node:tty

A component has no terminal of its own, and WASI 0.2 can only say whether its standard streams are terminals. node:tty therefore resolves against jco:node/tty@0.1.0, which addresses the embedding process’s descriptors as Node does: isatty(fd), a handle per descriptor and direction, raw mode, the window size, blocking reads, writes, and the environment used for color detection. When bundled source imports node:tty, Jco adds the import to the selected world and installs tty.wit and the shared types.wit under deps/jco-node-0.1.0.

It is denied by default: every operation, including isatty() on an in-range descriptor, throws ERR_JCO_TTY_ADAPTER_REQUIRED until the application maps a provider. jco-std ships one for Node:

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

With it, new tty.WriteStream(1) is the embedding process’s standard output when that is a terminal, and fails with Node’s own ERR_TTY_INIT_FAILED (with errno, syscall and info) when it is not. process.stdin and process.stdout remain unsupported on the process facade, so applications construct the streams from descriptors explicitly. Because readline and the REPL default terminal to output.isTTY, these streams are what makes an interactive session work:

import { ReadStream, WriteStream } from "node:tty";
import repl from "node:repl";

export function start() {
  const input = new ReadStream(0);
  const output = new WriteStream(1);
  repl.start({ prompt: "app> ", input, output, useGlobal: true });
  input.resume();
}

The port follows Node v24.20.0’s lib/tty.js and lib/internal/tty.js. The pinned unenv tty module answers isatty() === false and writes through console.log; it is not used.

Boundaries