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• downshift version: 3.2.10
• node version: 10.9.0
• npm (or yarn) version: 6.2.0

Relevant code or config

const Select = () => {
  const menuRef = useRef(null);

  return (
    <Downshift>
      {({ getMenuProps, getInputProps }) => (
        <div>
          <input {...getInputProps()} />
          <div {...getMenuProps({ ref: menuRef })}>...options</div>
        </div>
      )}
    </Downshift>
  );
};

What you did:

I passed an object-style ref to getMenuProps

What happened:

I got an error:

downshift.esm.js:236 Uncaught TypeError: fn.apply is not a function
    at downshift.esm.js:236
    at Array.forEach (<anonymous>)
    at downshift.esm.js:234
    ...

<!-- Please provide the full error message/screenshots/anything -->

Reproduction repository:

<!-- If possible, please create a repository that reproduces the issue with the minimal amount of code possible. -->

Problem description: Downshift tries to compose external refs, but it assumes all refs are functions.

Suggested solution: Augment ref composition to account for object-style refs. You can wrap Downshift's function ref in another function which calls it, then also sets externalRef.current = element (if external ref is an object) or calls external ref (if it's a function).

I'll create a PR soonish, if time allows. Otherwise this issue should help remind me that I need to.

@victormovchan TS allows you to mutate current if you type it with | null:

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react/index.d.ts#L932