Errors and their handling

JavaScriptJunior~1 min read

Basic handling: `try...catch...finally` and `throw`

javascript

try {
  risky();                       // code that might fail
} catch (err) {
  console.error(err.message);    // handling/logging
} finally {
  cleanup();                     // always runs (even on error)
}

Raising an error:

javascript

if (!user) {
  throw new Error('User not found');
}

Custom errors

javascript

class ValidationError extends Error {
  constructor(message, cause) {
    super(message, { cause });   // Node 16+/modern browsers: err.cause
    this.name = 'ValidationError';
  }
}
throw new ValidationError('Invalid email');

2) Asynchronous code

a) `async/await` + `try...catch`

javascript

async function load() {
  try {
    const res = await fetch('/api/data');
    if (!res.ok) throw new Error(`HTTP ${res.status}`);
    return await res.json();
  } catch (err) {
    // handling/retry/notification
    console.error('Load failed:', err);
    return null; // or rethrow: throw err
  }
}

b) Promises: `.catch()`

javascript

fetch('/api/data')
  .then(r => r.json())
  .catch(err => console.error('Request failed', err));

c) Parallel operations without failing the whole batch

javascript

const results = await Promise.allSettled(urls.map(u => fetch(u)));
for (const r of results) {
  if (r.status === 'fulfilled') console.log('OK', r.value);
  else console.warn('Failed', r.reason);
}

3) Global interceptors (the last line of defense)

Browser

javascript

// synchronous/uncaught exceptions
window.addEventListener('error', (event) => {
  console.error('Uncaught error:', event.error, event.message, event.filename, event.lineno);
});

// uncaught promise rejections
window.addEventListener('unhandledrejection', (event) => {
  console.error('Unhandled rejection:', event.reason);
});

Node.js

javascript

process.on('uncaughtException', (err) => {
  console.error('Uncaught Exception', err);
  // logging + a graceful process shutdown
  process.exit(1);
});

process.on('unhandledRejection', (reason) => {
  console.error('Unhandled Rejection', reason);
  // decide: log and shut down/continue per the app's policy
});

Global handlers are a safety net, not a replacement for local handling.

Short Answer

Interview ready

Premium

A concise answer to help you respond confidently on this topic during an interview.