node:repl

node:repl ports Node v24.20.0’s REPL on top of the readline port: repl.start(), REPLServer, the .break, .clear, .exit, .help, .editor keywords and defineCommand(), tab completion, in-memory history and reverse search, top-level await, recoverable multi-line input, _ and _error, and the 'exit' and 'reset' events. The pinned unenv repl module is stubs and is not used.

Applications keep the ordinary import and supply the streams:

import repl from "node:repl";

export function attach(input, output) {
  const server = repl.start({ prompt: "app> ", input, output, useGlobal: true });
  server.context.app = { version: "1.0.0" };
  server.on("exit", () => output.write("bye\n"));
  return server;
}

Bundle with jco componentize app.js --bundle --wit wit -o app.wasm. The REPL requires no WIT imports; input and output decide where the session goes. Without a process global they are required, since there is no stdin or stdout to fall back to.

Global scope only

Node’s default useGlobal: false runs each line in a separate vm context, a second realm with its own globals. No component engine can create one, so that option – given explicitly or omitted – is refused at construction with ERR_JCO_UNSUPPORTED_NODE_API. With useGlobal: true evaluation is an indirect eval, which is exactly vm.runInThisContext: replServer.context is globalThis, .clear is an alias for .break, and assigning to the context exposes values as documented. Node’s script scope keeps top-level let, const and class bindings across lines; an eval does not, so the REPL rewrites those declarations to persist them. The trade is that const is not enforced between lines and a later redeclaration is accepted – the same trade Node documents for lines containing await. REPL_MODE_STRICT is refused for the same reason: a strict-mode eval cannot bind declarations in the global scope at all.

Why acorn

Node’s REPL vendors acorn; this port depends on the same versions from npm (acorn@8.17.0, acorn-walk@8.3.5). A parser is needed for what the engine cannot answer: whether a line is incomplete (show the ... prompt) or wrong (print the error) – engine SyntaxError messages differ between SpiderMonkey and QuickJS and cannot drive that decision – rewriting top-level await into an async wrapper, and locating the expression to tab-complete. acorn also serves as the compile step Node performs with vm.Script, so syntax errors read the same on every engine. It is bundled only when node:repl is imported; a component without the REPL does not carry it. Bundle size grows by roughly 1.2 MB (QuickJS) to 1.4 MB (StarlingMonkey).

Boundaries