ReactJS Lists

Lists are very useful when it comes to developing the UI of any website. Lists are mainly used for displaying menus on a website, for example, the navbar menu. In regular JavaScript, we can use arrays for creating lists. 

Creating and traversing a list

We can create lists in React in a similar manner as we do in regular JavaScript i.e. by storing the list in an array. In order to traverse a list we will use the map() function.

Step 1: Create a list of elements in React in the form of an array and store it in a variable. We will render this list as an unordered list element in the browser.

Step 2: We will then traverse the list using the JavaScript map() function and updates elements to be enclosed between <li> </li> elements. 

Step 3: Finally we will wrap this new list within <ul> </ul> elements and render it to the DOM.

Write the below code in the index.js file of your react application:  

• 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') );

Output: The above code will render an unordered list as shown below 

ReactJS Lists

Rendering lists inside Components

In the above code in React, we directly rendered the list to the DOM. But usually, this is 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 a list 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: 

ReactJS Lists

The above code will give a warning in console of browser like:

ReactJS Lists Warning

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') );

Output: In the below-shown output you can see the rendered output is the same but this time without any warning in the console.

ReactJS Lists with Keys

Keys are used in React to identify which items in the list are changed, updated, or deleted. We will learn about keys in more detail in our ReactJS keys article.