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Devin Rasmussen
Devin Rasmussen

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Why list keys matter

As I was going through KCD's Beginner React course on egghead, I decided to explore more fully why the key matters in a React list.

Let's take as a quick example a list of inputs:

const [list, setList] = useState([
    {id: 1, value: 'agape'},
    {id: 2, value: 'philia'},
    {id: 3, value: 'storge'},
  ])
const remove = id => setList(list => list.filter(li => li.id !== id))
return <>
  {list.map(item => (
    <div style={{marginBottom: 20}}>
      <button onClick={() => remove(item.id)}>remove</button>
      <label>{item.value}</label>
      <input defaultValue={item.value} />
    </div>
  ))}
</>

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codesandbox

We'll of course notice that familiar warning in the console because of the absence of a key to identify each item returned from map.

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But precisely why?

If I click remove button on the bottom most item (storge) I notice it is removed appropriately from state and React removes it respectively from the DOM.

However, if I click to remove the second item (philia), something weird happens:

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Our React dev tools show us that the remove function correctly adjusted our state.

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So what is going on?

Well it's an indexing/state problem.

If you choose not to assign an explicit key to list items then React will default to using indexes as keys.

So if we are creating a list of items without keys:

<div>agape</div>
<div>philia</div>
<div>storge</div>

React has to think about these in some kind of order:

<div>agape</div> // key 0
<div>philia</div> // key 1
<div>storge</div> // key 2

But if we delete the second item, things get a little weird:

<div>agape</div> // key 0
<div>storge</div> // key 1

Now storge is at index 1 instead of 2. React says no problem. Index 1 from your JSX (storge) is the same as index 1 (philia) in the DOM. But that is not the case. This is why I said there is an index/state misalignment.

  1. removes the third DOM element (because there are only two)
  2. matches the "storge" JSX to the "philia" div and updates a portion of it that it sees is different (i.e., the label).

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Optimally we want React to nuke the correct DOM element (philia) and then simply update the storge DOM element. If we provide each list item with a key, React can do that because now it isn't depending on unstable indexes.

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Source/further reading

Lists and Keys
Reconciliation keys
Index as a key is an anti-pattern

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