Conditional component rendering in React - Step-by-Step Execution

Concept Flow - Conditional component rendering

Start Render

↓

Evaluate Condition

↓

Render Component A

↓

Display Output

React checks a condition during rendering and shows one component or another based on that condition.

Execution Sample

React

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

This component shows a welcome message if the user is logged in, otherwise it asks to sign in.

Execution Table

Step	isLoggedIn Value	Condition (isLoggedIn ?)	Component Rendered	Output Text
1	true	true	<h1>Welcome back!</h1>	Welcome back!
2	false	false	<h1>Please sign in.</h1>	Please sign in.
3	null	false	<h1>Please sign in.</h1>	Please sign in.
4	undefined	false	<h1>Please sign in.</h1>	Please sign in.

💡 Rendering ends after choosing the component based on the condition.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
isLoggedIn	undefined	true	false	null	undefined

Key Moments - 2 Insights

Why does the component render the 'Please sign in.' message when isLoggedIn is null or undefined?

Because in JavaScript, null and undefined are treated as false in conditions, so the else part of the conditional renders. See execution_table rows 3 and 4.

What happens if we forget to return a component in one branch?

React will render nothing for that branch, which might cause blank space or missing UI. The execution_table shows both branches always return a component.

Visual Quiz - 3 Questions

Look at the execution_table, what component is rendered when isLoggedIn is false?

A <h1>Please sign in.</h1>

B <h1>Welcome back!</h1>

C No component

D An error

💡 Hint

Check execution_table row 2 under 'Component Rendered'

At which step does the condition evaluate to true?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Condition (isLoggedIn ?)' column in execution_table

If isLoggedIn was changed to always be true, how would the output change?

A Always show 'Please sign in.'

B Always show 'Welcome back!'

C Show nothing

D Alternate messages each render

💡 Hint

Refer to variable_tracker and execution_table rows where isLoggedIn is true

Concept Snapshot

Conditional component rendering in React:
Use a condition inside the return to choose which component to show.
Syntax example: return condition ? <A /> : <B />;
If condition is true, render first component; else render second.
Useful for showing different UI based on state or props.

Full Transcript

This visual guide shows how React decides which component to render based on a condition. The example component Greeting takes a prop isLoggedIn. If true, it renders a welcome message. If false or falsy, it asks the user to sign in. The execution table traces different values of isLoggedIn and the resulting output. The variable tracker shows how isLoggedIn changes across steps. Key moments clarify why falsy values render the else branch and the importance of returning components in both branches. The quiz tests understanding of which component renders at each step and how changing isLoggedIn affects output.

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