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secure-exec supports three filesystem backends. The system driver controls which backend sandboxed code uses.

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In-memory

Fully in-memory filesystem backed by Maps. Works in both Node and browser. 
import { createInMemoryFileSystem } from "secure-exec";

const fs = createInMemoryFileSystem();
await fs.writeFile("/hello.txt", new TextEncoder().encode("hello"));
Use with a driver: 
import { createNodeDriver, createInMemoryFileSystem, allowAllFs } from "secure-exec";

const driver = createNodeDriver({
  filesystem: createInMemoryFileSystem(),
  permissions: { fs: allowAllFs },
});

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OPFS (browser)

Persistent filesystem using the Origin Private File System API. This is the default for createBrowserDriver(). 
import { createBrowserDriver } from "secure-exec/browser";

// OPFS (default)
const driver = await createBrowserDriver({ filesystem: "opfs" });

// In-memory fallback
const memDriver = await createBrowserDriver({ filesystem: "memory" });
OPFS does not support atomic rename operations.

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Node filesystem

Thin wrapper around Node.js fs/promises. Provides real host filesystem access (gated by permissions). 
import { NodeFileSystem } from "secure-exec";

const fs = new NodeFileSystem();

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VirtualFileSystem interface

All backends implement this interface:
MethodReturnsDescription
readFile(path)Promise<Uint8Array>Read file as bytes
readTextFile(path)Promise<string>Read file as text
readDir(path)Promise<string[]>List directory entries
readDirWithTypes(path)Promise<DirEntry[]>List entries with type info
writeFile(path, content)Promise<void>Write file
createDir(path)Promise<void>Create directory
mkdir(path)Promise<void>Create directory (alias)
exists(path)Promise<boolean>Check if path exists
stat(path)Promise<StatInfo>Get file metadata
removeFile(path)Promise<void>Delete a file
removeDir(path)Promise<void>Delete a directory
rename(old, new)Promise<void>Rename a file or directory