How target DOM with react useRef in map

Question

I looking for a solution about get an array of DOM elements with react useRef() hook.

example:

const Component = () =>  {    // In `items`, I would like to get an array of DOM element   let items = useRef(null);    return <ul>     {['left', 'right'].map((el, i) =>       <li key={i} ref={items} children={el} />     )}   </ul> } 

How can I achieve this?

Answer 1

useRef is just partially similar to React's ref(just structure of object with only field of current).

useRef hook is aiming on storing some data between renders and changing that data does not trigger re-rendering(unlike useState does).

Also just gentle reminder: better avoid initialize hooks in loops or if. It's first rule of hooks.

Having this in mind we:

1. create array and keep it between renders by useRef

2. we initialize each array's element by createRef()

3. we can refer to list by using .current notation

   const Component = () => {    let refs = useRef([React.createRef(), React.createRef()]);    useEffect(() => {     refs.current[0].current.focus()   }, []);    return (<ul>     {['left', 'right'].map((el, i) =>       <li key={i}><input ref={refs.current[i]} value={el} /></li>     )}   </ul>) } 

This way we can safely modify array(say by changing it's length). But don't forget that mutating data stored by useRef does not trigger re-render. So to make changing length to re-render we need to involve useState.

const Component = () => {    const [length, setLength] = useState(2);   const refs = useRef([React.createRef(), React.createRef()]);    function updateLength({ target: { value }}) {     setLength(value);     refs.current = refs.current.splice(0, value);     for(let i = 0; i< value; i++) {       refs.current[i] = refs.current[i] || React.createRef();     }     refs.current = refs.current.map((item) => item || React.createRef());   }    useEffect(() => {    refs.current[refs.current.length - 1].current.focus()   }, [length]);    return (<>     <ul>     {refs.current.map((el, i) =>       <li key={i}><input ref={refs.current[i]} value={i} /></li>     )}   </ul>   <input value={refs.current.length} type="number" onChange={updateLength} />   </>) } 

Also don't try to access refs.current[0].current at first rendering - it will raise an error.

Say

      return (<ul>         {['left', 'right'].map((el, i) =>           <li key={i}>             <input ref={refs.current[i]} value={el} />             {refs.current[i].current.value}</li> // cannot read property `value` of undefined         )}       </ul>) 

So you either guard it as

      return (<ul>         {['left', 'right'].map((el, i) =>           <li key={i}>             <input ref={refs.current[i]} value={el} />             {refs.current[i].current && refs.current[i].current.value}</li> // cannot read property `value` of undefined         )}       </ul>) 

or access it in useEffect hook. Reason: refs are bound after element is rendered so during rendering is running for the first time it is not initialized yet.

Answer 2

I'll expand on skyboyer's answer a bit. For performance optimization (and to avoid potential weird bugs), you might prefer to use useMemo instead of useRef. Because useMemo accepts a callback as an argument instead of a value, React.createRef will only be initialized once, after the first render. Inside the callback you can return an array of createRef values and use the array appropriately.

Initialization:

  const refs= useMemo(     () => Array.from({ length: 3 }).map(() => createRef()),     []   ); 

Empty array here (as a second argument) tells React to only initialize refs once. If ref count changes you may need to pass [x.length] as "a deps array" and create refs dynamically: Array.from({ length: x.length }).map(() => createRef()),

Usage:

  refs[i+1 % 3].current.focus();