In JS, the size of the Array is not directly related to the index keys
of the Array. In this respect, JS is memory conservative. So, you
can always pre-allocate an array of 50 entries and until you add a
51st entry, the runtime shouldn't have to grow the Array which will
then (often) make it sparse. In v8, however, you can trigger sparse
behavior (or hashed behavior) by using poor (sub-optimal) indexing as
well.
```
// Memory dense, sub-optimal access array
var i, a = new Array( 50 );
for( i = 0; i < 50; i++ )
{
a[ i * 50 ] = ( i + 1 ) * 50;
}
```
v8 initially expects that `a` will have indexes [0...49]. It's smart
enough to realize that it doesn't need to allocate more memory but
array accesses are forever slower once your second index becomes 50 in
the above code.
Also, as you saw, you can hint to runtime your intentions for the
Array but touching `.length`. This helps when you need to grow the
array but hurts if you truncate it. v8 reverts to sparse behavior in
the truncation case.
On Sat, Feb 20, 2016 at 7:11 AM, Michael Zhou
<[email protected]> wrote:
> From this perf https://jsperf.com/preallocating-array/27, using
> pre-allocated Array(size) seems the fastest.
>
> On Saturday, February 20, 2016 at 8:57:20 AM UTC-5, Dean McNamee wrote:
>>
>> https://youtu.be/UJPdhx5zTaw?t=17m43s
>>
>> Not sure if that advice still stands as it's a few years old, but my
>> experience has been, in the general case, it is unfortunately faster
>> to allocate an empty array and use push() than to preallocate an array
>> of the desired size and populate it. I have notes above a utility
>> function I wrote that said I measured a 2x improvement in using push
>> over Array(size), but I can't tell you now for what sort of size
>> values that was.
>>
>> As always, just try it and measure. I've also found using some of the
>> native methods helps in the process to understand why the performance
>> difference is there.
>>
>> You can probably use something like the native %HasFastProperties() to
>> check (this means running with --allow_natives_syntax).
>>
>> There is also %_HasFastPackedElements() in Hydrogen, but I didn't look
>> to see what the difference is.
>>
>> On Sat, Feb 20, 2016 at 1:05 PM, Michael Zhou
>> <[email protected]> wrote:
>> > If I have
>> >
>> > var arr = new Array(50); // Should be dense, I guess?
>> > arr[20] = 0;
>> > arr[40] = 1;
>> > arr[20] = undefined;
>> > var keys = Object.keys(arr);
>> >
>> > Will this access pattern make v8 think arr is a sparse array and change
>> > to
>> > using a hash table?
>> >
>> > Also, is copying from old back store to new back store unavoidable when
>> > I
>> > grow the array by doing
>> > for (var i = 50; i < 100; i++) {
>> > arr[i] = 0;
>> > }
>> >
>> > ? Thanks.
>> >
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