Explanation:
In React, each element in an array or an iterator should have a unique “key” prop. The key is used by React to efficiently update and re-render elements without re-creating them. However, relying on enumerating keys on a component instance can have performance issues, especially in production mode. To avoid this performance overhead, React intentionally leaves the keys empty in production mode.
Consider the following example:
const items = ['Apple', 'Banana', 'Orange'];
const itemList = items.map((item, index) => {
return <li key={index}>{item}</li>;
});
// ...
<ul>
{itemList}
</ul>
In the above example, we have an array of fruits. We are using the map function to create a list item element for each fruit. We provide the index as the key prop. However, relying on the index as the key can cause issues when the array changes its order or items are added/removed. React might fail to update or re-render the correct elements if the keys are not unique and stable.
To avoid relying on the index as the key, it is recommended to use a unique and stable identifier that can uniquely identify each item in the array. This could be an ID associated with each item, such as a database ID or a unique property of the item itself.
const items = [
{ id: 1, name: 'Apple' },
{ id: 2, name: 'Banana' },
{ id: 3, name: 'Orange' }
];
const itemList = items.map((item) => {
return <li key={item.id}>{item.name}</li>;
});
// ...
<ul>
{itemList}
</ul>
In the updated example, we have included a unique “id” property for each fruit object in the array. We use this ID as the key prop instead of relying on the index. Using a unique and stable identifier ensures that React can correctly update and re-render elements when the array changes.