How can I start a REPL that can access local variables in node.js?

Question

Just like from IPython import embed; embed() but for node.

I want to open a REPL shell programmatically and be able to at least read the local variables. Being able to change them as well is a plus.

Answer 1

You can build a REPL similar to the built-in Deno REPL and and evaluate expressions using the dangerous eval function. Through it you'll be able to access local variables and other things (e.g. window).

repl.ts

import { readLines, writeAll } from "https://deno.land/[email protected]/io/mod.ts";

export default async function repl(evaluate: (x: string) => unknown) {
  await writeOutput("exit using ctrl+d or close()\n");
  await writeOutput("> ");
  for await (const input of readInputs()) {
    try {
      const value = evaluate(input);
      const output = `${Deno.inspect(value, { colors: !Deno.noColor })}\n`;
      await writeOutput(output);
      await writeOutput("> ");
    } catch (error) {
      await writeError(error);
    }
  }
}

async function* readInputs(): AsyncIterableIterator<string> {
  yield* readLines(Deno.stdin);
}

async function writeOutput(output: string) {
  await writeAll(Deno.stdout, new TextEncoder().encode(output));
}

async function writeError(error: unknown) {
  await writeAll(Deno.stderr, new TextEncoder().encode(`Uncaught ${error}\n`));
}

repl_demo.ts

import repl from "./repl.ts";

let a = 1;
let b = 2;
let c = 3;

await repl((x) => eval(x));

example usage

% deno run repl_demo.ts
exit using ctrl+d or close()
> a
1
> a = 40
40
> a + b
42