Loopback Harness

Run your driver in the current process through an fs/promises-shaped API. This needs no kernel or privileges and works on every platform.

import { createLoopback } from "mountx";
import type { Loopback } from "mountx";

#createLoopback(driver)

function createLoopback(driver: FsDriver): Loopback;

The returned object has every optional driver method. A missing method throws ENOSYS. The object normalizes each path and resolves all capabilities. It also adds two whole-file helper methods.

interface Loopback extends Required<Omit<FsDriver, "capabilities" | "mountx">> {
  readonly driver: FsDriver;
  readonly capabilities: ResolvedCapabilities;
  readonly mountx: MountxExtensions | undefined;
  readFile(path: string): Promise<Uint8Array>;
  writeFile(path: string, data: string | Uint8Array): Promise<void>;
}

const fs = createLoopback(driver);

fs.capabilities; // every capability resolved
fs.driver; // the driver you passed
await fs.writeFile("/a.txt", "text"); // create + truncate
await fs.readFile("/a.txt"); // no handle dance

Driver authors test against this API. The Tier-0 conformance suite also uses it. If an operation works through createLoopback(), it works through a mount. The loopback harness is not a mock. It uses the same path normalization and capability layer as a real session. It does not include the kernel.

#Next

• Mounting shows how to expose the same driver as a directory.
• Capabilities explains the values in fs.capabilities.