Introduction

FreeCodeCamp is more than a learning platform; it is a large, open-source codebase with thousands of contributors and millions of lines of JavaScript and TypeScript. Managing such a complex project requires consistent architectural patterns, strict naming conventions, and thorough testing to avoid regressions and maintain reliability.

In this article, we present the most impactful code review rules extracted from FreeCodeCamp. Each rule demonstrates how careful design, consistent data flow, and robust error handling can help large projects stay organized and reduce the risk of subtle bugs over time.

The challenges

Maintaining code quality in FreeCodeCamp is challenging due to its large JavaScript and TypeScript codebase and thousands of contributors. Ensuring consistent patterns, predictable data flow, and reliable functionality requires structured review rules and automated checks.

Key challenges include inconsistent coding patterns, tightly coupled legacy modules, uneven test coverage, documentation drift, and a high volume of pull requests.

Clear standards, automated validation, and careful code review help keep the codebase maintainable, stable, and scalable as the project grows.

Why these rules matter

Consistent code review rules improve maintainability by enforcing uniform module structure, naming conventions, and predictable data flows, making tests more reliable and dependencies easier to track.

They also enhance security through input validation, error handling, and controlled side effects, while accelerating onboarding by helping new contributors understand module responsibilities and integration points quickly.

Bridging context to these rules

These rules are extracted from FreeCodeCamp’s repository and pull requests, reflecting recurring issues like unclear data flow, missing error handling, and inconsistent testing that impact stability and maintainability.

Each rule highlights a concrete pitfall, explains its impact on performance, clarity, or reliability, and includes ❌ non-compliant vs ✅ compliant JavaScript or TypeScript examples.

1. Avoid overuse of any type in TypeScript

Avoid using any type in TypeScript. Always define accurate and explicit types for variables, function parameters, and return values to ensure type safety and prevent runtime errors.

❌ Non-compliant:

let userData: any = fetchUserData();

✅ Compliant:

interface UserData {
  id: string;
  name: string;
  email: string;
}

let userData: UserData = fetchUserData();

Why this matters: Using any disables TypeScript’s type checking, which can allow runtime errors to occur and reduce the maintainability and reliability of the code. Explicit types make the code safer and easier for other developers to understand.

2. Prefer descriptive variable names over abbreviations

Always use clear and descriptive variable names. Avoid abbreviations or cryptic names that obscure the meaning of the code.

❌ Non-compliant:

const usr = getUser();

✅ Compliant:

const user = getUser();

Why this matters: Descriptive variable names make code easier to read, understand, and maintain. Poor naming can confuse developers and increase the risk of introducing bugs.

3. Avoid deeply nested loops or conditionals

Refactor code to prevent deep nesting in loops or conditionals. Use early returns or helper functions to flatten logic.

❌ Non-compliant:

if (user) {
  if (user.isActive) {
    if (user.hasPermission) {
      // Perform action
    }
  }
}

✅ Compliant:

if (!user) return;
if (!user.isActive) return;
if (!user.hasPermission) return;

// Perform action
processUserAction(user);

Why this matters: Deeply nested logic is hard to follow, maintain, and test. It complicates writing unit tests, especially for negative cases and early failures. Flattening control flow with early returns makes the code easier to reason about, improves test coverage, and reduces the chance of hidden edge-case bugs.

4. Ensure consistent error handling across the codebase

Always implement consistent error handling. Use centralized error functions or standardized patterns to handle exceptions uniformly.

❌ Non-compliant:

try {
  // Some code
} catch (e) {
  console.error(e);
}

✅ Compliant:

try {
  // Some code
} catch (error) {
  logError(error);
  throw new CustomError('An error occurred', { cause: error });
}

Why this matters: Consistent error handling makes debugging easier, prevents unexpected behaviors, and ensures reliability across the application.

5. Avoid hardcoding configuration values

Do not hardcode environment-specific values like URLs, ports, or secrets. Always use configuration files or environment variables.

❌ Non-compliant:

const apiUrl = 'https://api.example.com';

✅ Compliant:

const apiUrl = process.env.API_URL;

Why this matters: Hardcoded values reduce flexibility, make code less secure, and complicate deployment across different environments. Using configurations ensures maintainability and security.

6. Keep functions focused on a single responsibility

Ensure each function performs a single, well-defined task. Avoid functions that handle multiple responsibilities, as this can lead to confusion and difficulty in maintenance.

❌ Non-compliant:

function processUserData(user) {
  const validatedUser = validateUser(user);
  saveUserToDatabase(validatedUser);
  sendWelcomeEmail(validatedUser);
}

✅ Compliant:

function validateUser(user) {
  // validation logic
}

function saveUserToDatabase(user) {
  // saving logic
}

function sendWelcomeEmail(user) {
  // email sending logic
}

Why this matters: Functions with a single responsibility are easier to test, debug, and maintain. They promote code reuse and enhance readability.

7. Avoid using magic numbers

Replace magic numbers with named constants to improve code clarity and maintainability.

❌ Non-compliant:

const area = length * 3.14159 * radius * radius;

✅ Compliant:

const PI = 3.14159;
const area = length * PI * radius * radius;

Why this matters: Magic numbers can obscure the meaning of the code and make future modifications error-prone. Named constants provide context and reduce the risk of introducing bugs.

8. Minimize the use of global variables

Limit the use of global variables to reduce dependencies and potential conflicts in the codebase.

