What problem does this feature solve?

I propose a performance optimization.

Applications use plenty of arrays. Full array iteration is extremely common:

• v-for performs a for (i = 0; i < length; i ) array[i] loop when rendering;
• computed or watch often call .filter(), .sort(), .map() and co. All of these result in a full iteration;
• user code can also do a for loop or call .forEach().

This top use-case is tracked like any normal object, key by key. If I have a reactive array containing 100 items, then calling .filter in a computed will result in 102 new tracking entries: one for filter, one for length and 100 more from [0] to [99].
Each one results in a Set allocation one entry in the reverse deps array the required upkeep every time the effect runs again.

Because both (1) full array iteration is very common; and (2) arrays can be quite big, making tracking expensive; I think this may be worthy of a special optimization.

Here's one idea: detect full enumeration and only register a single symbol ALL_KEY in response.
It could work like this:

• Add arrayRanges: Map<Array, number> on effect.
• When track is called on an array target, if the key is numeric:
  • if target is not in activeEffect.arrayRanges and key is 0, add it with value 0.
  • if target is in arrayRanges and key === arrayRanges.get(target) 1 then increment arrayRanges.
• At the end of run:
  • for each arrayRanges that is equals to target.length - 1, create a dependency with symbol ALL_KEY.
  • Clear arrayRanges
• When calling trigger, if target is an array and key is numeric, additionally trigger ALL_KEY.

What does the proposed API look like?

No new public API