SOLID Principles in JavaScript and React

1. Single Responsibility Principle (SRP)

A class/component should have only one reason to change.

❌ Bad Example

class UserProfile {
  constructor(user) {
    this.user = user;
  }

  saveToDatabase() {
    // Database logic here
  }

  formatUserData() {
    // Data formatting logic
  }

  renderUserProfile() {
    // UI rendering logic
  }
}

✅ Good Example

// Data Service
class UserService {
  saveUser(user) {
    // Database logic here
  }
}

// Data Formatter
class UserFormatter {
  format(user) {
    // Data formatting logic
  }
}

// React Component
const UserProfile = ({ user }) => {
  const userService = new UserService();
  const userFormatter = new UserFormatter();

  const handleSave = () => {
    const formattedData = userFormatter.format(user);
    userService.saveUser(formattedData);
  };

  return (
    <div>
      <h1>{user.name}</h1>
      <button onClick={handleSave}>Save</button>
    </div>
  );
};

2. Open/Closed Principle (OCP)

Software entities should be open for extension but closed for modification.

❌ Bad Example

class Button {
  constructor(type) {
    this.type = type;
  }

  render() {
    if (this.type === 'primary') {
      return '<button class="primary">Click</button>';
    } else if (this.type === 'secondary') {
      return '<button class="secondary">Click</button>';
    }
    // Adding new button types requires modifying existing code
  }
}

✅ Good Example

// Abstract base class
class ButtonBase {
  getStyles() {
    throw new Error('Must implement getStyles');
  }

  render() {
    return <button className={this.getStyles()}>{this.props.children}</button>;
  }
}

// Extended button types
class PrimaryButton extends ButtonBase {
  getStyles() {
    return 'bg-blue-500 text-white';
  }
}

class SecondaryButton extends ButtonBase {
  getStyles() {
    return 'bg-gray-500 text-white';
  }
}

// New button types can be added without modifying existing code
class DangerButton extends ButtonBase {
  getStyles() {
    return 'bg-red-500 text-white';
  }
}

3. Liskov Substitution Principle (LSP)

Derived classes must be substitutable for their base classes.

❌ Bad Example

class Bird {
  fly() {
    return "I can fly!";
  }
}

class Penguin extends Bird {
  fly() {
    throw new Error("I can't fly!"); // Violates LSP
  }
}

✅ Good Example

class Bird {
  move() {
    throw new Error('Must implement move');
  }
}

class FlyingBird extends Bird {
  move() {
    return "I can fly!";
  }
}

class SwimmingBird extends Bird {
  move() {
    return "I can swim!";
  }
}

// React Example
const BirdComponent = ({ bird }) => {
  return <div>{bird.move()}</div>;
};

4. Interface Segregation Principle (ISP)

A client should not be forced to depend on interfaces it doesn't use.

❌ Bad Example

class UserActions {
  login(user) { /* ... */ }
  logout() { /* ... */ }
  updateProfile(data) { /* ... */ }
  deleteAccount() { /* ... */ }
}

// Component forced to implement all methods
class UserProfile extends React.Component {
  userActions = new UserActions();
  // Must include all methods even if not needed
}

✅ Good Example

// Separate interfaces
class AuthService {
  login(user) { /* ... */ }
  logout() { /* ... */ }
}

class ProfileService {
  updateProfile(data) { /* ... */ }
}

class AccountService {
  deleteAccount() { /* ... */ }
}

// Components can use only what they need
const LoginComponent = () => {
  const authService = new AuthService();
  
  return <button onClick={() => authService.login()}>Login</button>;
};

const ProfileEditor = () => {
  const profileService = new ProfileService();
  
  return <button onClick={() => profileService.updateProfile()}>Update</button>;
};

5. Dependency Inversion Principle (DIP)

High-level modules should not depend on low-level modules. Both should depend on abstractions.

❌ Bad Example

class NotificationService {
  constructor() {
    this.emailSender = new EmailSender(); // Direct dependency
  }

  sendNotification(message) {
    this.emailSender.send(message);
  }
}

✅ Good Example

// Abstract interface
class NotificationSender {
  send(message) {
    throw new Error('Must implement send');
  }
}

// Concrete implementations
class EmailSender extends NotificationSender {
  send(message) {
    // Send email
  }
}

class SMSSender extends NotificationSender {
  send(message) {
    // Send SMS
  }
}

// React Component using dependency injection
const NotificationComponent = ({ notificationSender }) => {
  const sendNotification = (message) => {
    notificationSender.send(message);
  };

  return (
    <button onClick={() => sendNotification('Hello!')}>
      Send Notification
    </button>
  );
};

// Usage
const App = () => {
  const emailSender = new EmailSender();
  const smsSender = new SMSSender();

  return (
    <>
      <NotificationComponent notificationSender={emailSender} />
      <NotificationComponent notificationSender={smsSender} />
    </>
  );
};

Practical React Example Combining All Principles

// 1. Single Responsibility Principle
const useUserData = () => {
  // Data management hook
};

const useUserValidation = () => {
  // Validation logic hook
};

// 2. Open/Closed Principle
const FormField = ({ validator, renderer, ...props }) => {
  // Extensible form field component
};

// 3. Liskov Substitution Principle
const Input = ({ type = 'text', ...props }) => {
  // Base input component
};

const EmailInput = (props) => <Input type="email" {...props} />;
const PasswordInput = (props) => <Input type="password" {...props} />;

// 4. Interface Segregation Principle
const useAuth = () => {
  // Authentication hook
};

const useProfile = () => {
  // Profile management hook
};

// 5. Dependency Inversion Principle
const UserForm = ({ onSubmit, validation, transformation }) => {
  const handleSubmit = async (data) => {
    const isValid = await validation(data);
    if (isValid) {
      const transformed = transformation(data);
      onSubmit(transformed);
    }
  };

  return <form onSubmit={handleSubmit}>{/* Form fields */}</form>;
};

// Complete Example
const UserProfile = () => {
  const { user, updateUser } = useUserData();
  const { validateUser } = useUserValidation();
  const { transformUserData } = useUserTransformation();

  return (
    <UserForm
      onSubmit={updateUser}
      validation={validateUser}
      transformation={transformUserData}
    >
      <EmailInput name="email" />
      <PasswordInput name="password" />
    </UserForm>
  );
};

Benefits of Following SOLID Principles

1. Maintainability

   • Easier to understand and modify code
   • Reduced technical debt
   • Better organization of components

2. Testability

   • Isolated components are easier to test
   • Clear dependencies make mocking simpler
   • Better unit test coverage

3. Flexibility

   • Easier to extend functionality
   • Better adaptation to changing requirements
   • Improved reusability of components

4. Scalability

   • Better handling of growing complexity
   • Easier team collaboration
   • More sustainable codebase