Error handling

When something goes wrong at runtime, JavaScript throws an error. Unhandled, it stops execution (you read these traces in the error-messages fundamentals lesson). try/catch lets you respond instead of crashing.

try / catch / finally

try {
  const data = JSON.parse(input);   // throws if input isn't valid JSON
  use(data);
} catch (err) {
  console.error("bad input:", err.message);
} finally {
  cleanup();   // runs either way — success or failure
}

If the try block throws, control jumps to catch with the error; finally always runs, which is the place for cleanup that must happen regardless.

Throw for invalid situations

When your code hits something invalid, throw an Error rather than returning a nonsense value — fail loudly and early (the "say what you mean" principle):

function setAge(age) {
  if (age < 0) {
    throw new Error("age cannot be negative");
  }
  return age;
}

Throw Error objects (not strings) — they carry a message and a stack trace, which is what makes debugging possible.

Errors in async code

Because await throws a rejected promise, the same try/catch works for async code — no special syntax:

try {
  const user = await loadUser(1);
} catch (err) {
  console.error("failed to load:", err.message);
}

Where to go next

Last in this module: fetch and APIs — talking to the network, pulling everything in this track together.