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.

The component rendering and side-effect logic are independent. So it would be a mistake to perform side-effects directly in the body of the 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 the side-effect? Welcome useEffect() — the hook that runs side-effects independently of rendering.

import { 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 if the dependencies have changed between renderings.

Put your side-effect logic into the callback function, then use the dependencies argument to control when you want the side-effect to run. That’s the sole purpose of useEffect().

2. The dependencies of useEffect()

dependencies argument of useEffect(callback, dependencies) lets you control when the side-effect runs. When dependencies are:

A) Not provided: the side-effect runs after every rendering.

import { useEffect } from 'react';

function MyComponent() {
  useEffect(() => {
    // Runs after EVERY rendering
  });  }

B) An empty array []: the side-effect runs once after the initial rendering.

import { useEffect } from 'react';

function MyComponent() {
  useEffect(() => {
    // Runs ONCE after initial rendering
  }, []);}

C) Has props or state values [prop1, prop2, ..., state1, state2]: the side-effect runs only when any depenendecy value changes.

import { useEffect, useState } from 'react';

function MyComponent({ prop }) {
  const [state, setState] = useState('');
  useEffect(() => {
    // Runs ONCE after initial rendering
    // and after every rendering ONLY IF `prop` or `state` changes
  }, [prop, state]);}

Let’s detail into the cases B) and C) since they’re used often.

3. The side-effect on component did mount

To invoke a side-effect once after the component mounting, use an empty dependencies array:

import { useEffect } from 'react';

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

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

useEffect(..., []) was supplied with an empty array as a 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

4. The side-effect on component did update

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

import { 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] has 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 RUNS

5. 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 { useEffect, useState } from 'react';

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

  useEffect(() => {
    async function fetchEmployees() {
      const response = await fetch(
        `/employees?q=${encodeURIComponent(query)}`
      );
      const fetchedEmployees = await response.json(response);
      setEmployees(fetchedEmployees);
    }
    fetchEmployees();
  }, [query]);

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

useEffect() starts a fetch request by calling fetchEmployees() async function after the initial mounting.

When the request completes, setEmployees(fetchedEmployees) updates the employees state with the just fetched employees list.

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

Note that the callback argument of useEffect(callback) cannot be an async function. But you can always define and then invoke an async function inside the callback itself:

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

  useEffect(() => {  // <--- CANNOT be an async function    async function fetchEmployees() {
      // ...
    }
    fetchEmployees(); // <--- But CAN invoke async functions  }, [query]);

  // ...
}

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

6. The side-effect cleanup

Some side-effects need cleanup: close a socket, clear timers.

If the callback of useEffect(callback) returns a function, then useEffect() considers this as an effect cleanup:

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 callback, useEffect() invokes 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.

Let’s see an example when the side-effect cleanup is useful.

The following component <RepeatMessage message="My Message" /> accepts a prop message. Then, every 2 seconds the message prop is logged to console:

import { useEffect } from 'react';

function RepeatMessage({ message }) {
  useEffect(() => {
    setInterval(() => {
      console.log(message);
    }, 2000);
  }, [message]);

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

Open the demo and type some messages. The console logs every 2 seconds any message that’s been ever typed into the input. However, you need to log only the latest message.

That’s the case to clean up the side-effect: cancel the previous timer when starting a new one. Let’s return a cleanup function that stops the previous timer:

import { useEffect } from 'react';

function RepeatMessage({ message }) {
  useEffect(() => {
    const id = setInterval(() => {
      console.log(message);
    }, 2000);
    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.

7. Conclusion

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

useEffect(callback, dependencies) invokes 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.

The next step to mastering useEffect() is to understand and avoid the infinite loop pitfall.

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