Looping over arrays to render lists of elements is a common need in React apps. However, there are some special considerations when rendering lists in JSX.

One important aspect is the key prop. React uses keys to uniquely identify list elements and optimize performance.

Let’s look at how to loop through arrays in JSX, and why keys are important:

Rendering Arrays in JSX

JSX makes looping straightforward - you can use JavaScript’s map() function directly:

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const people = [
  { id: 1, name: 'John'},
  { id: 2, name: 'Mary'},
  { id: 3, name: 'Peter'}
];

function App() {
  return (
    <ul>
      {people.map(person => {
        return <Person key={person.id} person={person} />
      })}
    </ul>
  )
}

This loops through the people array, rendering a <Person> component for each item.

The Importance of Keys

One important thing to note is the key prop passed to each <Person> element:

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<Person key={person.id} person={person} />

Keys help React differentiate elements in a list. If keys are missing, React may have trouble identifying list items when the list changes.

Keys should be:

  • Unique to each sibling
  • Stable across re-renders

Using a unique ID from the data as the key is usually best.

Issues from Missing Keys

Keys prevent issues when rendering list updates, like:

  • Duplicate keys - Causes performance issues
  • Unstable keys - Causes UI bugs like losing focus
  • No keys - Can cause elements to rearrange incorrectly

Not using keys is an anti-pattern in React.

When to Use index as Key

Sometimes data lacks unique IDs. As a last resort, you can use the element index as the key:

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{items.map((item, index) => (
  <Item key={index} item={item} />
))}

However, index keys can negatively impact performance. Elements may get re-ordered unnecessarily.

Ideally, rewrite data to have unique IDs whenever possible.

Summary

  • Use map() to loop over arrays in JSX
  • Provide a key prop to uniquely identify elements
  • key should be unique and stable
  • By default, use a unique ID as key
  • Index can work as key if no IDs, but not ideal

Keys may seem confusing at first, but understanding how React uses them will help you avoid performance issues and bugs in dynamic lists.