Initializer function in `useMap()`, `useSet()`, etc.

[benface]

New Features

It would be nice if useMap(), useSet(), etc. supported an initializer function for those initial values that can be expensive to compute, e.g.

const map = useMap(Object.entries(hugeObject).filter(...).map(([key]) => [key, ''])

Why should this feature be included?

~It can be done by keeping track of whether we're rendering for the first time with a useRef() and if so, setting the values...~

Actually, I think that would trigger a re-render immediately after the first render? A better solution is probably something like this:

let initialValue = useRef()

if (initialValue.current === undefined) {
  initialValue.current = expensiveComputation()
}

const map = useMap(initialValue.current)

But that's not as nice as the first snippet, right?

Thank you!