Conditional component rendering in React - Mini Project: Build & Apply

Conditional component rendering

📖 Scenario: You are building a simple React app that shows a greeting message only if the user is logged in.

🎯 Goal: Create a React component that conditionally renders a greeting message based on a login status.

📋 What You'll Learn

Create a state variable to track if the user is logged in

Create a boolean variable to represent the login status

Use conditional rendering to show a greeting message only when logged in

Render a button to toggle the login status

💡 Why This Matters

🌍 Real World

Conditional rendering is common in apps to show or hide parts of the UI based on user actions or data, like showing login status or notifications.

💼 Career

Understanding conditional rendering is essential for React developers to build dynamic and interactive user interfaces.

1

Set up the initial React component and state

Create a React functional component called Greeting. Inside it, use the useState hook to create a state variable called isLoggedIn and set its initial value to false.

React

import React, { useState } from 'react';

function Greeting() {
  // Create state variable isLoggedIn with initial value false
  
}

Hint

Use const [isLoggedIn, setIsLoggedIn] = useState(false); inside the component.

2

Add a button to toggle login status

Inside the Greeting component, add a button element. Set its onClick handler to toggle the isLoggedIn state between true and false using setIsLoggedIn.

React

import React, { useState } from 'react';

function Greeting() {
  const [isLoggedIn, setIsLoggedIn] = useState(false);
  // Add a button to toggle isLoggedIn state
}

Hint

Use <button onClick={() => setIsLoggedIn(!isLoggedIn)}> to toggle the login state.

3

Add conditional rendering for the greeting message

Inside the Greeting component's return, add a conditional rendering that shows a <p> element with the text "Welcome back!" only if isLoggedIn is true. Keep the toggle button visible always.

React

import React, { useState } from 'react';

function Greeting() {
  const [isLoggedIn, setIsLoggedIn] = useState(false);

  return (
    // Add conditional rendering for greeting message here
    
       setIsLoggedIn(!isLoggedIn)}>
        Toggle Login
      
    
  );
}

Hint

Use {isLoggedIn && <p>Welcome back!</p>} inside the return.

4

Add accessibility and semantic improvements

Wrap the button and greeting message inside a <section> element. Add an aria-live="polite" attribute to the <p> greeting message to announce changes to screen readers.

React

import React, { useState } from 'react';

function Greeting() {
  const [isLoggedIn, setIsLoggedIn] = useState(false);

  return (
    // Wrap content in a section and add aria-live to greeting
  );
}

Hint

Wrap the content in <section> and add aria-live="polite" to the greeting paragraph.

Practice

1. What does conditional rendering in React allow you to do?

easy

A. Create new components on the fly without JSX

B. Change the CSS styles of components dynamically

C. Automatically fetch data from APIs

D. Show or hide parts of the UI based on data or state

Solution

1. Step 1: Understand the purpose of conditional rendering

   Conditional rendering controls what parts of the UI appear based on conditions like state or props.

2. Step 2: Differentiate from other React features

   Changing styles or fetching data are different tasks; conditional rendering specifically controls visibility.

3. Final Answer:

   Show or hide parts of the UI based on data or state -> Option D

4. Quick Check:

   Conditional rendering = Show/hide UI [OK]

Hint: Conditional rendering controls UI visibility based on conditions [OK]

Common Mistakes:

• Confusing conditional rendering with styling changes
• Thinking it automatically fetches data
• Believing it creates components without JSX

2. Which of the following is the correct syntax for conditional rendering using a ternary operator in React JSX?

easy

A. if (condition) <ComponentA /> else <ComponentB />

B. {condition ? <ComponentA /> : <ComponentB />}

C. {if condition then <ComponentA /> else <ComponentB />}

D. {condition && <ComponentA /> else <ComponentB />}

Solution

1. Step 1: Recall JSX expression syntax

   JSX uses curly braces {} to embed JavaScript expressions, including ternary operators.

