try / catch / throw

An error in JavaScript is a value (usually an Error object) that interrupts the normal flow and bubbles up the call stack until someone catches it. The construct to handle this case is try / catch.

Throwing an error: throw

function dividi(a, b) {
  if (b === 0) {
    throw new Error('Divisione per zero');
  }
  return a / b;
}

throw immediately interrupts the current function.

Catching: try / catch

try {
  const r = dividi(10, 0);
  console.log('ok', r);
} catch (err) {
  console.log('errore:', err.message); // 'errore: Divisione per zero'
}

// Il programma continua qui, anche se c'è stato un errore.

Everything inside try is executed; if an error is thrown (via throw or by any operation that fails), control jumps to the catch block. The variable after catch (...) receives the error.

finally (optional)

Code that always runs, error or not. Useful to close resources:

try {
  // ... operazione rischiosa
} catch (err) {
  // ... log
} finally {
  // ... pulizia (sempre eseguita)
}

When to use it (and when not)

• Yes: I/O operations (JSON.parse on untrusted input, fetch, disk access), conversions that can fail.
• Maybe not: for the normal flow. If you know an input may be invalid, it is better to check it with an if than to throw and catch.

// Tipico: parsing JSON difensivo
function parseJsonSafe(text) {
  try {
    return { ok: true, value: JSON.parse(text) };
  } catch (err) {
    return { ok: false, error: err.message };
  }
}

parseJsonSafe('{"a":1}'); // { ok: true, value: {a:1} }
parseJsonSafe('non json'); // { ok: false, error: '...' }

Try it

Review exercise