Async error handling
Learn to catch errors in async functions with try/catch.
Catching async errors with try/catch
In a promise chain you use catch; with async/await you use a try...catch block. When an async function awaits a promise that is rejected, the error falls into the catch block.
async function loadPost() {
try {
const post = await fetchPost();
} catch (err) {
console.error(err);
}
}This structure mirrors error handling in synchronous code, which is why async/await is considered more readable.
Re-throwing an error
Sometimes you want to catch an error and then pass it on. To do that, you re-throw it with throw inside the catch block. The calling code then catches it with its own try/catch.
async function loadItem() {
try {
return await fetchItem();
} catch (err) {
console.error('Loading failed');
throw err;
}
}This pattern lets you log the error while still forwarding it to the caller.
Exercise Add error handling
Write async function loadOrder(). Inside a try block, call await fetchOrder() and assign the result to an order variable. Add a catch (err) block that logs the error with console.error(err).
Show example solution
async function loadOrder() {
try {
const order = await fetchOrder();
} catch (err) {
console.error(err);
}
}Test yourself
-
What does the following code do?
async function f() { try { const x = await Promise.reject(new Error('boom')); } catch (err) { console.log(err.message); } }- Prints 'boom'
- Throws an uncaught error
- Prints undefined
- Does nothing
Show answer
Prints 'boom' — Correct! The rejected promise's error falls into catch and its message is logged.
-
Add the catch block
async function loadItem() { try { const item = await fetchItem(); } ____ { console.error(err); } }Show answer
async function loadItem() { try { const item = await fetchItem(); } catch { console.error(err); } } -
Match error-handling concepts
- await Promise.reject(err)
- falls into the catch block
- try { await ... }
- continues inside try on success
- throw err
- forwards the error to the caller
- catch (err)
- receives the error as err