How do I swap array elements in an immutable fashion within a Redux reducer?

One of the key ideas of immutable updates is that while you should never directly modify the original items, it's okay to make a copy and mutate the copy before returning it.

With that in mind, this function should do what you want:

function immutablySwapItems(items, firstIndex, secondIndex) {
    // Constant reference - we can still modify the array itself
    const results= items.slice();
    const firstItem = items[firstIndex];
    results[firstIndex] = items[secondIndex];
    results[secondIndex] = firstItem;

    return results;
}

I wrote a section for the Redux docs called Structuring Reducers - Immutable Update Patterns which gives examples of some related ways to update data.

You could use map function to make a swap:

function immutablySwapItems(items, firstIndex, secondIndex) {
    return items.map(function(element, index) {
        if (index === firstIndex) return items[secondIndex];
        else if (index === secondIndex) return items[firstIndex];
        else return element;
    }
}

In ES2015 style:

const immutablySwapItems = (items, firstIndex, secondIndex) =>
  items.map(
    (element, index) =>
      index === firstIndex
        ? items[secondIndex]
        : index === secondIndex
        ? items[firstIndex]
        : element
  )