❌ Non-compliant:

let user = { name: 'Alice' };

function greetUser() {
  console.log(`Hello, ${user.name}`);
}

✅ Compliant:

function greetUser(user) {
  console.log(`Hello, ${user.name}`);
}

const user = { name: 'Alice' };
greetUser(user);

Why this matters: Global variables can create hidden dependencies and unpredictable side effects. They make it difficult to trace where data comes from or how it changes across the codebase. Passing data explicitly through function parameters keeps data flow clear and controlled, which improves modularity, debugging, and long-term maintainability.

9. Use template literals for string concatenation

Prefer template literals over string concatenation for better readability and performance.

❌ Non-compliant:

const message = 'Hello, ' + user.name + '! You have ' + user.notifications + ' new notifications.';

✅ Compliant:

const message = `Hello, ${user.name}! You have ${user.notifications} new notifications.`;

Why this matters: Template literals provide a cleaner syntax and improve readability, especially when dealing with complex strings or multiline content.

10. Implement proper input validation

Always validate user inputs to prevent invalid data from entering the system and to enhance security.

❌ Non-compliant:

function processUserInput(input) {
  // processing logic
}

✅ Compliant:

function validateInput(input) {
  if (typeof input !== 'string' || input.trim() === '') {
    throw new Error('Invalid input');
  }
}

function processUserInput(input) {
  validateInput(input);
  // processing logic
}

Why this matters: Input validation is crucial for preventing errors, ensuring data integrity, and protecting against security vulnerabilities such as injection attacks.

11. Maintain one logical change per pull request

Ensure each pull request (PR) implements a single logical change or feature; avoid combining unrelated fixes, refactors, and feature additions in one PR.

❌ Non-compliant:

# "Fix login + update homepage"
--- auth.js
+ if (!user) throw new Error('User not found');

--- HomePage.js
- <button>Start</button>
+ <button>Begin Journey</button>

✅ Compliant: (diff)

# PR 1: Fix login validation
+ if (!user) throw new Error('User not found');

# PR 2: Update homepage button
+ <button>Begin Journey</button>

Why this matters: Small, focused PRs simplify code review, reduce risk of unintended side-effects, and speed up merge cycles. AI tools can detect when unrelated files, modules, or domains change in the same PR — something linters cannot determine.

12. Use domain-aligned naming for APIs and services

Name APIs, services, and modules according to the business domain (e.g., challengeService.createSubmission not handler1.doIt); names should clearly reflect entity and action.

❌ Non-compliant:

// backend/services/handler.js
export async function doIt(data) {
  return await process(data);
}

// routes/index.js
router.post('/submit', handler.doIt);

✅ Compliant:

// backend/services/challengeService.js
export async function createSubmission({ userId, challengeId, answer }) {
  return await challengeModel.create({ userId, challengeId, answer });
}

// routes/challenges.js
router.post('/submissions', challengeService.createSubmission);

Why this matters: Domain-aligned naming makes code self-documenting, aids clarity for new contributors, and aligns with business logic. Only an AI aware of semantic context (entity names, service layers) can detect mis-aligned or generic naming across modules.

13. Ensure tests cover failures and edge cases

Write tests not only for the “happy path” but also for error conditions, edge cases, and boundary values; confirm that each critical module has both positive and negative tests.

❌ Non-compliant:

describe('login', () => {
  it('should succeed with correct credentials', async () => { … });
});

✅ Compliant:

describe('login', () => {
  it('should succeed with correct credentials', async () => { … });
  it('should fail with incorrect password', async () => { … });
  it('should lock account after 5 failed attempts', async () => { … });
});

Why this matters: Business-critical logic often breaks when edge cases are missed, like invalid inputs, timeouts, or failed logins. Testing both success and failure paths ensures the application behaves reliably in real-world conditions and prevents regressions that are hard to catch later.

14. Avoid mixing layers: UI components should not perform business logic

Keep UI components (React, front-end) free of business logic and database/service calls; delegate these tasks to dedicated services or hooks.

❌ Non-compliant:

// FreeCodeCamp-style
function CurriculumCard({ user, challenge }) {
  if (!user.completed.includes(challenge.id)) {
    saveCompletion(user.id, challenge.id);
  }
  return <Card>{challenge.title}</Card>;
}

✅ Compliant:

function CurriculumCard({ user, challenge }) {
  return <Card>{challenge.title}</Card>;
}

// In service:
async function markChallengeComplete(userId, challengeId) {
  await completionService.create({ userId, challengeId });
}

Why this matters: Mixing business logic into UI components blurs the boundaries between layers and makes future changes risky. It also forces front-end developers to understand backend logic and slows collaboration. Keeping responsibilities separate ensures each layer can evolve independently and reduces the risk of introducing subtle bugs when refactoring.

Conclusion

By analyzing FreeCodeCamp’s repository, we extracted practical code review rules that help keep its large codebase organized, readable, and maintainable. These 20 rules reflect real practices from years of contributions, even though they are not enforced by any automated tool.

Applying these lessons in your own projects can improve clarity, reduce bugs, and make collaboration smoother. They provide a strong foundation for scaling your code safely, ensuring that as the project grows, the code remains reliable and easy to work with.

Code review is more than checking syntax. It is about maintaining the quality and integrity of your code over time. Learning from a mature open-source project like FreeCodeCamp gives developers concrete guidance to improve any codebase.