Usando REPL desde un programa

Es posible crear un bucle REPL en cualquier punto de nuestro programa - quizá para depurarlo. Para ello usamos la función repl.start . Esta función retorna una instancia REPLServer. Acepta como argumento un objeto options que toma los siguientes valores:

1. prompt - the prompt and stream for all I/O. Defaults to > .
2. input - the readable stream to listen to. Defaults to process.stdin.
3. output - the writable stream to write readline data to. Defaults to process.stdout.
4. terminal - pass true if the stream should be treated like a TTY, and have ANSI/VT100 escape codes written to it. Defaults to checking isTTY on the output stream upon instantiation.
5. eval - function that will be used to eval each given line. Defaults to an async wrapper for eval().
6. useColors - a boolean which specifies whether or not the writer function should output colors. If a different writer function is set then this does nothing. Defaults to the repl's terminal value.
7. useGlobal - if set to true, then the repl will use the global object, instead of running scripts in a separate context. Defaults to false.
8. ignoreUndefined - if set to true, then the repl will not output the return value of command if it's undefined. Defaults to false.
9. writer - the function to invoke for each command that gets evaluated which returns the formatting (including coloring) to display. Defaults to util.inspect.

[~/Dropbox/src/javascript/node.js/repl(master)]$ cat repl.js 
var repl = require("repl");

connections = 0;

repl.start({
  prompt: "node via stdin> ",
  input: process.stdin,
  output: process.stdout
});

[~/Dropbox/src/javascript/node.js/repl(master)]$ node repl.js 
node via stdin> 2+3
5
node via stdin> .exit

el bucle REPL proporciona acceso a las variables de ámbito global. Es posible hacer explícitamente visible una variable al REPL asignándosela al context asociado con el REPLServer. Por ejemplo:

[~/Dropbox/src/javascript/node.js/repl(master)]$ cat repl2.js 
var repl = require("repl");

z = 4
repl.start({
  prompt: "node via stdin> ",
  input: process.stdin,
  output: process.stdout
}).context.m = "message";

Las variables en el objeto context se ven como locales al REPL:

[~/Dropbox/src/javascript/node.js/repl(master)]$ node repl2.js 
node via stdin> z
4
node via stdin> m
'message'