Summary - Error handling

Build a user sign-up flow with a custom error class and async error handling.

Summary - Error handling

In this group you learned to create custom error classes, propagate errors by wrapping them with cause and rethrowing, catch multiple patterns with instanceof, use async error strategies and handle errors globally.

Now you will combine all of these in a user sign-up flow: throwing a custom error, async registration and setting up a global handler.

Exercise Sign-up flow

Write an async function signUp. If the email parameter is empty, throw new ValidationError('Email required'). Then in a try block call await registerUser(email). In a catch (err) block call console.error(err).

Show example solution

async function signUp(email) {
  if (!email) {
    throw new ValidationError('Email required');
  }
  try {
    await registerUser(email);
  } catch (err) {
    console.error(err);
  }
}

Exercise Set up the global handler

Write a global error listener. With window.addEventListener('error', handler) send event.message to the reportError function. With window.addEventListener('unhandledrejection', handler) send event.reason to the reportError function.

Show example solution

window.addEventListener('error', (event) => {
  reportError(event.message);
});
window.addEventListener('unhandledrejection', (event) => {
  reportError(event.reason);
});

Test yourself

Match concepts to their definitions

extends Error

Custom error class
throw err

Error propagation
instanceof

Type check
unhandledrejection

Global catching

Error class skeleton

class AuthError ____ {
  constructor(message) {
    super(message);
    this.name = 'AuthError';
  }
}

Show answer

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

What happens when await rejects in an async function?
- The promise auto-fulfils
- It is silently ignored
- The error is caught by try/catch
- window.onload fires
Show answer

The error is caught by try/catch — Correct! An await rejection is caught by try/catch.

Next lesson: The JSON format

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