#Errors and exit codes

There are no custom error classes in Fairyfly. You get QuickJS's built-ins — Error, TypeError, SyntaxError, RangeError — plus a handful of APIs that reject with plain strings.

#Uncaught errors

An exception that escapes your script prints the message and stack to stderr, then the process exits 1:

// ff boom.js
throw new Error("boom");

$ ff boom.js
Error: Error: boom
    at <eval> (boom.js:1:11)

$ echo $?
1

Note the doubled Error: Error: prefix, the at <eval> frame carrying file:line:column, and the blank line before the shell prompt.

try / catch works as usual — for the APIs listed below.

#What throws (and what doesn't)

Sync fs swallows errors today. A missing file, a bad directory, an unreadable path — all return undefined with no exception. Always check fs.exists() first, or use the Promise variants, which report the real error:

// Reliable: async variants reject with the error name
try {
  const text = await fs.readFileAsync("config.json", "utf8");
} catch (e) {
  console.error("cannot read config:", e.message); // "FileNotFound"
}

// Sync: guard with exists()
if (fs.exists("config.json")) {
  const text = fs.readFile("config.json");
}

#Unhandled promise rejections are silent

In script context a rejected Promise with no handler prints nothing and the process exits 0:

Promise.reject(new Error("rej"));   // no output, exit 0
async function f() { throw new Error("x"); }
f();                               // no output, exit 0

Always attach a .catch, or await inside a try. In an HTTP handler the runtime does catch it — a rejected handler Promise becomes a 500 response.

#fetch rejects with strings

Failed fetches reject with a string, not an Error, so e.message is undefined:

try {
  await fetch("notaurl");
} catch (e) {
  console.log(typeof e);        // "string"
  console.log(e);               // "Invalid URL" | "Network error" | "Too many redirects"
}

A diagnostic line also goes to stderr:

[fetch] request failed: ConnectionRefused url=http://127.0.0.1:1/

#Server-side failures

#Exit codes

process.exit(300);
// $ ff exit.js; echo $?
// 255

#Message catalogue

Strings you'll see in practice — useful for assertions in tests:

#Gotchas

• e.message is undefined for fetch rejections — check typeof e.
• A successful fs.*Async call hangs the process afterwards (it prints, then never exits) — Known issue, see fs.
• Sync fs failures are silent; don't write try/catch around them expecting an exception.
• There is no process.on("uncaughtException") and no unhandledRejection hook — there's nothing to subscribe to.
• Errors thrown inside a Worker surface on the parent via onerror.

#See also

• Event loop
• Limitations
• Compatibility
• Testing