Conditional component rendering in React - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is conditional component rendering in React?

It means showing or hiding parts of the UI based on certain conditions, like a light switch turning on or off depending on if it's dark or bright.

beginner

How do you use the && operator for conditional rendering in React?

You write a condition before &&, and if it's true, React shows the component after it. If false, React shows nothing, like saying 'If hungry && have food, then eat.'

intermediate

What is the difference between using if-else and ternary operator for rendering in React?

If-else is used outside JSX to decide what to render. Ternary is a short way inside JSX to pick between two things, like choosing between tea or coffee in one line.

intermediate

Why should you avoid using if statements directly inside JSX?

Because JSX expects expressions, not statements. Using if inside JSX causes errors. Instead, use ternary or prepare variables before JSX, like planning your outfit before going out.

beginner

What happens if you return null in a React component?

Returning null means React renders nothing for that component, like closing your eyes to ignore something.

Which operator is commonly used in React to render a component only if a condition is true?

A &&

B ||

C ? :

D !

The && operator lets you render something only when the condition before it is true.

What does this JSX code do? {`{isLoggedIn ? : }`}

A Always shows Dashboard

B Shows Dashboard if isLoggedIn is true, else Login

C Shows Login if isLoggedIn is true, else Dashboard

D Shows nothing

The ternary operator picks Dashboard when isLoggedIn is true, otherwise Login.

What will React render if a component returns null?

A An error

B A default component

C The string 'null'

D An empty space (renders nothing)

Returning null means React renders nothing for that component.

Why can't you use an if statement directly inside JSX?

A JSX only accepts expressions, not statements

B If statements are deprecated in React

C JSX only allows functions

D If statements cause syntax errors in JavaScript

JSX expects expressions like ternary or logical operators, not statements like if.

Which is the best way to conditionally render multiple components based on different conditions?

A Use && operator only

B Use nested ternary operators inside JSX

C Use multiple if-else blocks outside JSX

D Use switch-case inside JSX

Using if-else outside JSX helps keep code clear and avoids complex nested ternaries.

Explain how you can show a React component only when a user is logged in.

Think about using a simple true/false check to decide what to show.

Describe why returning null from a React component is useful in conditional rendering.

Imagine closing your eyes to ignore something on screen.

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