useEffect when all dependencies have changed?

Question

Currently useEffect is fired when just one of the dependencies have changed.

How could I update it / use it to fire back when both ( or all ) of the dependencies have changed?

Answer 1

You'll need to add some logic to call your effect when all dependencies have changed. Here's useEffectAllDepsChange that should achieve your desired behavior.

The strategy here is to compare the previous deps with the current. If they aren't all different, we keep the previous deps in a ref an don't update it until they are. This allows you to change the deps multiple times before the the effect is called.

import React, { useEffect, useState, useRef } from "react";

// taken from https://usehooks.com/usePrevious/
function usePrevious(value) {
  const ref = useRef();

  useEffect(() => {
    ref.current = value;
  }, [value]);
  
  return ref.current;
}

function useEffectAllDepsChange(fn, deps) {
  const prevDeps = usePrevious(deps);
  const changeTarget = useRef();

  useEffect(() => {
    // nothing to compare to yet
    if (changeTarget.current === undefined) {
      changeTarget.current = prevDeps;
    }

    // we're mounting, so call the callback
    if (changeTarget.current === undefined) {
      return fn();
    }

    // make sure every dependency has changed
    if (changeTarget.current.every((dep, i) => dep !== deps[i])) {
      changeTarget.current = deps;

      return fn();
    }
  }, [fn, prevDeps, deps]);
}

export default function App() {
  const [a, setA] = useState(0);
  const [b, setB] = useState(0);

  useEffectAllDepsChange(() => {
    console.log("running effect", [a, b]);
  }, [a, b]);

  return (
    <div>
      <button onClick={() => setA((prev) => prev + 1)}>A: {a}</button>
      <button onClick={() => setB((prev) => prev + 1)}>B: {b}</button>
    </div>
  );
}

An alternate approach inspired by Richard is cleaner, but with the downside of more renders across updates.

function useEffectAllDepsChange(fn, deps) {
  const [changeTarget, setChangeTarget] = useState(deps);

  useEffect(() => {
    setChangeTarget(prev => {
      if (prev.every((dep, i) => dep !== deps[i])) {
        return deps;
      }

      return prev;
    });
  }, [deps]);

  useEffect(fn, changeTarget);
}