Async errors

Learn async errors and try/catch.

Async errors

Errors in asynchronous functions return inside a Future. They are caught with try/await/catch.

When a Future returns an error, the await call throws: try { await fetchUser(); } catch (e) { print(e); }.

main must now be async: void main() async { ... }. This is required to use await.

An async error example

An async function can throw: Future<void> fetchUser() async { throw Exception('offline'); }.

The caller: try { await fetchUser(); } catch (e) { print(e); }. await waits for the Future; if there is an error it falls into catch.

This looks the same as synchronous try/catch; the only difference is the await keyword.

Exercise Async error handling

Define Future<void> fetchUser() async { throw Exception('offline'); }. Make main async: call it with try { await fetchUser(); } catch (e) { print(e); }.

Show example solution

Future<void> fetchUser() async {
  throw Exception('offline');
}

void main() async {
  try {
    await fetchUser();
  } catch (e) {
    print(e);
  }
}

Test yourself

Catch with await

try { await fetch(); } ____ (e) { print(e); }

Show answer

try { await fetch(); } catch (e) { print(e); }

Async error concepts

await

Waits the Future
catch

Catches the error
Future

Async value
async

Async function
How is an error caught in an async function?
- on Error catch
- if (error)
- try { await ... } catch
- switch (error)
Show answer

try { await ... } catch — the await call goes inside try, catch handles it.

Next lesson: Summary - Advanced error handling

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