useRef

const ref = useRef(initialValue)

• Viite
  • useRef(initialValue)
• Käyttö
  • Referencing a value with a ref
  • Manipulating the DOM with a ref
  • Avoiding recreating the ref contents
• Vianmääritys
  • I can’t get a ref to a custom component

Viite

useRef(initialValue)

Call useRef at the top level of your component to declare a ref.

import { useRef } from 'react';

function MyComponent() {
  const intervalRef = useRef(0);
  const inputRef = useRef(null);
  // ...

See more examples below.

Parametrit

• initialValue: The value you want the ref object’s current property to be initially. It can be a value of any type. This argument is ignored after the initial render.

Palautukset

useRef returns an object with a single property:

• current: Initially, it’s set to the initialValue you have passed. You can later set it to something else. If you pass the ref object to React as a ref attribute to a JSX node, React will set its current property.

On the next renders, useRef will return the same object.

Rajoitukset

• You can mutate the ref.current property. Unlike state, it is mutable. However, if it holds an object that is used for rendering (for example, a piece of your state), then you shouldn’t mutate that object.
• When you change the ref.current property, React does not re-render your component. React is not aware of when you change it because a ref is a plain JavaScript object.
• Do not write or read ref.current during rendering, except for initialization. This makes your component’s behavior unpredictable.
• In Strict Mode, React will call your component function twice in order to help you find accidental impurities. This is development-only behavior and does not affect production. Each ref object will be created twice, but one of the versions will be discarded. If your component function is pure (as it should be), this should not affect the behavior.

Käyttö

Referencing a value with a ref

Call useRef at the top level of your component to declare one or more refs.

import { useRef } from 'react';

function Stopwatch() {
  const intervalRef = useRef(0);
  // ...

useRef returns a ref object with a single current property initially set to the initial value you provided.

On the next renders, useRef will return the same object. You can change its current property to store information and read it later. This might remind you of state, but there is an important difference.

Changing a ref does not trigger a re-render. This means refs are perfect for storing information that doesn’t affect the visual output of your component. For example, if you need to store an interval ID and retrieve it later, you can put it in a ref. To update the value inside the ref, you need to manually change its current property:

function handleStartClick() {
  const intervalId = setInterval(() => {
    // ...
  }, 1000);
  intervalRef.current = intervalId;
}

Later, you can read that interval ID from the ref so that you can call clear that interval:

function handleStopClick() {
  const intervalId = intervalRef.current;
  clearInterval(intervalId);
}

By using a ref, you ensure that:

• You can store information between re-renders (unlike regular variables, which reset on every render).
• Changing it does not trigger a re-render (unlike state variables, which trigger a re-render).
• The information is local to each copy of your component (unlike the variables outside, which are shared).

Changing a ref does not trigger a re-render, so refs are not appropriate for storing information you want to display on the screen. Use state for that instead. Read more about choosing between useRef and useState.

function MyComponent() {
  // ...
  // 🚩 Don't write a ref during rendering
  myRef.current = 123;
  // ...
  // 🚩 Don't read a ref during rendering
  return <h1>{myOtherRef.current}</h1>;
}

function MyComponent() {
  // ...
  useEffect(() => {
    // ✅ You can read or write refs in effects
    myRef.current = 123;
  });
  // ...
  function handleClick() {
    // ✅ You can read or write refs in event handlers
    doSomething(myOtherRef.current);
  }
  // ...
}

Manipulating the DOM with a ref

It’s particularly common to use a ref to manipulate the DOM. React has built-in support for this.

First, declare a ref object with an initial value of null:

import { useRef } from 'react';

function MyComponent() {
  const inputRef = useRef(null);
  // ...

Then pass your ref object as the ref attribute to the JSX of the DOM node you want to manipulate:

  // ...
  return <input ref={inputRef} />;

After React creates the DOM node and puts it on the screen, React will set the current property of your ref object to that DOM node. Now you can access the <input>’s DOM node and call methods like focus():

  function handleClick() {
    inputRef.current.focus();
  }

React will set the current property back to null when the node is removed from the screen.

Read more about manipulating the DOM with refs.

Avoiding recreating the ref contents

React saves the initial ref value once and ignores it on the next renders.

function Video() {
  const playerRef = useRef(new VideoPlayer());
  // ...

Although the result of new VideoPlayer() is only used for the initial render, you’re still calling this function on every render. This can be wasteful if it’s creating expensive objects.

To solve it, you may initialize the ref like this instead:

function Video() {
  const playerRef = useRef(null);
  if (playerRef.current === null) {
    playerRef.current = new VideoPlayer();
  }
  // ...

Normally, writing or reading ref.current during render is not allowed. However, it’s fine in this case because the result is always the same, and the condition only executes during initialization so it’s fully predictable.

function Video() {
  const playerRef = useRef(null);

  function getPlayer() {
    if (playerRef.current !== null) {
      return playerRef.current;
    }
    const player = new VideoPlayer();
    playerRef.current = player;
    return player;
  }

  // ...

Vianmääritys

I can’t get a ref to a custom component

If you try to pass a ref to your own component like this:

const inputRef = useRef(null);

return <MyInput ref={inputRef} />;

You might get an error in the console:

By default, your own components don’t expose refs to the DOM nodes inside them.

To fix this, find the component that you want to get a ref to:

export default function MyInput({ value, onChange }) {
  return (
    <input
      value={value}
      onChange={onChange}
    />
  );
}

And then wrap it in forwardRef like this:

import { forwardRef } from 'react';

const MyInput = forwardRef(({ value, onChange }, ref) => {
  return (
    <input
      value={value}
      onChange={onChange}
      ref={ref}
    />
  );
});

export default MyInput;

Then the parent component can get a ref to it.

Read more about accessing another component’s DOM nodes.