eslint react-hooks/rulesOfHooks: add rule to check for non-function return from useEffect

[jlbooker]

Feature request: Could we add an eslint rule to eslint-plugin-react-hooks to show an error when a non-function value is returned from within a useEffect call?

For the function passed to the useEffect hook, valid return statements are:
return; (no value)
return undefined; (unnecessarily verbose, but still valid)
return () => { [...cleanup function... ] }

Returning anything else will cause the error: Uncaught TypeError: destroy is not a function at runtime.

The dev UX of this isn't great, as it can be hard to find the return statement from the backtrace, even with source maps enabled. Particularly in regards to upgrading to React 18 -- It seems React 17 was more tolerant of incorrect return values, but as I'm upgrading a codebase to React 18, these errors are now coming to light.

React version: 18+

[vkurchatkin]

This has nothing to do with rules of hooks. What you want is type checking, that is what TypeScript is for

[jlbooker]

@vkurchatkin Yes, a realize a full and complete solution to this requires a type checking system to fully evaluate the return type. However, it seems there are at least a few cases we could check for with lint rules, and that would handle 95% of the basic cases. For example, I came upon uses return "none"; and return null;, which are clearly not allowed and easily caught by a lint rule. Anything other than a bare return, undefined, or an arrow function is probably an error.

[noahtallen]

I opened a related issue in the DT repo, for reference: DefinitelyTyped/DefinitelyTyped#65890. Unfortunately, the types as they exist aren't strong enough to prevent crashes.

[noahtallen]

Looks like it is simply impossible to catch this type of issue with TS, which is suuuper unfortunate.

Honestly, I really wish that React didn't crash here. IMO, it's unlikely that most expect a return value to do anything at all. For example, inline callbacks:

// returns whatever `doSomething` returns, or false, but I wouldn't expect the return value to do anything

useEffect( () => checkSomething && doSomething() );

[maximejen]

Any update on this issue ?
With React 18 it looks like return undefined; or doing a return ; is also crashing, which was not the case in React 17.
We have a huge code base, and there is no way I can do a regexp that complicated, and a global search of all useEffects is not even an option.

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