Referencing Values with Refs

When you want a component to “remember” some information, but you don’t want that information to trigger new renders, you can use a ref—it’s like a secret “pocket” for storing information in your component!

You will learn

  • How to add a ref to your component
  • How to update a ref’s value
  • How refs are different from state
  • How to use refs safely

Adding a ref to your component

You can add a ref to your component by importing the useRef Hook from React:

import { useRef } from 'react';

Inside your component, call the useRef Hook and pass the initial value that you want to reference as the only argument. For example, here is a ref to the value 0:

const ref = useRef(0);

useRef returns an object like this:

{ 
  current: 0 // The value you passed to useRef
}
An arrow with 'current' written on it stuffed into a pocket with 'ref' written on it.

You can access the current value of that ref through the ref.current property. This value is intentionally mutable, meaning you can both read and write to it. (This is what makes it an “escape hatch” from React’s one-way data flow—more on that below!)

Here, a button will increment ref.current on every click:

import { useRef } from 'react';

export default function Counter() {
  let ref = useRef(0);

  function handleClick() {
    ref.current = ref.current + 1;
    alert('You clicked ' + ref.current + ' times!');
  }

  return (
    <button onClick={handleClick}>
      Click me!
    </button>
  );
}

The ref points to a number, but, like state, you could point to anything: a string, an object, or even a function. Unlike state, ref is a plain JavaScript object with the current property that you can read and modify.

Note that the component doesn’t re-render with every increment. Like state, refs are retained by React between re-renders. However, setting state re-renders a component. Changing a ref does not!

Example: building a stopwatch

You can combine refs and state in a single component. For example, let’s make a stopwatch that the user can start or stop by pressing a button. In order to display how much time has passed since the user pressed “Start,” you will need to keep track of when the Start button was pressed and what the current time is. This information is used for rendering, so you’ll keep it in state:

const [startTime, setStartTime] = useState(null);
const [now, setNow] = useState(null);

When the user presses “Start,” you’ll use setInterval in order to update the time every 100 milliseconds:

import { useState } from 'react';

export default function Stopwatch() {
  const [startTime, setStartTime] = useState(null);
  const [now, setNow] = useState(null);

  function handleStart() {
    // Start counting.
    setStartTime(Date.now());
    setNow(Date.now());

    setInterval(() => {
      // Update the current time every 100ms.
      setNow(Date.now());
    }, 100);
  }

  let secondsPassed = 0;
  if (startTime != null && now != null) {
    secondsPassed = (now - startTime) / 1000;
  }

  return (
    <>
      <h1>Time passed: {secondsPassed.toFixed(3)}</h1>
      <button onClick={handleStart}>
        Start
      </button>
    </>
  );
}

When the “Stop” button is pressed, you need to cancel the existing interval so that it stops updating the now state variable. You can do this by calling clearInterval, but you need to give it the interval ID that was previously returned by the setInterval call when the user pressed Start. You need to keep the interval ID somewhere. Since the interval ID is not used for rendering, you can keep it in a ref:

import { useState, useRef } from 'react';

export default function Stopwatch() {
  const [startTime, setStartTime] = useState(null);
  const [now, setNow] = useState(null);
  const intervalRef = useRef(null);

  function handleStart() {
    setStartTime(Date.now());
    setNow(Date.now());
    intervalRef.current = setInterval(() => {
      setNow(Date.now());
    }, 10);
  }

  function handleStop() {
    clearInterval(intervalRef.current);
  }

  let secondsPassed = 0;
  if (startTime != null && now != null) {
    secondsPassed = (now - startTime) / 1000;
  }

  return (
    <>
      <h1>Time passed: {secondsPassed.toFixed(3)}</h1>
      <button onClick={handleStart}>
        Start
      </button>
      <button onClick={handleStop}>
        Stop
      </button>
    </>
  );
}

When a piece of information is used for rendering, keep it in state. When a piece of information is only needed by event handlers and changing it doesn’t require a re-render, using a ref may be more efficient.

Differences between refs and state

Perhaps you’re thinking refs seem less “strict” than state—you can mutate them instead of always having to use a state setting function, for instance. But in most cases, you’ll want to use state. Refs are an “escape hatch” you won’t need often. Here’s how state and refs compare:

refsstate
useRef(initialValue) returns { current: initialValue }useState(initialValue) returns the current value of a state variable and a state setter function ( [value, setValue])
Doesn’t trigger re-render when you change it.Triggers re-render when you change it.
Mutable—you can modify and update current’s value outside of the rendering process.“Immutable”—you must use the state setting function to modify state variables to queue a re-render.
You shouldn’t read (or write) the current value during rendering.You can read state at any time. However, each render has its own snapshot of state which does not change.

