How can I use multiple refs for an array of elements with hooks?

Question

As far as I understood I can use refs for a single element like this:

const { useRef, useState, useEffect } = React;    const App = () => {    const elRef = useRef();    const [elWidth, setElWidth] = useState();      useEffect(() => {      setElWidth(elRef.current.offsetWidth);    }, []);      return (      <div>        <div ref={elRef} style={{ width: "100px" }}>          Width is: {elWidth}        </div>      </div>    );  };    ReactDOM.render(    <App />,    document.getElementById("root")  );

<script src="https://unpkg.com/react@16/umd/react.production.min.js"></script>  <script src="https://unpkg.com/react-dom@16/umd/react-dom.production.min.js"></script>    <div id="root"></div>

How can I implement this for an array of elements? Obviously not like that: (I knew it even I did not try it:)

const { useRef, useState, useEffect } = React;    const App = () => {    const elRef = useRef();    const [elWidth, setElWidth] = useState();      useEffect(() => {      setElWidth(elRef.current.offsetWidth);    }, []);      return (      <div>        {[1, 2, 3].map(el => (          <div ref={elRef} style={{ width: `${el * 100}px` }}>            Width is: {elWidth}          </div>        ))}      </div>    );  };    ReactDOM.render(    <App />,    document.getElementById("root")  );

<script src="https://unpkg.com/react@16/umd/react.production.min.js"></script>  <script src="https://unpkg.com/react-dom@16/umd/react-dom.production.min.js"></script>    <div id="root"></div>

I have seen this and hence this. But, I'm still confused about how to implement that suggestion for this simple case.

Answer 1

As you cannot use hooks inside loops, here is a solution in order to make it work when the array changes over the time.

I suppose the array comes from the props :

const App = props => {     const itemsRef = useRef([]);     // you can access the elements with itemsRef.current[n]      useEffect(() => {        itemsRef.current = itemsRef.current.slice(0, props.items.length);     }, [props.items]);      return props.items.map((item, i) => (       <div            key={i}            ref={el => itemsRef.current[i] = el}            style={{ width: `${(i + 1) * 100}px` }}>         ...       </div>     )); } 

Answer 2

A ref is initially just { current: null } object. useRef keeps the reference to this object between component renders. current value is primarily intended for component refs but can hold anything.

There should be an array of refs at some point. In case the array length may vary between renders, an array should scale accordingly:

const arrLength = arr.length; const [elRefs, setElRefs] = React.useState([]);  React.useEffect(() => {   // add or remove refs   setElRefs((elRefs) =>     Array(arrLength)       .fill()       .map((_, i) => elRefs[i] || createRef()),   ); }, [arrLength]);  return (   <div>     {arr.map((el, i) => (       <div ref={elRefs[i]} style={...}>         ...       </div>     ))}   </div> ); 

This piece of code can be optimized by unwrapping useEffect and replacing useState with useRef but it should be noted that doing side effects in render function is generally considered a bad practice:

const arrLength = arr.length; const elRefs = React.useRef([]);  if (elRefs.current.length !== arrLength) {   // add or remove refs   elRefs.current = Array(arrLength)     .fill()     .map((_, i) => elRefs.current[i] || createRef()); }  return (   <div>     {arr.map((el, i) => (       <div ref={elRefs.current[i]} style={...}>         ...       </div>     ))}   </div> );