Separation of concerns in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a React component that only renders a greeting message.

React

function Greeting() {
  return <h1>{"Hello, friend!"}</h1>;
}

Drag options to blanks, or click blank then click option'

AHello, friend!

B{Hello, friend!}

C"Hello, friend!"

D<Hello, friend!>

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the string causes syntax errors.

Using curly braces without quotes tries to interpret as JavaScript expression.

Adding extra quotes displays them on screen.

In JSX, text content between tags must be a JavaScript string literal like "Hello, friend!" without extra quotes around the entire string.

2fill in blank

medium

Complete the code to separate the button's click logic into a handler function.

React

function Clicker() {
  function [1]() {
    alert('Clicked!');
  }
  return <button onClick={handleClick}>Click me</button>;
}

Drag options to blanks, or click blank then click option'

AclickHandler

BhandleClick

ConClick

Dclick

Attempts:

3 left

💡 Hint

Common Mistakes

Naming the function differently than the onClick attribute causes errors.

Using reserved words like 'onClick' as function names.

The function name must match the event handler used in JSX for clarity and correctness.

3fill in blank

hard

Fix the error in the code by completing the import statement for React hooks.

React

import React, { [1] } from 'react';

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

Drag options to blanks, or click blank then click option'

AuseRef

BuseEffect

CuseContext

DuseState

Attempts:

3 left

💡 Hint

Common Mistakes

Importing the wrong hook causes runtime errors.

Forgetting to import useState leads to 'useState is not defined' error.

The useState hook is needed to create state in the Counter component.

4fill in blank

hard

Fill both blanks to create a component that separates display and logic.

React

function Display({ [1] }) {
  return <p>{message}</p>;
}

function Container() {
  const message = 'Hello from container!';
  return <Display [2]={message} />;
}

Drag options to blanks, or click blank then click option'

Amessage

Btext

Cmsg

Dcontent

Attempts:

3 left

💡 Hint

Common Mistakes

Using different prop names causes 'undefined' errors.

Forgetting to pass the prop from Container to Display.

The prop name must match between the Display component's parameter and the Container's prop passed.

5fill in blank

hard

Fill all three blanks to create a reusable button component with separated concerns.

React

function Button({ [1], [2], [3] }) {
  return <button onClick={onClick} aria-label=[3]>{label}</button>;
}

function App() {
  function handleClick() {
    alert('Button pressed');
  }
  return <Button [1]="Press me" [2]={handleClick} [3]="Press me button" />;
}

Drag options to blanks, or click blank then click option'

Alabel

BonClick

CariaLabel

DclickHandler

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect prop names breaks the component.

Forgetting accessibility props reduces usability.

Props 'label', 'onClick', and 'ariaLabel' separate button text, click logic, and accessibility label.

Practice

1. What does separation of concerns mean in React development?

easy

A. Dividing the app so each part handles one specific task

B. Writing all code in a single component for simplicity

C. Mixing UI and data logic in the same function

D. Using only class components instead of functions

Solution

1. Step 1: Understand the concept of separation of concerns

   It means each part of the app should focus on one clear responsibility, like UI or data handling.

2. Step 2: Identify the correct description

   Dividing the app so each part handles one specific task correctly describes dividing the app so each part handles one specific task, which matches the concept.

3. Final Answer:

   Dividing the app so each part handles one specific task -> Option A

4. Quick Check:

   Separation of concerns = dividing tasks clearly [OK]

Hint: Think: one part, one job in your app [OK]

Common Mistakes:

• Thinking all code should be in one component
• Mixing UI and logic in the same place
• Confusing separation with using classes

2. Which of the following is the correct way to separate UI and logic in React?

easy

A. Put all UI and logic inside one component's render method

B. Create a UI component and a separate hook for logic

C. Write logic inside HTML tags directly

D. Use inline styles to handle logic

Solution

1. Step 1: Identify proper separation of UI and logic

   React encourages using components for UI and hooks or functions for logic.

