add dependency array to examples

[donavon]

trafficstars

The usePrevious example is missing a dependency on value.

[reactjs-bot]

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[donavon]

BTW, This was prompted by this Twitter thread.

https://twitter.com/GabeRicard/status/1198316044957421568

[rimunroe]

This hook shouldn’t need a dependency array, and I don’t think we want people adding dependency arrays when they don’t need to.

Specifically: there’s no semantic reason to limit the effect to only run when the identity of the argument changes since repeated assignments of the same value have no effect.

[RebekahEloise]

The useEffect hook takes 2 arguments, a callback, and a dependency array. Therefore, it seems reasonable to include a dependency array for these two instances of useEffect.