Question

I am testing different methods to initialise a large javascript array with zeros. So far a simple for loop with push(0) seems to outperform the other approaches by far and large (see http://jsperf.com/initialise-array-with-zeros), but I am having doubts about the validity of this test.

In practice you would create such a large array only once and cache it, so that later when you need a large initialised array again you can simply slice it. Therefore I believe the most important evaluation is the time it takes the first time this code is executed, rather than an average over many trials.

Does anyone disagree? Or does anybody know how/where I can test the timings of only one round?

Edit: In response to some misconceptions as to the rationale of allocating an array with so many zeros I would like to clarify two things.

1. There will be no sparsity. I need to create more than one large array and use them for computations. These copies will be filled with floats and the chance for a float to be exactly zero is negligible.
2. Not all computations are performed sequentially over the array. I believe that a function that generates the array in the process would be inefficient compared to overwriting values in an array that is passed by reference (see e.g. gl-matrix.js).

My solution is therefore to create one large zero-filled array once and then take a slice() whenever a new array is needed, then pass that copy by reference to any function to work with it. Slice is super-duper-mega fast in any browser.

Now, although you may still have concerns why I want to do this, what I am really interested in is if it is at all possible to evaluate the performance of different initialisation methods at the first time run. I want to have this timing because in my situation I will certainly only run this once.

And yes, my jsperf code likely misses some solutions. So if you have an approach that I didn’t think of, please feel free to add it! Thanks!

Answer 1

Testing the operation only once is very complicated, as the performance varies a lot depending on what else the computer is doing. You would have to run that single test a lot of times, and reset to the same conditions between each test. The reason that jsperf runs the test a lot of times is to get a good average to weed out the anomalies.

You should test this in different browsers, to see which method is the best overall. You will see that you get very varying results.

In Internet Explorer, the fastest methods is actually neither of the ones you tested, but a simple loop that assigns the zeroes:

for (var i = 0; i < numzeros; i++) zeros[i] = 0;