Error handling: try, catch, finally and throw

What is an error?

An error (also called an exception) is an object that describes a problem. When code throws one, the current function stops immediately and the error travels up the call stack until some code catches it. If nothing catches it, the program crashes (in Node) or the console shows "Uncaught …" (in the browser).

const user = null;
console.log(user.name);
// TypeError: Cannot read properties of null (reading 'name')

Built-in error types

Every error has a name ('TypeError'), a message (the human text) and usually a stack (the list of function calls that led to it, with file names and line numbers). Read the first line of the stack that points to YOUR code.

try, catch and finally

function parseAmount(text) {
  try {
    const n = JSON.parse(text);          // may throw SyntaxError
    console.log('parsed');
    return n;
  } catch (error) {
    console.log('Could not parse:', error.message);
    return 0;
  } finally {
    console.log('done');                 // runs in every case
  }
}
parseAmount('42');    // parsed, done
parseAmount('4 2');   // Could not parse: …, done

Order: the try block runs; if it throws, the rest of try is skipped and catch runs; finally runs at the end whether there was an error or not, even after a return. Use finally for clean-up: closing a file, hiding a loading spinner.

Throwing your own errors

function withdraw(balance, amount) {
  if (typeof amount !== 'number' || amount <= 0) {
    throw new RangeError(`Invalid amount: ${amount}`);
  }
  if (amount > balance) throw new Error('Insufficient balance');
  return balance - amount;
}

Custom error classes and cause

class NotFoundError extends Error {
  constructor(what) {
    super(`${what} not found`);
    this.name = 'NotFoundError';
  }
}
try {
  try {
    throw new NotFoundError('PNR 4521');
  } catch (inner) {
    throw new Error('Booking lookup failed', { cause: inner });  // wrap with cause
  }
} catch (e) {
  console.log(e.message, '←', e.cause.message);
  // Booking lookup failed ← PNR 4521 not found
}

With a custom class you can react to specific problems with instanceof: if (e instanceof NotFoundError) show a 404 page, otherwise re-throw.

Where to handle errors

• Validate input at the boundaries (forms, API requests, file reads) and throw clear errors early.
• Catch errors where you can do something useful: show a message, retry, use a default.
• Let unexpected errors bubble up to one top-level handler that logs them.