A “key” is a special string attribute you need to include when creating lists of elements in React. Keys are used in React to identify which items in the list are changed, updated or deleted. In other words we can say that keys are used to give an identity to the elements in the lists. The next thing that comes in mind is that what should be good to be choose as key for the items in lists. It is recommended to use a string as a key that uniquely identifies the items in the list. Below example shows a list with string keys:
Keys help React identify which items have changed, are added, or are removed. Keys should be given to the elements inside the array to give the elements a stable identity:
const numbers = [1, 2, 3, 4, 5];
const listItems = numbers.map((number) =>
<li key={number.toString()}>
{number}
</li>
);
The best way to pick a key is to use a string that uniquely identifies a list item among its siblings. Most often you would use IDs from your data as keys:
const todoItems = todos.map((todo) =>
<li key={todo.id}>
{todo.text}
</li>
);
When you don’t have stable IDs for rendered items, you may use the item index as a key as a last resort:
const todoItems = todos.map((todo, index) =>
// Only do this if items have no stable IDs
<li key={index}>
{todo.text}
</li>
);
We don’t recommend using indexes for keys if the order of items may change. This can negatively impact performance and may cause issues with component state. If you choose not to assign an explicit key to list items then React will default to using indexes as keys.
