React setState hook from scroll event listener

In your code I see several issues:

1) [] in useEffect means it will not see any changes of state, like changes of goingUp. It will always see initial value of goingUp

2) debounce does not work so. It returns a new debounced function.

3) usually global variables is an anti-pattern, thought it works just in your case.

4) your scroll listener is not passive, as mentioned by @skyboyer.

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

const App = () => {
  const prevScrollY = useRef(0);

  const [goingUp, setGoingUp] = useState(false);

  useEffect(() => {
    const handleScroll = () => {
      const currentScrollY = window.scrollY;
      if (prevScrollY.current < currentScrollY && goingUp) {
        setGoingUp(false);
      }
      if (prevScrollY.current > currentScrollY && !goingUp) {
        setGoingUp(true);
      }

      prevScrollY.current = currentScrollY;
      console.log(goingUp, currentScrollY);
    };

    window.addEventListener("scroll", handleScroll, { passive: true });

    return () => window.removeEventListener("scroll", handleScroll);
  }, [goingUp]);

  return (
    <div>
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
      <div style={{ background: "orange", height: 100, margin: 10 }} />
    </div>
  );
};

export default App;

https://codesandbox.io/s/react-setstate-from-event-listener-q7to8

In short, you need to add goingUp as the dependency of useEffect.

If you use [], you will only create/remove a listener with a function(handleScroll, which is created in the initial render). In other words, when re-render, the scroll event listener is still using the old handleScroll from the initial render.

  useEffect(() => {
    window.addEventListener("scroll", handleScroll);
    return () => window.removeEventListener("scroll", handleScroll);
  }, [goingUp]);

Using custom hooks

I recommend move the whole logic into a custom hooks, which can make your code more clear and easy to reuse. I use useRef to store the previous value.

export function useScrollDirection() {
  const prevScrollY = useRef(0)
  const [goingUp, setGoingUp] = useState(false);

  const handleScroll = () => {
    const currentScrollY = window.scrollY;
    if (prevScrollY.current < currentScrollY && goingUp) {
      setGoingUp(false);
    }
    if (prevScrollY.current > currentScrollY && !goingUp) {
      setGoingUp(true);
    }
    prevScrollY.current = currentScrollY;
  };

  useEffect(() => {
    window.addEventListener("scroll", handleScroll);
    return () => window.removeEventListener("scroll", handleScroll);
  }, [goingUp]); 
  return goingUp ? 'up' : 'down';
}