equalityF is not a function

[gshilin]

After release 7.2.3 I got error "equalityFn is not a function" in a component that beforehand worked perfect. In debugger I see that in useSelector.js in function checkForUpdates value of equalityFn is Array with one element "null":
equalityFn: Array(1)
0: null

I don't know how to reproduce it in a short example.

React: 17.0.2
ReactDOM: 17.0.2
Redux: 4.0.5
React Redux: 7.2.3

[markerikson]

Hmm. The code passes tests and looks valid when I glance at it again. It's going to be hard to investigate this without a reproduction example.

[gshilin]

Found it: for some reason I supplied an array as an equalityFn, probably that was an attempt for some optimization. And it didn't crash in 7.2.2.

[gshilin]

Its call was inside of try-catch:

try {
        var newSelectedState = latestSelector.current(store.getState());

        if (equalityFn(newSelectedState, latestSelectedState.current)) {
          return;

[markerikson]

I suppose we could add a couple dev-only checks to useSelector to verify that both selector and equalityFn are functions :)

I think what might have been happening before is that it threw an error in the store subscription, but we swallow that error and always queue up a re-render for this component. Then, we re-ran the selector while rendering, it returns a result fine, no equality check, and rendering continues as usual.

Now, same thing happens, but the equality check happens while rendering and explodes again.

[gshilin]

It would be nice to raise an exception when a useSelector is created with wrong properties. At least it would be easy to find it.

[gshilin]

This is the comment from prev version:

// we ignore all errors here, since when the component
// is re-rendered, the selectors are called again, and
// will throw again, if neither props nor store state
// changed

[markerikson]

Yep, agreed. If you've got some time, could you try filing a PR that would throw an error if either selector or equalityFn are not actually functions, in dev mode only?

[gshilin]

Yep, will try tomorrow