How to fix this?

To resolve the unique key error in React, follow these steps:

Step 1: Identify the List Rendering Code

Locate the part of your code where you are rendering a list. This is typically where you use the .map() function to iterate over an array and return JSX elements.

Step 2: Ensure Each Element Has a Unique Key

Modify the .map() function to include a unique key prop for each element in the list. The key should be a string that uniquely identifies each list item.

Step 3 : Suppose you have an array of user objects, and each user has a unique ID. Your code might look something like this:

const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
  { id: 3, name: 'Charlie' }
];

function UserList() {
  return (
    <ul>
      {users.map(user => (
        <li key={user.id}>{user.name}</li>
      ))}
    </ul>
  );
}

Step 4 : Use IDs as Keys If Possible

If your data items have unique IDs, use them as keys. This is the most reliable way to ensure keys are unique.

Step 5: Avoid Using Indexes as Keys If Possible

Only use indexes as keys if there are no unique identifiers and the list is static (i.e., items are not added, removed, or reordered). Using indexes can lead to performance issues and bugs in dynamic lists.

Note: Either we can use indexes as unique key or id as the key.

Example:

Then run the application using the following command:

npm start

Output:

In React, keys should be unique within a list to help React efficiently identify and update elements during rendering. Unique keys enable React to distinguish between different list items and optimize re-rendering, ensuring accurate component state management and preventing unexpected behaviour or errors.