How could I use map to render a list of items but grouped at every 3 items?

Question

I have a long list of items in an array. These need to be rendered in a component, which should be all fine and dandy. However, I need these to be wrapped by another component at every 3 items.

So, if I were to just render the items it would look like this:

return (
  <div>
    items.map((x, index) => {
      <span key={index}>{x}</span>
    })
  </div>
)

But basically, I want every three items to be wrapped inside a div with a special class, so something like this:

return (
  <div>
    <div className='group-of-3'>
      <span>Item 1</span>
      <span>Item 2</span>
      <span>Item 3</span>
    </div>
    <div className='group-of-3'>
      <span>Item 4</span>
      <span>Item 5</span>
      <span>Item 6</span>
    </div>
    .
    .
    .
  </div>
)

What would be the ideal way to do this? Keep in mind that the amount of items does change, so doing it manually is out of the question.

Answer 1

Using plain JavaScript, you can use Array.reduce() to create subarrays from your items. Then map on your items twice:

const group = (items, n) => items.reduce((acc, x, i) => {
  const idx = Math.floor(i / n);
  acc[idx] = [...(acc[idx] || []), x];
  return acc;
}, []);

function Example({ items }) {
  return (
    <div>{group(items, 3).map(children =>
        <div className='group-of-3'>
          {children.map((x, i) => <span key={i}>{x}</span>)}
        </div>
    )}</div>
  );
}

ReactDOM.render(
  <Example items={[1, 2, 3, 4, 5, 6, 7, 8]} />,
  document.getElementById('root')
);

.group-of-3 {
  border: 1px solid black;
  padding: 5px;
  margin: 5px;
  width: 50px;
  text-align: center;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>
<div id="root"></div>