[Feature Request]: Add array of updated deps indices to useEffect hooks arg

[myckhel]

I would like to request adding the indices of updated dependencies of useEffect, useLayoutEffect as arg of the callback.

For example:

if dep1 and dep3 changes, then indices of the dependencies that triggers the update should be passed to the callback as argument.

useEffect((updatedIndices) => {

  console.log(updatedIndices); // Array [1,3]

}, [dep1, dep2, dep3])

I think this feature will be useful for most of us all.

This is just one example use case

Allow component modify prop internally

Good

const InternalState = ({value, checked}) => {
  const [_checked, setChecked] = useState(checked)
  const [_value, setValue] = useState(value)
  
  // better
  useEffect((indices) => {
    if(indices === 1){
      setValue(value + ' appends');
     }
    
     if(indices === 2){
      setChecked(checked + ' appends');
     }
  }, [value, checked])

}

Can improve

  useEffect(() => {
      setValue(value + ' appends');
  }, [value])
  
  // can improve
  useEffect(() => {
      setChecked(checked + ' appends')
  }, [checked])

Bad

  useEffect(() => {
    // possible unnecessary state update if value not changed
    setValue(value + ' appends');

    // possible unnecessary state update if checked not changed
    setChecked(checked + ' appends');
  }, [value, checked])

My actual use case would be too complicated to show here.

[gaearon]

Hi, thanks for your suggestion. RFCs should be submitted as pull requests, not issues. I will close this issue but feel free to resubmit in the PR format.