React does not re-render updated array state

Question

I have a simple array in below and I just change index of elements in array by a handler:

const [items, setItems] = useState([
    { id: 5, val: "Italy" },
    { id: 2, val: "Germany" },
    { id: 3, val: "Brazil" },
    { id: 4, val: "Canada" }
]);

My Handler:

const handlePosition = (old_index, new_index) => {
    if (new_index >= items.length) {
      var k = new_index - items.length + 1;
      while (k--) {
        items.push(undefined);
      }
    }
    items.splice(new_index, 0, items.splice(old_index, 1)[0]);

    setItems(items);

    console.log(items);
};

I try to render the items array as follows:

<ul>
  {items.map((item, index) => (
    <li key={item.id}>
      {item.val}
        <button onClick={() => handlePosition(index, index + 1)}>DOWN</button>
        <button onClick={() => handlePosition(index, index - 1)}>UP</button>
    </li>
  ))}
</ul>

Now, My array changed properly and updates the state but not re-render.

Here is a demo of my project in CodeSandBox: Link

user2740650 · Accepted Answer · 2020-05-19 17:11:15Z

26

It's because the instance of the items array is not changing.

Try this:

setItems([...items]);

That copies the array to a new identical array. React will see that it's different and will re-render.

edited May 19, 2020 at 17:11

answered May 19, 2020 at 17:02

user2740650

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2 Comments

Sound in Worship Over a year ago

I now understand ...

Jammar Over a year ago

My good god, the way I coded it this shouldn't in my logical sense have mattered BUT IT DID. thank you!

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