Why Is New Array Slow?

when comparing the operation var fat_cats = cats.slice() to var fat_cats = new Array(cats.length) the performance differences are confusing. In firefox and chrome new Array is

Solution 1:

Figured it out by looking at the source code for V8's array functions.

If an array has more than 1000 elements and .slice is called, a function called SmartSlice is used, verses the SimpleSlice function used otherwise.

SimpleSlice is implemented as a for loop copy (exactly the same as the code in the array copy test case). SmartSlice, on the other hand, uses sparse arrays to represent the data.

In a test case where the number of elements is dropped from 10,000 to below 1000, they are exactly the same performance (within the margin of error), whereas in a better-controlled test case with fewer variation and more than 1000 elements, the SmartSlice method is ~36% faster than the naïve copy.

---

While this explains the V8 side of things perfectly, I do not know why Firefox is also slower on new arrays than sliced arrays, even at smaller sizes - unless they have a similar optimization in place (perhaps for all slice functions).

EDIT

This kept bugging me, so I downloaded the Firefox source and checked out js/src/jsarray.cpp!array_slice, and Firefox does have a similar optimization: the result of .slice is a DenseCopiedArray or DenseAllocatedArray, which is apparently similar to the V8 sparse array.