chore(runtime-core): use new Array.fill to populate array

[pikax]

using Array.fill seems a bit faster than looping.

Array.fill is not supported on IE maybe use a polyfill when the time comes?

[CyberAP]

using Array.fill seems a bit faster than looping.

It's faster because Array length is settled.

[yyx990803]

This is even faster (and won't require a polyfill when we do IE11 support):

const newIndextoOldIndexMap = new Array(toBePatched)
for (i = 0; i < toBePatched; i++) newIndexToOldIndexMap[i] = 0

[pikax]

on chrome seems not to be as fast 🤔

https://jsperf.com/array-generate-and-populate

EDIT:

chrome:

safary:

[CyberAP]

This is even faster (and won't require a polyfill when we do IE11 support):

const newIndextoOldIndexMap = new Array(toBePatched)
for (i = 0; i < toBePatched; i++) newIndexToOldIndexMap[i] = 0

Kinda equal https://jsperf.com/array-fill-vs-loop

[pikax]

I'm concern on how our benchmarks are yielding different results 🤔

[CyberAP]

I'm concern on how our benchmarks are yielding different results 🤔

You last test assigns to window.i

[CyberAP]

By the way right now fill is hilariously slow in Firefox.

Maybe it'd be better to extract that into a shared function? Although I don't know if there are any more places where a fixed array is filled.

[yyx990803]

@pikax your tests are using a global i, which incurs a global variable check on every i access.

There's no other places that need this so no need for extraction.

[pikax]

Nice catch, I copied the code from the source code, probably better as you said @yyx990803

[vue-bot]

Hey @pikax, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