Functional components in React - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a functional component in React?

A functional component is a JavaScript function that returns React elements to describe what should appear on the screen. It is simple and uses hooks for state and side effects.

beginner

How do you define a functional component that shows a greeting message?

You write a function that returns JSX. For example:<br><pre>function Greeting() {<br>  return &lt;h1&gt;Hello!&lt;/h1&gt;;<br>}</pre>

intermediate

Why do we use hooks like useState in functional components?

Hooks like useState let functional components remember information (state) between renders, making them interactive without needing class components.

intermediate

What is the difference between a functional component and a class component?

Functional components are simpler functions returning JSX and use hooks for features. Class components use ES6 classes and lifecycle methods. Functional components are now preferred.

intermediate

Can functional components have side effects like fetching data? How?

Yes, by using the useEffect hook inside the functional component, you can run code after rendering, like fetching data or updating the document title.

What does a functional component in React return?

A A string

B JSX elements

C A number

D A class instance

Functional components return JSX elements that describe the UI.

Which hook is used to add state to a functional component?

A useState

B useContext

C useEffect

D useReducer

useState is the hook that lets you add state to functional components.

How do you write a simple functional component named Hello that shows 'Hi'?

A function Hello() { return <h1>Hi</h1>; }

B class Hello extends React.Component { render() { return 'Hi'; } }

C const Hello = () => 'Hi';

D function Hello() { console.log('Hi'); }

A functional component returns JSX, so option A is correct.

Which hook lets you perform side effects like data fetching in functional components?

A useCallback

B useState

C useMemo

D useEffect

useEffect runs side effects after rendering.

Why are functional components preferred over class components in modern React?

A They do not support state

B They use more memory

C They are simpler and use hooks for features

D They require more code

Functional components are simpler and use hooks, making them easier to write and maintain.

Explain what a functional component is and how it differs from a class component in React.

Think about how React components can be written as functions or classes.

Describe how you would add state and side effects to a functional component.

Hooks are the key to adding features in functional components.

Practice

1. What is a React functional component?

easy

A. A JavaScript function that returns JSX to render UI

B. A class that extends React.Component

C. A CSS style sheet for React components

D. A special HTML tag used in React

Solution

1. Step 1: Understand React component types

   React components can be functions or classes that return UI elements.

2. Step 2: Identify functional component definition

   A functional component is a plain JavaScript function returning JSX, not a class or style.

3. Final Answer:

   A JavaScript function that returns JSX to render UI -> Option A

4. Quick Check:

   Functional component = function returning JSX [OK]

Hint: Functional components are just functions returning JSX [OK]

Common Mistakes:

• Confusing functional components with class components
• Thinking functional components are CSS or HTML tags
• Believing functional components must extend React.Component

2. Which of the following is the correct syntax for a React functional component named Greeting?

easy

A. const Greeting = Hello!;

B. function Greeting() { return Hello!; }

C. class Greeting extends React.Component { render() { return Hello!; } }

D. function Greeting { return Hello!; }

Solution

1. Step 1: Check function declaration syntax

   function Greeting() { return

   Hello!

   ; } correctly declares a function with parentheses and returns JSX.

2. Step 2: Identify syntax errors in other options

   const Greeting =

   Hello!

   ; assigns JSX directly without a function wrapper; class Greeting extends React.Component { render() { return

   Hello!

   ; } } is a class component, not functional; function Greeting { return

   Hello!

   ; } misses parentheses in function declaration.

3. Final Answer:

   function Greeting() { return

   Hello!

   ; } -> Option B

4. Quick Check:

   Function syntax with parentheses and return JSX [OK]

Hint: Functional components need parentheses after function name [OK]

Common Mistakes:

• Omitting parentheses in function declaration
• Confusing class components with functional components
• Assigning JSX directly without a function

3. What will be rendered by this React functional component?

function Welcome(props) {
  return <h2>Hello, {props.name}!</h2>;
}

// Usage: <Welcome name="Alice" />

medium

A. <Welcome name="Alice" />

B. Hello, {props.name}!

C. Error: props is undefined

D. <h2>Hello, Alice!</h2>

Solution

1. Step 1: Understand props usage in functional components

   The component receives props and uses props.name inside JSX to display the name.

2. Step 2: Substitute props.name with passed value

   Since name="Alice" is passed, {props.name} becomes "Alice" in the output.

3. Final Answer:

   <h2>Hello, Alice!</h2> -> Option D

4. Quick Check:

   props.name replaced by "Alice" in JSX output [OK]

Hint: Props values replace placeholders inside JSX [OK]

Common Mistakes:

• Thinking JSX shows literal {props.name} text
• Assuming props is undefined without passing
• Confusing component usage with rendered output

4. Identify the error in this React functional component:

function Counter() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

medium

A. Missing return statement

B. Incorrect JSX syntax inside return

C. useState is not imported from React

D. setCount should not be called inside onClick

Solution

1. Step 1: Check useState usage

   useState is a React hook and must be imported explicitly from 'react'.

2. Step 2: Verify other code parts

   The return statement and JSX syntax are correct; calling setCount in onClick is valid.

3. Final Answer:

   useState is not imported from React -> Option C

4. Quick Check:

   Hooks require import from React [OK]

Hint: Always import useState from React before using it [OK]

Common Mistakes:

• Forgetting to import useState
• Thinking setCount can't be called in event handlers
• Assuming JSX syntax errors without checking imports

5. You want to create a functional component that shows a list of items passed as props and highlights the selected item on click. Which approach correctly uses hooks and props together?

hard

A. Use useState to track selected item and map props.items to render list with onClick handlers updating state

B. Use class component with this.state and this.props to manage selected item and render list

C. Use useEffect to directly modify props.items and highlight selected item

D. Render props.items without state and rely on CSS :hover to highlight selection

Solution

1. Step 1: Understand state and props roles

   Props provide data (items), state tracks UI changes (selected item).

2. Step 2: Choose correct React hooks usage

   useState is used to track selected item; mapping props.items with onClick updates state correctly.

3. Step 3: Evaluate other options

   Class components are outdated here; useEffect should not modify props; CSS :hover can't track selection state.

4. Final Answer:

   Use useState to track selected item and map props.items to render list with onClick handlers updating state -> Option A

5. Quick Check:

   State for selection + props for data = Use useState to track selected item and map props.items to render list with onClick handlers updating state [OK]

Hint: Use useState for UI changes, props for data input [OK]

Common Mistakes:

• Trying to modify props directly
• Using class components instead of functional with hooks
• Relying on CSS hover for persistent selection