Separation of concerns in React

Introduction

Separation of concerns means keeping different parts of your code focused on one job. This makes your app easier to understand and fix.

When building a React app with multiple features like buttons, lists, and forms.

When you want to make your code easier to test and change later.

When working with a team so everyone can work on different parts without confusion.

When you want to reuse parts of your app in other projects.

When you want to keep your UI, data, and logic separate for clarity.

Syntax

React

function ComponentA() {
  // Handles UI only
  return <div>UI here</div>;
}

function ComponentB() {
  // Handles data or logic
  const data = fetchData();
  return <ComponentA data={data} />;
}

Each component should have one clear job, like showing UI or handling data.

Use props to pass data between components to keep them separate.

Examples

This Button component only shows a button and calls a function when clicked. It does not handle what happens on click.

React

function Button({ onClick, label }) {
  return <button onClick={onClick}>{label}</button>;
}

The App component handles the logic for what happens when the button is clicked. It passes this to Button as a prop.

React

function App() {
  const handleClick = () => alert('Clicked!');
  return <Button onClick={handleClick} label="Click me" />;
}

Sample Program

This Counter app splits the UI and logic into separate parts. CounterDisplay only shows the count. CounterButtons only shows buttons and calls functions when clicked. The main Counter component holds the count state and logic to change it.

This makes the code clear and easy to change or reuse parts.

React

import React, { useState } from 'react';

function CounterDisplay({ count }) {
  return <h1>Count: {count}</h1>;
}

function CounterButtons({ onIncrement, onDecrement }) {
  return (
    <div>
      <button onClick={onIncrement} aria-label="Increase count">+</button>
      <button onClick={onDecrement} aria-label="Decrease count">-</button>
    </div>
  );
}

export default function Counter() {
  const [count, setCount] = useState(0);

  const increment = () => setCount(c => c + 1);
  const decrement = () => setCount(c => c - 1);

  return (
    <main>
      <CounterDisplay count={count} />
      <CounterButtons onIncrement={increment} onDecrement={decrement} />
    </main>
  );
}

Important Notes

Keep components small and focused on one job.

Use props to share data and actions between components.

Separation helps with testing, reusing, and fixing code.

Summary

Separation of concerns means each part of your app does one clear thing.

Use small components for UI and separate logic or data handling.

This makes your React app easier to read, test, and maintain.

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