What's useEffect execution order and its internal clean-up logic in react hooks?

One thing that's not clear in the above answers is the order in which the effects run when you have multiple components in the mix. We've been doing work that involves coordination between a parent and it's children via useContext so the order matters more to us. useLayoutEffect and useEffect work in different ways in this regard.

useEffect runs the clean up and the new effect before moving to the next component (depth first) and doing the same.

useLayoutEffect runs the clean ups of each component (depth first), then runs the new effects of all components (depth first).

render parent
render a
render b
layout cleanup a
layout cleanup b
layout cleanup parent
layout effect a
layout effect b
layout effect parent
effect cleanup a
effect a
effect cleanup b
effect b
effect cleanup parent
effect parent

const Test = (props) => {
  const [s, setS] = useState(1)

  console.log(`render ${props.name}`)

  useEffect(() => {
    const name = props.name
    console.log(`effect ${props.name}`)
    return () => console.log(`effect cleanup ${name}`)
  })

  useLayoutEffect(() => {
    const name = props.name
    console.log(`layout effect ${props.name}`)
    return () => console.log(`layout cleanup ${name}`)
  })

  return (
    <>
      <button onClick={() => setS(s+1)}>update {s}</button>
      <Child name="a" />
      <Child name="b" />
    </>
  )
}

const Child = (props) => {
  console.log(`render ${props.name}`)

  useEffect(() => {
    const name = props.name
    console.log(`effect ${props.name}`)
    return () => console.log(`effect cleanup ${name}`)
  })

  useLayoutEffect(() => {
    const name = props.name
    console.log(`layout effect ${props.name}`)
    return () => console.log(`layout cleanup ${name}`)
  })

  return <></>
}

Put these three lines of code in a component and you'll see their order of priority.

  useEffect(() => {
    console.log('useEffect')
    return () => {
      console.log('useEffect cleanup')
    }
  })

  window.requestAnimationFrame(() => console.log('requestAnimationFrame'))

  useLayoutEffect(() => {
    console.log('useLayoutEffect')
    return () => {
      console.log('useLayoutEffect cleanup')
    }
  })

useLayoutEffect > requestAnimationFrame > useEffect

The problem you're experiencing is caused by loopRaf requesting another animation frame before the cleanup function for useEffect is executed.

Further testing has shown that useLayoutEffect is always called before requestAnimationFrame and that its cleanup function is called before the next execution preventing overlaps.

Change useEffect to useLayoutEffect and it should solve your problem.

useEffect and useLayoutEffect are called in the order they appear in your code for like types just like useState calls.

You can see this by running the following lines:

  useEffect(() => {
    console.log('useEffect-1')
  })
  useEffect(() => {
    console.log('useEffect-2')
  })
  useLayoutEffect(() => {
    console.log('useLayoutEffect-1')
  })
  useLayoutEffect(() => {
    console.log('useLayoutEffect-2')
  })

There are two different hooks that you would need to set your eyes on when working with hooks and trying to implement lifecycle functionalities.

As per the docs:

useEffect runs after react renders your component and ensures that your effect callback does not block browser painting. This differs from the behavior in class components where componentDidMount and componentDidUpdate run synchronously after rendering.

and hence using requestAnimationFrame in these lifecycles works seemlessly but has a slight glitch with useEffect. And thus useEffect should to be used to when the changes that you have to make do not block visual updates like making API calls that lead to a change in DOM after a response is received.

Another hook that is less popular but is extremely handy when dealing with visual DOM updates is useLayoutEffect. As per the docs

The signature is identical to useEffect, but it fires synchronously after all DOM mutations. Use this to read layout from the DOM and synchronously re-render. Updates scheduled inside useLayoutEffect will be flushed synchronously, before the browser has a chance to paint.

So, if your effect is mutating the DOM (via a DOM node ref) and the DOM mutation will change the appearance of the DOM node between the time that it is rendered and your effect mutates it, then you don’t want to use useEffect. You’ll want to use useLayoutEffect. Otherwise the user could see a flicker when your DOM mutations take effect which is exactly the case with requestAnimationFrame

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

const {useState, useLayoutEffect} = React;

//import ReactDOM from "react-dom";

function App() {
  const [startSeconds, setStartSeconds] = useState("");
  const [progress, setProgress] = useState(0);

  useLayoutEffect(() => {
    setStartSeconds(Math.random());

    const interval = setInterval(() => {
      setStartSeconds(Math.random());
    }, 1000);

    return () => clearInterval(interval);
  }, []);

  useLayoutEffect(
    () => {
      let raf = null;

      const onFrame = () => {
        const currentProgress = startSeconds / 120.0;
        setProgress(Math.random());
        // console.log(currentProgress);
        loopRaf();
        if (currentProgress > 100) {
          stopRaf();
        }
      };

      const loopRaf = () => {
        raf = window.requestAnimationFrame(onFrame);
        // console.log('Assigned Raf ID: ', raf);
      };

      const stopRaf = () => {
        console.log("stopped", raf);
        window.cancelAnimationFrame(raf);
      };

      loopRaf();

      return () => {
        console.log("Cleaned Raf ID: ", raf);
        // console.log('init', raf);
        // setTimeout(() => console.log("500ms later", raf), 500);
        // setTimeout(()=> console.log('5s later', raf), 5000);
        stopRaf();
      };
    },
    [startSeconds]
  );

  let t = [];
  for (let i = 0; i < 1000; i++) {
    t.push(i);
  }

  return (
    <div className="App">
      <h1>Hello CodeSandbox</h1>
      <text>{progress}</text>
      {t.map(e => (
        <span>{progress}</span>
      ))}
    </div>
  );
}

ReactDOM.render(<App />,
document.querySelector("#root"));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.7.0-alpha.2/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.7.0-alpha.2/umd/react-dom.production.min.js"></script>
<div id="root"></div>