React Hook useCallback received a function whose dependencies are unknown. Pass an inline function instead

user_3963067 0
Viewed 11851

Consider this example:

 let memoizedCb = React.useCallback(
    memoize((param) => () => someFunction(param)),
    []
  );

where memoize is from external library like "fast-memoize". Above code gives warning:

React Hook useCallback received a function whose dependencies are unknown. Pass an inline function instead

I found this thread which if we adapt to my use case would suggest this as solution (if I am not wrong):

const memoizedCb = React.useMemo(
  () => memoize((param) => () => someFunction(param)),
  []
);

What is the warning about? why does useMemo fix this problem?

NOTE: someFunction is defined outside the function component so it is not needed as a dependency.

usecallback react-hooks reactjs javascript
4 Answers

It seems the warning is there because useCallback (and also useMemo see below) expect inline function as argument (don't know why though).

So in the solution from the question:

const memoizedCb = React.useMemo(
  () => memoize((param) => () => someFunction(param)),
  []
);

they used useMemo to imitate useCallback functionality while also passing an inline function to useMemo as the warning required:

React Hook useCallback received a function whose dependencies are unknown. Pass an inline function instead

That warning is not specific to useCallback, you get same warning if you just replace useCallback with useMemo in the first example:

 // For testing
 // Gives same warning
 let memoizedCb = React.useMemo(
    memoize((param) => () => someFunction(param)),
    []
 );
user_39630670

As a more general explanation the below will produce a warning:

const someFuncWrap = (fn) => (e) => fn(e)
const memoizedFunc = useCallback(someFuncWrap(foo), [someFuncWrap, foo]);

Changing the useCallback to a useMemo with an inline function passes.

const someFuncWrap = (fn) => (e) => fn(e)
const memoizedFunc = useMemo(() => someFuncWrap(foo), [someFuncWrap, foo]);

This is because to parse the hook eslint needs to see it in the following form:

useMemo(() => foo, [foo]);
useCallback((bar) => foo(bar), [foo]);
user_102598130

As far as I know, the useCallback expects an inline function. If you pass a returned function from another function, eslint won't be able to figure out what are the dependancies of the function, so it will show this warning.

Here I think, eslint is failing to determine the dependancies of somefunction and thus it cannot evaluate the react-hooks/exhaustive-deps rule because to evaluate this rule eslint should be able to identify is there any dependancy like state, props for the function passed to useCallback hook. So eslint is asking you to pass an inline function which it will understand and can evalute for lint rules.

user_79175870

From React docs

useCallback will return a memoized version of the callback that only changes if one of the dependencies has changed. This is useful when passing callbacks to optimized child components that rely on reference equality to prevent unnecessary renders

You are seeing the warning because you are not passing any dependencies when using useCallback hook.

Also,

useCallback(fn, deps) is equivalent to useMemo(() => fn, deps).

user_70043880
Related