Here we are having a list of todo items that is passed to the props of our component.

Each todo item has a text and id property. Imagine that the id property comes from a backend datastore and is a unique numeric value:

todos = [
  {
    id: 1,
    text: 'value 1'
  },
  {
    id: 2,
    text: 'value 2'
  },
  {
    id: 3,
    text: 'value 3'
  },
  {
    id: 4,
    text: 'value 4'
  },
];

We set the key attribute of each iterated list element to todo-${todo.id} so that react can identify it internally:

render() {
  const { todos } = this.props;
  return (
    <ul>
      { todos.map((todo) =>
        <li key={ `todo-${todo.id}` }>
          { todo.text }
        </li>
      }
    </ul>
  );
}

If you don't have unique database ids at hand, you could also use the numeric index of your array like this:

render() {
  const { todos } = this.props;
  return (
    <ul>
      { todos.map((todo, index) =>
        <li key={ `todo-${index}` }>
          { todo.text }
        </li>
      }
    </ul>
  );
}

Remarks

Using the array index as a key is generally not recommended when the array is going to change over time. From the React Docs:

As a last resort, you can pass item's index in the array as a key. This can work well if the items are never reordered, but reorders will be slow.

A good example about this: https://medium.com/@robinpokorny/index-as-a-key-is-an-anti-pattern-e0349aece318 (opens new window)