Error Handling & Debugging

try / catch / finally

try {
  const data = JSON.parse(invalidJSON);
} catch (err) {
  if (err instanceof SyntaxError) {
    console.error('Invalid JSON:', err.message);
  } else {
    throw err; // re-throw unexpected errors
  }
} finally {
  console.log('Always runs');
}

Custom Error Classes

class ValidationError extends Error {
  constructor(message, field) {
    super(message);
    this.name = 'ValidationError';
    this.field = field;
  }
}

try {
  throw new ValidationError('Invalid email', 'email');
} catch (err) {
  if (err instanceof ValidationError) {
    console.log(err.field); // 'email'
  }
}

Async Error Handling

// Promises
fetch('/api/data')
  .then(res => res.json())
  .catch(err => console.error('Fetch failed:', err));

// Async/Await
async function fetchData() {
  try {
    const res = await fetch('/api/data');
    if (!res.ok) throw new Error(`HTTP ${res.status}`);
    return await res.json();
  } catch (err) {
    console.error(err);
  }
}