Errors

Throw what node:fs throws. fsError() builds it, errnoOf() puts it on the wire, and the kernel gets the code you meant.

import { ERRNO_CODES, errnoOf, fsError, isFsError, rangeError } from "mountx";

#fsError(code, options?)

function fsError(code: ErrnoCode, options?: FsErrorOptions): FsError;

Builds an error byte-for-byte identical to node:fs's — same message, code, negative errno, syscall, path, dest. It reaches the kernel untranslated.

throw fsError("ENOENT", { syscall: "stat", path });
throw fsError("EXDEV", { syscall: "rename", path, dest });

An error forwarded from a real fs call is already this shape — let it through unchanged.

#isFsError(error, code?)

function isFsError(error: unknown, code?: ErrnoCode): error is FsError;

The type guard, optionally narrowing on a specific code.

#errnoOf(error)

function errnoOf(error: unknown): number;

What the transports call to turn anything thrown into a wire errno. Unknown values become EIO, which is how the exactly one reply per request rule survives a driver throwing something unexpected.

#ERRNO_CODES

The errno table, transcribed once, with Linux values — they are the numbers on the wire, not necessarily your host's. ErrnoCode is keyof typeof ERRNO_CODES.

#rangeError(name, expected, value)

Node's ERR_OUT_OF_RANGE, for drivers validating read/write arguments the way node:fs does.

#Next

• Loopback harness — run the driver and see what comes back out.
• Writing a driver — the same subject as prose.