As far as I can tell, trying to access an element of array that
> doesn't exist is kinda slow... but why? :-)
The test you point to really doesn't test the difference between caching the
length property or not, the loops structure are completely different, one
executing an AssignmentExpression in the ForExpression, and the other
reading the value of a Property in the ForExpression and executing the
AssignmentExpression in the loops body - it might seem trivial, but for the
runtime, these are not interchangeable due to optimization and so on.
But the answer is really just that the two browser use different runtimes,
and so they have different strategies to executing code - one might be
fastest at one kind of operation, while the other is fastest on another.
If you want the fastest loop, do this:
var i, obj, name;
while (obj = arr[i++]) {
name = obj.name;}
It's a whole lot faster (http://jsperf.com/if-with-i/4) due to only having a
single expression to execute in the control section (`for` having two) with the
optimization this allows for.
Sean
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