← ⚡ JavaScript & the browser·Module C7 · Lesson 6

TaskWrite async function loadOrFallback(p) that takes a Promise p. In try/catch: return await p on success; in catch log 'fallback: ' + err.message and return 0. Call it with Promise.resolve(42) (log result) and Promise.reject(new Error('down')) (log result).

Async errors — try/catch around await

125 XP9 min

Theory

try/catch wraps awaits naturally

An await inside try/catch behaves exactly like a synchronous throw. If the awaited Promise rejects, the catch catches it.

async function loadUser(id) {
  try {
    const r = await fetch(\`/api/users/${id}\`);
    if (!r.ok) throw new Error("HTTP " + r.status);
    return await r.json();
  } catch (err) {
    console.error("loadUser failed:", err);
    return null;
  }
}

fetch + the HTTP-error trap

fetch only rejects on network errors. A 404 response does NOT reject — the Promise fulfills, you have to check r.ok (or status code) yourself. Forgetting this is the #1 fetch bug.

const r = await fetch("/api/x");
if (!r.ok) throw new Error(\`HTTP ${r.status}\`);

Returning vs throwing

return null from a catch turns "missing data" into a checkable value. throw propagates to the caller. Pick based on whether the caller should handle it.

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