React – Add unique key to array map

Question

I would like to add a unique key to the returned elements this function provides.

function RowList() {
  const rows = [<Row0 />, <Row1 />, <Row2 />, <Row3 />];
  return (
    <>
      {rows.map(row => (
        <tr key={?}>{row}</tr>
      ))}
    </>
  );
}

I have tried:

function Rows() {
  const rows = [<Row0 />, <Row1 />, <Row2 />, <Row3 />];
  return (
    <>
      {rows.map(row => (
        <tr key={row}>{row}</tr>
      ))}
    </>
  );
}

But [object Object] is returned as the key.

I also will not be able to do something like

let x = 0
function Rows() {
  const rows = [<Row0 />, <Row1 />, <Row2 />, <Row3 />];
  return (
    <>
      {rows.map(row => (
        <tr key={x = x + 1}>{row}</tr>
      ))}
    </>
  );
}

As I will need to be able to remove and add back to the array later.

Answer 1

As you know, you can't just do this:

// DON'T DO THIS
{rows.map((row, index) => (
  <tr key={index}>{row}</tr>
))}

As the documentation says, that's a "last resort" and really only useful for static lists. You've said your list won't be static.

It's fairly unusual to have an array of already-created elements like that rather than an array of the data for the elements. If you can avoid it, I would, and give the data entries enduring ID values that you can use as keys, eg ( name is obviously a stand-in for actual data):

class RowInfo {
  static id = 0;
  constructor(name) {
    this.name = name;
    this.id = RowInfo.id++;
  }
}

function RowList() {
  const rows = [new RowInfo("one"), new RowInfo("two"), new RowInfo("three"), new RowInfo("four")];
  return (
    <>
      {rows.map(({id, name}) => (
        <tr key={id}><Row name={name}/></tr>
      ))}
    </>
  );
}

That assumes that they should all be the same type of component, of course, which may not be true.

If you can't do that and must pre-create the actual elements, I'd probably create wrapper objects:

class RowInfo {
   static id = 0;
   constructor(element) {
     this.element = element;
     this.id = RowInfo.id++;
   }
}
function RowList() {
  const rows = [new RowInfo(<Row0 />), new RowInfo(<Row1 />), new RowInfo(<Row2 />), new RowInfo(<Row3 />)];
  return (
    <>
      {rows.map(({id, element}) => (
        <tr key={id}>{element}</tr>
      ))}
    </>
  );
}

Or if they don't have any props you need to specify, you can let React keep track of them, as that's part of its job:

class RowInfo {
   static id = 0;
   constructor(Comp) {
     this.Comp = Comp;
     this.id = RowInfo.id++;
   }
}
function RowList() {
  const rows = [new RowInfo(Row0), new RowInfo(Row1), new RowInfo(Row2), new RowInfo(Row3)];
  return (
    <>
      {rows.map(({id, Comp}) => (
        <tr key={id}><Comp/></tr>
      ))}
    </>
  );
}

Here's a live example of that one:

 const Row0 = () => <div>Row 0</div>; const Row1 = () => <div>Row 1</div>; const Row2 = () => <div>Row 2</div>; const Row3 = () => <div>Row 3</div>; const {Fragment} = React; class RowInfo { static id = 0; constructor(Comp) { this.Comp = Comp; this.id = RowInfo.id++; } } // Have to use <Fragment></Fragment> in the below instead of <></> because // Stack Snippet's version of Babel is out of date and // doesn't understand <></>. function RowList() { const rows = [new RowInfo(Row0), new RowInfo(Row1), new RowInfo(Row2), new RowInfo(Row3)]; return ( <Fragment> {rows.map(({id, Comp}) => ( <tr key={id}><Comp/></tr> ))} </Fragment> ); } ReactDOM.render(<RowList/>, document.getElementById("root"));

 <div id="root"></div> <script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.6/umd/react.production.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.6/umd/react-dom.production.min.js"></script>