[Feature Request] Allow dependency length to change in hooks

Not allowing dependency lists to change size limits the usefulness of useMemo in this particular use case but I imagine there are other similar use cases.

To be clear I am talking about the error triggered here:

https://github.com/facebook/react/blob/024a764310e64ef38963df16970364b9942a60cd/packages/react-reconciler/src/ReactFiberHooks.js#L322

For instance, in my app I have a bunch of items and the user can select an unlimited amount of them, in another component I want to compute an expensive derived value based on this selection that is relevant only to this component, a good use case for useMemo.

However it is not currently possible to use useMemo and I am forced to compute this derived data outside of this component even though I am only interested in doing so whilst this component is mounted.

I don’t understand why a change in dependency list length cannot be assumed to be a change in the dependencies itself?

I believe this can be implemented by changing the above to:

if (prevDeps.length !== nextDeps.length) {
    return false;
}

[fabiospampinato]

Yes I still care about this.

[jrmyio]

I agree with OP, I also don’t understand why instead of throwing an error prevDeps.length !== nextDeps.length can’t just count as a change in the dependencies itself.

I want to be able to memoize an array but spreading over the values inside the dependency would throw an error if the array changes in size.

A temporary solution could be to use JSON.stringify(arr) inside hook’s deps, but this will only work for serializable data.