The above code will log the below output to the console:
[3, 4, 5, 6, 7]
The below code illustrates this:
The above code will render an unordered list as shown in below output:
Rendering lists inside Components
In the above code in React, we had directly rendered the list to the DOM. But usually this not a good practice to render lists in React. We already have talked about the uses of Components and had seen that everything in React is built as individual components. Consider the example of a Navigation Menu. It is obvious that in any website the items in a navigation menu are not hard coded. This item is fetched from the database and then displayed as lists in the browser. So from the component’s point of view, we can say that we will pass a list to a component using props and then use this component to render the list to the DOM. We can update the above code in which we have directly rendered the list to now a component that will accept an array as props and returns an unordered list.
You can see in the above output that the unordered list is successfully rendered to the browser but a warning message is logged to the console.
Warning: Each child in an array or iterator should have a unique "key" prop
The above warning message says that each of the list items in our unordered list should have a unique key. A “key” is a special string attribute you need to include when creating lists of elements in React. We will discuss about keys in detail in further articles. For now, let’s just assign a string key to each of our list items in the above code.
Below is the updated code with keys:
This code will give the same output as that of the previous code but this time without any warning. 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. We will learn about keys in more detail in our next article.