useRef

Question 1

useRef

Answer 1

useRef is a React Hook that lets you reference a value that’s not needed for rendering.

Question 2

Referencing a value with a ref

Answer 2

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

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.

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:

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 that you want to display on the screen. Use state for that instead.

Question 3

Do not write or read ref.current during rendering.

Answer 3

React expects that the body of your component behaves like a pure function:

If the inputs (props, state, and context) are the same, it should return exactly the same JSX.

• Calling it in a different order or with different arguments should not affect the results of other calls.
• Reading or writing a ref during rendering breaks these expectations.

You can read or write refs from event handlers or effects instead.

If you have to read or write something during rendering, use state instead.

When you break these rules, your component might still work, but most of the newer features we’re adding to React will rely on these expectations.

Question 4

Manipulating the DOM with a ref

Answer 4

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

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.

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

Question 5

Avoiding recreating the ref contents

Answer 5

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.

Question 6

Reference

Answer 6

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

Parameters

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.

Returns

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.

Caveats

• 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. This means that each ref object will be created twice, and one of the versions will be discarded. If your component function is pure (as it should be), this should not affect the logic of your component.

Question 7

Troubleshooting

Answer 7

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

const inputRef = useRef(null);

return

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 (

);
}

And then wrap it in forwardRef like this:

import { forwardRef } from ‘react’;

const MyInput = forwardRef(({ value, onChange }, ref) => {
  return (

);
});

export default MyInput;

Then the parent component can get a ref to it.