node:fs

node:fs and node:fs/promises use one jco:node/fs@0.1.0 host capability. When either specifier occurs in bundled source, Jco adds a missing import to the selected world, installs fs.wit under deps/jco-node-0.1.0, and prints a CLI warning to alert to the fact that a WIT dependency has been added.

The default filesystem host provider returns a typed denial result, which the guest reconstructs as ERR_JCO_FS_ADAPTER_REQUIRED. To use the passthrough NodeJS host provider you can map it in:

jco transpile component.wasm \
  --map 'jco:node/fs@0.1.0=@bytecodealliance/jco-std/wasi/0.2.x/node/24.x.x/fs/host/node'

The resulting call path for filesystem function is:

1. guest node:fs
2. WIT capability
3. host adapter
4. NodeJS builtins

The Node provider delegates to Node 24’s synchronous operations; guest callback APIs queue their callbacks on a microtask, and promise APIs share the same descriptor state through promise FileHandles.

Common file and directory operations, metadata, directory entries, scalar and vector descriptor I/O, and their callback/promise facades are supported.

Warning

APIs whose contract requires long-lived streams or event sources are not yet supported – including ReadStream, WriteStream, Utf8Stream, watch, watchFile, and openAsBlob.

These functions currently throw ERR_JCO_UNSUPPORTED_NODE_API because the typed WIT interface does not model those resources.