A Simple Explanation of React.useEffect()

I am impressed by the expressiveness of React hooks. You can do so much by writing so little.

But the brevity of hooks has a price — they’re relatively difficult to get started. Especially useEffect() — the hook that manages side-effects in functional React components.

In this post, you’ll learn how and when to use useEffect() hook.

1. useEffect() is for side-effects

A functional React component uses props and/or state to calculate the output. If the functional component makes calculations that don’t target the output value, then these calculations are named side-effects.

Examples of side-effects are fetch requests, manipulating DOM directly, using timer functions like setTimeout(), and more.

Normally the component rendering and side-effect invocation logic are independent.

It would be a mistake to perform side-effects directly in the body of the functional component. How often the component renders isn’t something you can control — if React wants to render the component, you cannot stop it.

function Greet({ name }) {
  const message = `Hello, ${name}!`; // Calculates output

  // Bad!
  document.title = 'Greetings page'; // Side-effect!
  return <div>{message}</div>;       // Calculates output
}

How to decouple rendering from side-effect? Welcome useEffect() — the hook that runs side-effects independently of rendering.

import React, { useEffect } from 'react';

function Greet({ name }) {
  const message = `Hello, ${name}!`;   // Calculates output

  useEffect(() => {
    // Good!
    document.title = 'Greetings page'; // Side-effect!  }, []);

  return <div>{message}</div>;         // Calculates output
}

useEffect() hook accepts 2 arguments:

useEffect(callback[, dependencies]);

• callback is the callback function containing side-effect logic. useEffect() executes the callback function after React has committed the changes to the screen.
• dependencies is an optional array of dependencies. useEffect() executes callback only when the dependencies have changed between renderings.

dependencies array lets you control when the side-effect runs. When dependencies are:

• Not provided: the side-effect runs after each rendering
• An empty array []: the side-effect runs once after the initial rendering
• Has props or state values [prop1, prop2, ..., state1, state2]: the side-effect runs only when any value in the dependencies change.

In simple words, put the side-effect logic inside the callback argument, and use dependencies argument to control when the side-effect should run. That’s the sole purpose of useEffect().

2. Side-effect on component did mount

There are side-effects that you’d like to invoke once after the mounting of the component.

To do so, indicate an empty dependencies array [] to useEffect(..., []):

import { useEffect } from 'react';

function Greet({ name }) {
  const message = `Hello, ${name}!`;

  useEffect(() => {
    // Run once, after mounting
    document.title = 'Greetings page';
  }, []);
  return <div>{message}</div>;
}

useEffect(..., []) was supplied with an empty array as dependencies argument. When configured in such a way, the useEffect() is going to execute the callback just once, after initial mounting.

Even if the component re-renders with different name property, the side-effect runs only once after the first render:

// First render
<Greet name="Eric" />   // Side-effect RUNS

// Second render, name prop changes
<Greet name="Stan" />   // Side-effect does NOT RUN

// Third render, name prop changes
<Greet name="Butters"/> // Side-effect does NOT RUN

3. Side-effect on component did update

Each time the side-effect uses props or state values, you must indicate these values as dependencies:

import React, { useEffect } from 'react';

function MyComponent({ prop }) {
  const [state, setState] = useState();

  useEffect(() => {
    // Side-effect uses `prop` and `state`
  }, [prop, state]);
  return <div>....</div>;
}

The useEffect(callback, [prop, state]) invokes the callback after the changes are being committed to DOM and if and only if any value in the dependencies array [prop, state] have changed.

Using the dependencies argument of useEffect() you control when to invoke the side-effect, independently from the rendering cycles of the component. Again, that’s the essence of useEffect() hook.

Let’s improve the Greet component by using name prop in the document title:

import { useEffect } from 'react';

function Greet({ name }) {
  const message = `Hello, ${name}!`;

  useEffect(() => {
    document.title = `Greetings to ${name}`; 
  }, [name]);
  return <div>{message}</div>;
}

name prop is mentioned in the dependencies argument of useEffect(..., [name]). useEffect() hook runs the side-effect after initial rendering, and on later renderings only if the name value changes.

// First render
<Greet name="Eric" />   // Side-effect RUNS

// Second render, name prop changes
<Greet name="Stan" />   // Side-effect RUNS

// Third render, name prop doesn't change
<Greet name="Stan" />   // Side-effect does NOT RUN

// Fourth render, name prop changes
<Greet name="Butters"/> // Side-effect RUN

4. Fetching data

useEffect() can perform data fetching side-effect.

The following component FetchEmployeesByQuery fetches the employees list over the network. The query prop filters the fetched employees:

import React, { useState } from 'react';
import { fetchEmployees } from "./fetchEmployees";

function FetchEmployeesByQuery({ query }) {
  const [employees, setEmployees] = useState([]);

  useEffect(() => {
    const fetch = async () => {
      setEmployees(await fetchEmployees(query));
    };
    fetch();
  }, [query]);

  return (
    <div>
      {employees.map(name => <div>{name}</div>)}
    </div>
  );
}

Inside <FetchEmployeesByQuery query="query">, useEffect() starts a fetch request by calling the asynchronous function fetch() right after the initial mounting.

When the request completes, setEmployees(await fetchEmployees(query)) updates the component state with the newly fetched employees list.

On later renderings, if the query prop changes, useEffect() hook starts a new fetching process for a new query value.

To run only once the fetch request when the component mounts, simply indicate an empty dependencies list: useEffect(fetchSideEffect, []).

5. Side-effect cleanup

There are side-effects that need cleanup. useEffect() invokes the clean up function you return from the callback function:

useEffect(() => {
  // Side-effect...

  return function cleanup() {    // Side-effect cleanup...  };}, dependencies);

Cleanup works the following way:

A) After initial rendering, useEffect() invokes the callback having the side-effect. cleanup function is not invoked.

B) On later renderings, before invoking the next side-effect, useEffect() is going to invoke the cleanup function from the previous side-effect execution (to clean up everything after the previous side-effect), then runs the current side-effect.

C) Finally, after unmounting the component, useEffect() invokes the cleanup function from the latest side-effect.

For example, let’s log a message to console every 3 seconds:

import React, { useEffect } from 'react';

function RepeatMessage({ message }) {
  useEffect(() => {
    setInterval(() => {
      console.log(message);
    }, 1000);
  }, [message]);

  return <div>I'm logging to console "{message}"</div>;
}

Open the demo and type different messages — the console logs every 3 seconds each message ever typed.

You need to stop the logging of previous messages. That’s the right case to cleanup the side-effect: cancel the previous timer when starting a new one.

Let’s return a cleanup function that clears the previous timer:

import React, { useEffect } from 'react';

function RepeatMessage({ message }) {
  useEffect(() => {
    const id = setInterval(() => {
      console.log(message);
    }, 3000);
    return () => {      clearInterval(id);    };  }, [message]);

  return <div>I'm logging to console "{message}"</div>;
}

Open the demo and type some messages: only the latest message logs to console.

6. Conclusion

useEffect(callback, dependencies) is the hook that manages the side-effects in functional components. callback argument is the place to put the side-effect logic. dependencies is a list of dependencies of your side-effect: being props or state values.

useEffect() will make sure to invoke the callback after initial mounting, and on later renderings if any value inside dependencies has changed.

Because useEffect() hook heavily relies on closures, you might need to get them well too. Also be aware of stale closures issue.

Still have questions about useEffect() hook? Ask in the comments below!