2. Step 2: Match options to this pattern

   Create a UI component and a separate hook for logic correctly separates UI in a component and logic in a hook, following best practices.

3. Final Answer:

   Create a UI component and a separate hook for logic -> Option B

4. Quick Check:

   UI + logic separated = Create a UI component and a separate hook for logic [OK]

Hint: Use hooks for logic, components for UI [OK]

Common Mistakes:

• Putting logic inside render method
• Trying to write logic inside HTML
• Using styles to control logic

3. Given this React code, what will be the output?

function UserName({ name }) {
  return <h1>User: {name}</h1>;
}

function UserLogic() {
  const user = { name: 'Anna' };
  return <UserName name={user.name} />;
}

medium

A. <h1>User: </h1>

B. Error: user is undefined

C. <h1>User: Anna</h1>

D. UserName component not found

Solution

1. Step 1: Trace the data flow from UserLogic to UserName

   UserLogic creates a user object with name 'Anna' and passes user.name as prop to UserName.

2. Step 2: Check UserName output

   UserName receives name='Anna' and renders <h1>User: Anna</h1> correctly.

3. Final Answer:

   <h1>User: Anna</h1> -> Option C

4. Quick Check:

   Prop passed correctly = <h1>User: Anna</h1> [OK]

Hint: Follow props from parent to child component [OK]

Common Mistakes:

• Assuming user is undefined outside UserLogic
• Expecting empty output if prop missing
• Confusing component names

4. What is wrong with this React code regarding separation of concerns?

function Profile() {
  const [user, setUser] = React.useState(null);
  React.useEffect(() => {
    fetch('/api/user')
      .then(res => res.json())
      .then(data => setUser(data));
  }, []);
  return <div>{user.name}</div>;
}

medium

A. Fetching data and UI rendering are mixed in one component

B. useEffect hook is used incorrectly

C. State should not be used in functional components

D. The fetch URL is invalid

Solution

1. Step 1: Identify concerns in the Profile component

   Profile handles both fetching user data and rendering UI, mixing logic and UI.

2. Step 2: Understand separation of concerns principle

   Best practice is to separate data fetching logic from UI components, e.g., using custom hooks.

3. Final Answer:

   Fetching data and UI rendering are mixed in one component -> Option A

4. Quick Check:

   Mixed logic and UI = Fetching data and UI rendering are mixed in one component [OK]

Hint: Keep data fetching separate from UI components [OK]

Common Mistakes:

• Thinking useEffect is wrong here
• Believing state can't be used in functions
• Assuming fetch URL is the problem

5. You want to improve this React app's separation of concerns:

function Dashboard() {
  const [data, setData] = React.useState(null);
  React.useEffect(() => {
    fetch('/api/data')
      .then(res => res.json())
      .then(json => setData(json));
  }, []);
  return <div>{data ? <Display data={data} /> : 'Loading...'}</div>;
}

Which change best improves separation of concerns?

hard

A. Remove useEffect and fetch data synchronously

B. Combine Display and Dashboard into one component

C. Fetch data inside Display component instead

D. Move data fetching into a custom hook and keep Dashboard for UI only

Solution

1. Step 1: Analyze current code responsibilities

   Dashboard handles both fetching data and rendering UI, mixing concerns.

2. Step 2: Choose best separation approach

   Moving data fetching to a custom hook isolates logic, leaving Dashboard focused on UI.

3. Final Answer:

   Move data fetching into a custom hook and keep Dashboard for UI only -> Option D

4. Quick Check:

   Separate logic in hooks, UI in components = Move data fetching into a custom hook and keep Dashboard for UI only [OK]

Hint: Use custom hooks for logic, components for display [OK]

Common Mistakes:

• Merging components reduces separation
• Fetching data inside UI components mixes concerns
• Removing useEffect breaks async data loading