How do you handle errors in JavaScript, including async code?

Question

Accepted Answer

Synchronous errors use **`try/catch/finally`**, and you should `throw` `Error` objects (not strings) so you get a stack trace.

```js
try {
  const data = JSON.parse(input);
} catch (err) {
  if (err instanceof SyntaxError) handleBadJson(err);
  else throw err;        // re-throw what you can't handle
} finally {
  cleanup();             // always runs
}
```

## Async error handling differs by style

```js
// async/await → wrap awaits in try/catch
async function load() {
  try {
    return await fetch(url).then(r => r.json());
  } catch (err) { showError(err); }
}

// promises → use .catch
fetch(url).then(use).catch(showError);
```

## A common trap: try/catch doesn't catch async callbacks

```js
try {
  setTimeout(() => { throw new Error("boom"); }, 0); // ❌ NOT caught
} catch (e) { /* never runs — the throw happens later, outside this stack */ }
```

The `setTimeout` callback runs on a later tick, so the surrounding `try` is long gone. Handle the error *inside* the callback.

## Global safety nets

```js
window.addEventListener("unhandledrejection", e => log(e.reason)); // browser
process.on("unhandledRejection", err => log(err));                 // Node
```

## Best practices

- Throw `Error` (or custom subclasses) for stack traces and `instanceof` checks.
- Catch **specifically** where you can recover; let unexpected errors bubble.
- Never silently swallow errors (`catch (e) {}`). Always log or rethrow.