error: Do not use Array index in keys

Solution 1:

When you use index of an array as a key, React will optimize and not render as expected. What happens in such a scenario can be explained with an example.

Suppose the parent component gets an array of 10 items and renders 10 components based on the array. Suppose the 5th item is then removed from the array. On the next render the parent will receive an array of 9 items and so React will render 9 components. This will show up as the 10th component getting removed, instead of the 5th, because React has no way of differentiating between the items based on index.

Therefore always use a unique identifier as a key for components that are rendered from an array of items.

You can generate your own unique key by using any of the field of the child object that is unique as a key. Normal, any id field of the child object can be used if available.

Edit : You will only be able to see the behavior mentioned above happen if the components create and manage their own state, e.g. in uncontrolled textboxes, timers etc. E.g. React error when removing input component

Solution 2:

The issue with using key={index} happens whenever the list is modified. React doesn't understand which item was added/removed/reordered since index is given on each render based on the order of the items in the array. Although, usually it's rendered fine, there are still situations when it fails.

Here is my example that I came across while building a list with input tags. One list is rendered based on index, another one based on id. The issue with the first list occurs every time you type anything in the input and then remove the item. On re-render React still shows as if that item is still there. This is 💯 UI issue that is hard to spot and debug.

enter image description here

    class List extends React.Component {
      constructor() {
        super();
        this.state = {
          listForIndex: [{id: 1},{id: 2}],
          listForId: [{id: 1},{id: 2}]
        }
      }
      renderListByIndex = list => {
        return list.map((item, index) => {
          const { id } = item;
          return (
              <div key={index}>
                <input defaultValue={`Item ${id}`} />
                <button 
                  style={{margin: '5px'}} 
                  onClick={() => this.setState({ listForIndex: list.filter(i => i.id !== id) })}
                 >Remove</button>
              </div>
          )
        })
      }
      renderListById = list => {
        return list.map((item) => {
          const { id } = item;
          return (
              <div key={id}>
                <input defaultValue={`Item ${id}`} />
                <button 
                  style={{margin: '5px'}} 
                  onClick={() => this.setState({ listForId: list.filter(i => i.id !== id) })}
                 >Remove</button>
              </div>
          )
        })
      }
      render() {
        const { listForIndex, listForId } = this.state;
        return (
          <div className='flex-col'>
            <div>
              <strong>key is index</strong>
              {this.renderListByIndex(listForIndex)}
            </div>
            <div>
              <strong>key is id</strong>
              {this.renderListById(listForId)}
            </div>
          </div>
        )
      }
    }

    ReactDOM.render(
      <List />,
      document.getElementById('root')
    );

.flex-col {
  display: flex;
  flex-direction: row;
}

.flex-col > div {
  flex-basis: 50%;
  margin: .5em;
  padding: .5em;
  border: 1px solid #ccc;
 }

<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">
        <!-- This element's contents will be replaced with your component. -->
    </div>

error: Do not use Array index in keys

Solution 1:

Solution 2:

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