Warning: Each child in a list should have a unique "key" prop

Question

I'm building an app using the Google Books API and I appear to be passing a unique key to each child in the list, but the error won't go away. I must be doing something wrong but I'm not sure what.

const BookList = (props) => {
    //map over all of the book items to create a new card for each one in the list
    const books = props.books.data.items.map((book) => {
        console.log(book.id);
        return (
            <div className="col col-lg-4 grid-wrapper">
                <BookCard
                    key={book.id}
                    image={book.volumeInfo.imageLinks.thumbnail}
                    title={book.volumeInfo.title}
                    author={book.volumeInfo.authors[0]}
                    description={book.volumeInfo.description}
                    previewLink={book.volumeInfo.previewLink}
                    buyLink={book.saleInfo.buyLink}
                />
            </div>
        );
    });

    return <div>{books}</div>;
};

Notice that after the return in const books I have a console.log(book.id), which will display all 10 unique id keys in the console. But when I try to pass it to the child of this component using key={book.id}, I get this error.

Answer 1

The key needs to go on the outermost returned element. In your specific case, that means changing this:

            <div className="col col-lg-4 grid-wrapper">
                <BookCard 
                    key={book.id}

to this:

            <div className="col col-lg-4 grid-wrapper" key={book.id}>
                <BookCard 

Answer 2

I was using React fragments in my map() call in their simple syntax form, and was running into the same warnings with the code below:

<>
  <h3>{employee.department}</h3>
  <TableRow
    key={employee.id}
    cellValues={["Name", "Title"]} />

  <TableRow
    key={employee.id}
    cellValues={[employee.name, employee.title]}
  />
</>

Building off the accepted answer, I realized I needed the outermost element to have the ID. I learned of an alternate syntax for React fragments that allows one to put an ID on it. The resulting code below caused the warnings to go away:

<React.Fragment key={employee.id}>
  <h3>{employee.department}</h3>
  <TableRow
    cellValues={["Name", "Title"]} />

  <TableRow
    cellValues={[employee.name, employee.title]}
  />
</React.Fragment>