<For>

Syntax: <For each={array}>{(item, index) => Block}</For>
Example: <For each={[1, 2, 3]}>{(item) => myBlock({ item })}</For>

The <For /> component is used to render a list of blocks. It takes an array as the each prop and a function as its children. The function is called for each item in the array and is passed the item and its index as arguments.

It's the best way to loop over an array. As the array changes, <For /> updates or moves items in the DOM rather than recreating them. Let's look at an example:

import { For } from 'million/react';
 
function App() {
  const [items, setItems] = useState([1, 2, 3]);
 
  return (
    <>
      <button onClick={() => setItems([...items, items.length + 1])}>
        Add item
      </button>
      <ul>
        <For each={items}>{(item) => <li>{item}</li>}</For>
      </ul>
    </>
  );
}
 
export default App;

as prop

The as prop can be used to specify the tag name of the For. By default, it is slot.

<For each={items} as="div">
  {(item) => <li>{item}</li>}
</For>

Optimizing <For />

Internally, <For /> will not reuse blocks in order to avoid unknown behavior. This means that if you have a <For /> with 1000 items, it will recreate 1000 blocks.

However, if you know that your items aren't dependent on any values except the item passed to the function, you can use the memo prop to tell Million to reuse blocks. This will improve performance and reduce memory usage.

<For each={items} memo>
  {(item) => <li>{item}</li>}
</For>

Hydration mismatch

If you are using Million.js on the server, you may encounter a hydration mismatch error. This is because Million.js uses a different algorithm for rendering on the server than it does on the client. To fix this, you can disable SSR.

<For each={[Math.random(), Math.random(), Math.random()]} ssr={false}>
  {(item) => <li>{item}</li>}
</For>