ReactJS - Conditional Rendering

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Conditional rendering in React

Conditional rendering is used to show / hide a certain part of the UI to the user depending on the situation. For example, if the user is not logged into a web application, the web application will show the login button. When the user logged into the web application, the same link will be replaced by the welcome message.

Let us learn the options provided by the React to support conditional rendering.

Methods of conditional rendering

React provides multiple ways to do conditional rendering in a web application. They are as follows −

• Conditional statement

• JSX / UI variable

• Logical && operator in JSX

• Conditional operator in JSX

• null return value

Conditional statement

Conditional statement is simple and straight forward option to render a UI based on a condition. Let us consider that the requirement is to write a component which will show either login link or welcome message based on the user's login status. Below is the implementation of the component using conditional rendering,

function Welcome(props) {
   if(props.isLoggedIn) {
      return <div>Welcome, {props.userName}</div>
   } else {
      return <div><a href="/login">Login</a></div>
   }
}

Here,

• Used isLoggedIn props to check whether the user is already logged in or not.

• Returned welcome message, if the user is already logged in.

• Returned login link, if the user is not logged in.

The component can be used as shown below C−

function App() {
   return (
      <div className="container">
         <div style={{ padding: "10px" }}>
            <div>
               <Welcome isLoggedIn={true} userName={'John'} />
            </div>
         </div>
      </div>
   );
}

The component will render the welcome message as shown below −

JSX / UI variable

React allows the storing of JSX element into a variable and use it whenever necessary. Developer can create the necessary JSX through conditional statement and store it in a variable. Once the UI is stored in a variable, it can be rendered as shown below −

function Welcome(props) {
   let output = null;
   if(props.isLoggedIn) {
      output = <div>Welcome, {props.userName}</div>
   } else {
      output = <div><a href="/login">Login</a></div>
   }
   return output
}

Here, we have used the variable output to hold the UI. The component can be used as shown below −

function App() {
   return (
      <div className="container">
         <div style={{ padding: "10px" }}>
            <div>
               <Welcome isLoggedIn={true} userName={'John'} />
            </div>
         </div>
      </div>
   );
}

The component will render the welcome message as shown below −

Logical && operator

React allows any expression to be used in the JSX code. In Javascript, condition are applied from left to right. If the left most condition is false, then the next condition will not be processed. Developer can utilize this feature and output the message in the JSX itself as shown below −

function ShowUsers(props) {
   return (
      <div>
         <ul>
            {props.users && props.users.length > 0 &&
               props.users.map((item) => 
                  (
                     <li>{item}</li>
                  )
               )}
         </ul>
      </div>
   );
}
export default ShowUsers;

Here,

• First of all, props.users will be checked for availability. If props.users is null, then the condition will not be processed further

• Once, props.users is available, then the length of the array will be checked and only if the length is greater then zero, the condition will be processed further

• Finally, props.users will be looped over through map function and the users information will be rendered as unordered list.

The component can be used as shown below −

function App() {
   const users = ['John', 'Peter']
   return (
      <div className="container">
         <div style={{ padding: "10px" }}>
            <div>
               <ShowUsers users={users} />
            </div>
         </div>
      </div>
   );
}

The component will render the users as shown below −

Conditional operator in JSX

Since React allows any javascript expression in JSX, developer can use conditional operator (a =b ? x : y) in the JSX and render the necessary UI element only as shown below −

function Welcome(props) {
   if(props.isLoggedIn) {
      return props.isLoggedIn ?
      <div>Welcome, {props.userName}</div> : <div><a href="/login">Login</a></div>
   }
}

Here, we have used the conditional operator to show either welcome message or login link. The component can be used as shown below −

function App() {
   return (
      <div className="container">
         <div style={{ padding: "10px" }}>
            <div>
               <Welcome isLoggedIn={true} userName={'John'} />
            </div>
         </div>
      </div>
   );
}

The component will render the welcome message as shown below −

The null return value

React renders a component only when the component returns a UI element. Otherwise, it silently skip the rendering without any error message. Developer can exploit this feature and render certain UI only when a condition is met.

function Welcome(props) {
   return props.isLoggedIn ? <div>Welcome, {props.userName}</div> : null
}

Here,

• We have used conditional operator to either show / hide the welcome message

• null does not render any UI

The component can be used as shown below −

function App() {
   return (
      <div className="container">
         <div style={{ padding: "10px" }}>
            <div>
               <div>Welcome component will not output any content</div>
                  <Welcome isLoggedIn={false} />
               </div>
            </div>
         </div>
      </div>
   );
}

The component will render the welcome message as shown below −

Summary

React provides multiple way to conditionally render a UI element. Method has to be chosen by the developer by analyzing the situation