Why is the cleanup function from `useEffect` called on every render?

React performs the cleanup when the component unmounts.

I'm not sure where you read this but this statement is incorrect. React performs the cleanup when the dependencies to that hook changes and the effect hook needs to run again with new values. This behaviour is intentional to maintain the reactivity of the view to changing data. Going off the official example, let's say an app subscribes to status updates from a friends' profile. Being the great friend you are, you are decide to unfriend them and befriend someone else. Now the app needs to unsubscribe from the previous friend's status updates and listen to updates from your new friend. This is natural and easy to achieve with the way useEffect works.

 useEffect(() => { 
    chatAPI.subscribe(props.friend.id);

    return () => chatAPI.unsubscribe(props.friend.id);
  }, [ props.friend.id ])

By including the friend id in the dependency list, we can indicate that the hook needs to run only when the friend id changes.

In your example you have specified the array in the dependency list and you are changing the array at a set interval. Every time you change the array, the hook reruns.

You can achieve the correct functionality simply by removing the array from the dependency list and using the callback version of the setState hook. The callback version always operates on the previous version of the state, so there is no need to refresh the hook every time the array changes.

  useEffect(() => {
    const id = setInterval(() => setArray(array => [ ...array, "hello" ]), 3000);

    return () => {
      console.log("unmount");
      clearInterval(id);
    };
  }, []);

Some additional feedback would be to use the id directly in clearInterval as the value is closed upon (captured) when you create the cleanup function. There is no need to save it to a ref.

The React docs have an explanation section exactly on this.

In short, the reason is because such design protects against stale data and update bugs.

The useEffect hook in React is designed to handle both the initial render and any subsequent renders (here's more about it).

---

Effects are controlled via their dependencies, not by the lifecycle of the component that uses them.

Anytime dependencies of an effect change, useEffect will cleanup the previous effect and run the new effect.

Such design is more predictable - each render has its own independent (pure) behavioral effect. This makes sure that the UI always shows the correct data (since the UI in React's mental model is a screenshot of the state for a particular render).

The way we control effects is through their dependencies.

To prevent cleanup from running on every render, we just have to not change the dependencies of the effect.

In your case concretely, the cleanup is happening because array is changing, i.e. Object.is(oldArray, newArray) === false

useEffect(() => {
  // ...
}, [array]);
//  ^^^^^ you're changing the dependency of the effect

You're causing this change with the following line:

useEffect(() => {
  const id = setInterval(() => {
    setArray(array.concat("hello")); // <-- changing the array changes the effect dep
  }, 3000);
  myRef.current = id;

  return () => {
    clearInterval(myRef.current);
  };
}, [array]); // <-- the array is the effect dep