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Why push method is significantly slower than putting values via array indices in Javascript

I don't understand why this test :

http://jsperf.com/push-method-vs-setting-via-key

Shows that

 a.push(Math.random());

is over ten times slower than

 a[i] = Math.random();

Could you explain why this is the case ? What magic "push" do that make it so slow ? (or so slow compared to other valid method of doing that).

EDIT

NOTE: The push test is biased. I increase size of the array every iteration! Read carefully accepted answer!

Benchmark results

Solution

  • Could you explain why this is the case?

    Because your test is flawed. The push does always append to the existing a array making it much larger, while the second test does only use the first 1000 indices. Using the setup is not enough here, you would have to reset the a array before every for-loop: http://jsperf.com/push-method-vs-setting-via-key/3.

    Apart from that, the method call to push might have a little overhead, and determining the current array length might need additional time compared to using the index of the for-loop.

    Usually there is no reason not to use push - the method is there for exactly that operation and makes some code easier to read. While a few people think one version is faster than the other, both are equally optimized in browsers. See Why is array.push sometimes faster than array[n] = value? and Using the push method or .length when adding to array? - results vary so wide that it's actually irrelevant. Use what is better to understand.