Clean Code: Managing Side Effects with Functional Programming

Side effects can make code unpredictable and hard to maintain. Let's explore how functional programming helps us handle them effectively.

What Are Side Effects?

A function creates a side effect if it does anything beyond taking inputs and returning outputs. Some common side effects include:

• Changing global variables
• Modifying input parameters
• Writing to files or databases
• Making API calls
• Updating the DOM
• Logging to console

Why Side Effects Can Cause Problems

Here's an example that shows problematic side effects:

let userProfile = {
  name: "Alice Johnson",
  email: "[e-mail]",
  preferences: {
    theme: "dark",
    notifications: true
  }
};

function updateUserTheme(newTheme) {
  userProfile.preferences.theme = newTheme;
}

function toggleNotifications() {
  userProfile.preferences.notifications = !userProfile.preferences.notifications;
}

// Multiple functions modifying the same global state
updateUserTheme("light");
toggleNotifications();

console.log(userProfile); // State is unpredictable

This code has several issues:

1. It uses global state
2. Multiple functions can change the same data
3. Changes become difficult to track
4. Testing gets complicated

A Better Solution with Pure Functions

Here's an improved version using functional programming principles:

const createUserProfile = (name, email, theme, notifications) => ({
  name,
  email,
  preferences: {
    theme,
    notifications
  }
});

const updateTheme = (profile, newTheme) => ({
  ...profile,
  preferences: {
    ...profile.preferences,
    theme: newTheme
  }
});

const toggleNotifications = (profile) => ({
  ...profile,
  preferences: {
    ...profile.preferences,
    notifications: !profile.preferences.notifications
  }
});

// Usage
const initialProfile = createUserProfile(
  "Alice Johnson",
  "[e-mail]",
  "dark",
  true
);

const updatedProfile = updateTheme(initialProfile, "light");
const finalProfile = toggleNotifications(updatedProfile);

console.log(initialProfile); // Original state unchanged
console.log(finalProfile);   // New state with updates

Practical Example: File Operations

Here's how to handle necessary side effects in file operations using functional programming:

// Separate pure business logic from side effects
const createUserData = (user) => ({
  id: user.id,
  name: user.name,
  createdAt: new Date().toISOString()
});

const createLogEntry = (error) => ({
  message: error.message,
  timestamp: new Date().toISOString(),
  stack: error.stack
});

// Side effect handlers (kept at the edges of the application)
const writeFile = async (filename, data) => {
  const serializedData = JSON.stringify(data);
  await fs.promises.writeFile(filename, serializedData);
  return data;
};

const appendFile = async (filename, content) => {
  await fs.promises.appendFile(filename, content);
  return content;
};

// Usage with composition
const saveUser = async (user) => {
  const userData = createUserData(user);
  return writeFile('users.json', userData);
};

const logError = async (error) => {
  const logData = createLogEntry(error);
  return appendFile('error.log', JSON.stringify(logData) + '\n');
};

Handling Side Effects with Functional Programming

1. Pure Functions

   // Pure function - same input always gives same output
   const calculateTotal = (items) => 
     items.reduce((sum, item) => sum + item.price, 0);

   // Side effect wrapped in a handler function
   const processPurchase = async (items) => {
     const total = calculateTotal(items);
     await saveToDatabase(total);
     return total;
   };

1. Function Composition

   const pipe = (...fns) => (x) => 
     fns.reduce((v, f) => f(v), x);

   const processUser = pipe(
     validateUser,
     normalizeData,
     saveUser
   );

1. Data Transformation

   const transformData = (data) => {
     const addTimestamp = (item) => ({
       ...item,
       timestamp: new Date().toISOString()
     });

     const normalize = (item) => ({
       ...item,
       name: item.name.toLowerCase()
     });

     return data
       .map(addTimestamp)
       .map(normalize);
   };

Testing Pure Functions

Testing becomes much simpler with pure functions:

describe('User Profile Functions', () => {
  const initialProfile = createUserProfile(
    'Alice',
    '[e-mail]',
    'dark',
    true
  );

  test('updateTheme returns new profile with updated theme', () => {
    const newProfile = updateTheme(initialProfile, 'light');

    expect(newProfile).not.toBe(initialProfile);
    expect(newProfile.preferences.theme).toBe('light');
    expect(initialProfile.preferences.theme).toBe('dark');
  });

  test('toggleNotifications flips the notifications setting', () => {
    const newProfile = toggleNotifications(initialProfile);

    expect(newProfile.preferences.notifications).toBe(false);
    expect(initialProfile.preferences.notifications).toBe(true);
  });
});

Final Thoughts

Functional programming offers powerful tools for managing side effects:

• Keep core logic pure and predictable
• Handle side effects at the edges of your application
• Use composition to combine functions
• Return new data instead of modifying existing state

These practices lead to code that's easier to test, understand, and maintain.

---

How do you handle side effects in your functional code? Share your approaches in the comments!