ReactJS | Lists

Lists are very useful when it comes to developing the UI of any website. Lists are mainly used for displaying menus in a website, for example, the navbar menu. In regular JavaScript, we can use arrays for creating lists. We can create lists in React in a similar manner as we do in regular JavaScript. We will see how to do this in detail further in this article.
Let’s first see how we can traverse and update any list in regular JavaScript. We can use the map() function in JavaScript for traversing the lists.

Below JavaScript code illustrate using map() function to traverse lists:  

Javascript

<script type="text/javascript">
    var numbers = [1,2,3,4,5];

    const updatedNums = numbers.map((number)=>{
        return (number + 2);
    });

    console.log(updatedNums);
</script>

The above code will log the below output to the console:  

[3, 4, 5, 6, 7]

Let us now create a list of elements in React. We will render the list numbers in the above code as an unordered list element in the browser rather than simply logging in to the console. To do this, we will traverse the list using the JavaScript map() function and updates elements to be enclosed between <li> </li> elements. Finally we will wrap this new list within <ul> </ul> elements and render it to the DOM.

The below code illustrates this:  

Javascript

import React from 'react';
import ReactDOM from 'react-dom';

const numbers = [1,2,3,4,5];

const updatedNums = numbers.map((number)=>{
    return <li>{number}</li>;
});

ReactDOM.render(
    <ul>
        {updatedNums}
    </ul>,
    document.getElementById('root')
);

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. 

Javascript

import React from 'react';
import ReactDOM from 'react-dom';

// Component that will return an
// unordered list
function Navmenu(props)
{
    const list = props.menuitems;

    const updatedList = list.map((listItems)=>{
        return <li>{listItems}</li>;
    });

    return(
        <ul>{updatedList}</ul>
    );
}

const menuItems = [1,2,3,4,5];

ReactDOM.render(
    <Navmenu menuitems = {menuItems} />,
    document.getElementById('root')
);

Output: 

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:  

Javascript

import React from 'react';
import ReactDOM from 'react-dom';

// Component that will return an
// unordered list
function Navmenu(props)
{
    const list = props.menuitems;

    const updatedList = list.map((listItems)=>{
        return(
                <li key={listItems.toString()}>
                    {listItems}
                </li>
            );
    });

    return(
        <ul>{updatedList}</ul>
    );
}

const menuItems = [1,2,3,4,5];

ReactDOM.render(
    <Navmenu menuitems = {menuItems} />,
    document.getElementById('root')
);

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.