Here is a counter button that’s implemented with state:

import { useState } from 'react';

export default function Counter() {
  const [count, setCount] = useState(0);

  function handleClick() {
    setCount(count + 1);
  }

  return (
    <button onClick={handleClick}>
      You clicked {count} times
    </button>
  );
}

Because the count value is displayed, it makes sense to use a state value for it. When the counter’s value is set with setCount(), React re-renders the component and the screen updates to reflect the new count.

If you tried to implement this with a ref, React would never re-render the component, so you’d never see the count change! See how clicking this button does not update its text:

import { useRef } from 'react';

export default function Counter() {
  let countRef = useRef(0);

  function handleClick() {
    // This doesn't re-render the component!
    countRef.current = countRef.current + 1;
  }

  return (
    <button onClick={handleClick}>
      You clicked {countRef.current} times
    </button>
  );
}

This is why reading ref.current during render leads to unreliable code. If you need that, use state instead.

Deep Dive

How does useRef work inside?

When to use refs

Typically, you will use a ref when your component needs to “step outside” React and communicate with external APIs—often a browser API that won’t impact the appearance of the component. Here are a few of these rare situations:

If your component needs to store some value, but it doesn’t impact the rendering logic, choose refs.

Best practices for refs

Following these principles will make your components more predictable:

  • Treat refs as an escape hatch. Refs are useful when you work with external systems or browser APIs. If much of your application logic and data flow relies on refs, you might want to rethink your approach.
  • Don’t read or write ref.current during rendering. If some information is needed during rendering, use state instead. Since React doesn’t know when ref.current changes, even reading it while rendering makes your component’s behavior difficult to predict. (The only exception to this is code like if (!ref.current) ref.current = new Thing() which only sets the ref once during the first render.)

Limitations of React state don’t apply to refs. For example, state acts like a snapshot for every render and doesn’t update synchronously. But when you mutate the current value of a ref, it changes immediately:

ref.current = 5;
console.log(ref.current); // 5

This is because the ref itself is a regular JavaScript object, and so it behaves like one.

You also don’t need to worry about avoiding mutation when you work with a ref. As long as the object you’re mutating isn’t used for rendering, React doesn’t care what you do with the ref or its contents.

Refs and the DOM

You can point a ref to any value. However, the most common use case for a ref is to access a DOM element. For example, this is handy if you want to focus an input programmatically. When you pass a ref to a ref attribute in JSX, like <div ref={myRef}>, React will put the corresponding DOM element into myRef.current. You can read more about this in Manipulating the DOM with Refs.

Recap

  • Refs are an escape hatch to hold onto values that aren’t used for rendering. You won’t need them often.
  • A ref is a plain JavaScript object with a single property called current, which you can read or set.
  • You can ask React to give you a ref by calling the useRef Hook.
  • Like state, refs let you retain information between re-renders of a component.
  • Unlike state, setting the ref’s current value does not trigger a re-render.
  • Don’t read or write ref.current during rendering. This makes your component hard to predict.

Challenge 1 of 4:
Fix a broken chat input

Type a message and click “Send”. You will notice there is a three second delay before you see the “Sent!” alert. During this delay, you can see an “Undo” button. Click it. This “Undo” button is supposed to stop the “Sent!” message from appearing. It does this by calling clearTimeout for the timeout ID saved during handleSend. However, even after “Undo” is clicked, the “Sent!” message still appears. Find why it doesn’t work, and fix it.

import { useState } from 'react';

export default function Chat() {
  const [text, setText] = useState('');
  const [isSending, setIsSending] = useState(false);
  let timeoutID = null;

  function handleSend() {
    setIsSending(true);
    timeoutID = setTimeout(() => {
      alert('Sent!');
      setIsSending(false);
    }, 3000);
  }

  function handleUndo() {
    setIsSending(false);
    clearTimeout(timeoutID);
  }

  return (
    <>
      <input
        disabled={isSending}
        value={text}
        onChange={e => setText(e.target.value)}
      />
      <button
        disabled={isSending}
        onClick={handleSend}>
        {isSending ? 'Sending...' : 'Send'}
      </button>
      {isSending &&
        <button onClick={handleUndo}>
          Undo
        </button>
      }
    </>
  );
}