Javascript perf to clean an array

Question

I just took a look to that:

http://jsperf.com/array-destroy/32

I don't understand how the first one:

arr.length = 0;

Can be slower than:

while (arr.length > 0) {
  arr.shift();
}

Someone could link/explain why?

Answer 1

In the test setup, a large array is created. Once the test begins, the array is emptied, and the test repeats itself. However, every time after the first run of the test, the array is already empty. To perform this test accurately, you have to create a new array each time. Try this:

http://jsperf.com/array-destroy/67

I modified the test to return a new array each time. The results are as expected. splice and length are fastest because they instantly modify the length of the array without a loop.