2. Step 2: Identify correct ternary operator usage

   The ternary operator syntax is condition ? exprIfTrue : exprIfFalse, which matches {condition ? <ComponentA /> : <ComponentB />}.

3. Final Answer:

   {condition ? <ComponentA /> : <ComponentB />} -> Option B

4. Quick Check:

   Ternary in JSX = condition ? true : false [OK]

Hint: Use curly braces and ? : for ternary in JSX [OK]

Common Mistakes:

• Using if-else statements directly inside JSX
• Writing invalid JavaScript syntax inside JSX
• Confusing logical AND with ternary operator

3. Given the code below, what will be rendered if isLoggedIn is false?

function Greeting({ isLoggedIn }) {
  return (
    <div>
      {isLoggedIn ? <h1>Welcome back!</h1> : <h1>Please sign up.</h1>}
    </div>
  );
}

medium

A. Welcome back!

B. Syntax error

C. Please sign up.

D. Nothing will render

Solution

1. Step 1: Check the value of isLoggedIn

   The value is false, so the condition in the ternary operator is false.

2. Step 2: Evaluate the ternary operator

   Since condition is false, the expression after the colon is rendered: <h1>Please sign up.</h1>.

3. Final Answer:

   <h1>Please sign up.</h1> -> Option C

4. Quick Check:

   isLoggedIn false = show sign up message [OK]

Hint: False condition shows expression after colon in ternary [OK]

Common Mistakes:

• Assuming true branch renders when condition is false
• Thinking nothing renders on false
• Confusing syntax with runtime error

4. Identify the error in this React component that tries to conditionally render a message:

function Message({ show }) {
  return (
    <div>
      {show && <p>Hello World!</p> else <p>No message</p>}
    </div>
  );
}

medium

A. Using else inside JSX expression is invalid

B. Missing parentheses around JSX elements

C. Incorrect use of logical AND operator

D. Component must return a string, not JSX

Solution

1. Step 1: Review JSX expression syntax

   JSX expressions inside curly braces must be valid JavaScript expressions; 'else' cannot be used inside them.

2. Step 2: Identify invalid use of else

   The code uses 'else' after a logical AND expression, which is not valid JavaScript syntax.

3. Final Answer:

   Using else inside JSX expression is invalid -> Option A

4. Quick Check:

   JSX expressions cannot contain else keyword [OK]

Hint: No else keyword inside JSX expressions; use ternary instead [OK]

Common Mistakes:

• Trying to use else with logical AND
• Forgetting ternary operator syntax
• Assuming JSX can contain statements like else

5. You want to render a list of notifications only if the list is not empty. Which React pattern correctly handles this conditional rendering?

const notifications = [];

function NotificationList() {
  return (
    <div>
      {/* Choose the correct conditional rendering here */}
      {notifications.length > 0 ? (
        notifications.map((note, index) => <p key={index}>{note}</p>)
      ) : (
        <p>No notifications</p>
      )}
    </div>
  );
}

hard

A. Use ternary operator checking notifications.length > 0

B. Use only logical AND: {notifications.length && notifications.map(...)}

C. Use if statement inside JSX to check length

D. Render notifications.map(...) without any condition

Solution

1. Step 1: Understand the need for else case

   We want to show a message if there are no notifications, so we need both true and false cases.

2. Step 2: Evaluate each option

   Use ternary operator checking notifications.length > 0 uses a ternary operator to handle both cases correctly. Use only logical AND: {notifications.length && notifications.map(...)} only renders when length is truthy but shows nothing if empty. Use if statement inside JSX to check length is invalid syntax inside JSX. Render notifications.map(...) without any condition always renders map, which errors if empty.

3. Final Answer:

   Use ternary operator checking notifications.length > 0 -> Option A

4. Quick Check:

   Ternary handles both cases in JSX [OK]

Hint: Use ternary to handle both empty and non-empty lists [OK]

Common Mistakes:

• Using if statements directly inside JSX
• Using logical AND without else case
• Rendering map on empty array without check