Advanced

Error Handling

try / catch / finally and custom Error classes.

1. Explanation

Wrap risky code in try { } and handle failures in catch (err). finally always runs. Throw your own errors with throw new Error('message'). Errors have name, message and stack. For async code, await inside try/catch.

2. Example

Try it yourself

function divide(a, b) {
  if (b === 0) throw new Error("Cannot divide by zero");
  return a / b;
}

try {
  console.log(divide(10, 2));
  console.log(divide(10, 0));
} catch (err) {
  console.log("Error:", err.message);
} finally {
  console.log("done");
}

Console

Click Run to see output.

3. Expected output

output

5
Error: Cannot divide by zero
done

4. Common mistakes

✗Swallowing errors with an empty catch block
✗Throwing strings instead of Error objects (no stack trace)
✗Forgetting to await inside try — the rejection escapes the catch

5. Practice questions

Throw a custom error when input is not a number
Catch a JSON.parse failure
Wrap an async function in try/catch and log the error

→ Try more practice problems