The REPL

Node's REPL (Read-Eval-Print-Loop) represents the Node shell. To enter the shell prompt, enter Node via your terminal without passing a filename:

$ node

You now have access to a running Node process, and may pass JavaScript commands to this process. Additionally, if you enter an expression, the REPL will echo back the value of the expression. As a simple example of this, you can use the REPL as a pocket calculator:

$ node
> 2+2
4

Enter the 2+2 expression, and Node will echo back the value of the expression, 4. Going beyond simple number literals, you can use this behavior to query, set, and again, query the values of variables:

> a
ReferenceError: a is not defined
  at repl:1:1
  at sigintHandlersWrap (vm.js:22:35)
  at sigintHandlersWrap (vm.js:96:12)
  at ContextifyScript.Script.runInThisContext (vm.js:21:12)
  at REPLServer.defaultEval (repl.js:346:29)
  at bound (domain.js:280:14)
  at REPLServer.runBound [as eval] (domain.js:293:12)
  at REPLServer.<anonymous> (repl.js:545:10)
  at emitOne (events.js:101:20)
  at REPLServer.emit (events.js:188:7)
> a = 7
7
> a
7

Node's REPL is an excellent place to try out, debug, test, or otherwise play with JavaScript code.

As the REPL is a native object, programs can also use instances as a context in which to run JavaScript interactively. For example, here we create our own custom function sayHello, add it to the context of a REPL instance, and start the REPL, emulating a Node shell prompt:

require('repl').start("> ").context.sayHello = function() {
  return "Hello";
};

Enter sayHello() at the prompt, and the function will send Hello to standard out.

Let's take everything we've learned in this chapter and create an interactive REPL that allows us to execute JavaScript on a remote server:

1. Create two files, client.js and server.js, and type in the following code.
2. Run each in its own terminal window, keeping both windows side by side on your screen:

// File client.js
let net = require("net");
let sock = net.connect(8080);
process.stdin.pipe(sock);
sock.pipe(process.stdout);

// File server.js
let repl = require("repl")
let net = require("net")
net.createServer((socket) => {
  repl
  .start({
    prompt: "> ",
    input: socket,
    output: socket,
    terminal: true
  }).on('exit', () => {
    socket.end();
  })
}).listen(8080);

The client.js program creates a new socket connection to port 8080 through net.connect, and pipes any data coming from standard in (your terminal) through to that socket. Similarly, any data arriving from the socket is piped to standard out (back to your terminal). With this code, we've created a way to take terminal input and send it via a socket to port 8080, listening for any data that the socket may send back to us.

The other program, server.js, closes the loop. This program uses net.createServer and .listen to create and start a new TCP server. The callback the code passes to net.createServer receives a reference to the bound socket. Within the enclosure of that callback, we instantiate a new REPL instance, giving it a nice prompt (> here, but could be any string), indicating that it should both listen for input from, and broadcast output to, the passed socket reference, indicating that the socket data should be treated as terminal data (which has special encoding).

We can now type something like console.log("hello") into the client terminal, and see hello displayed.

To confirm that the execution of our JavaScript commands is occurring in the server instance, type console.log(process.argv) into the client, and the server will display an object containing the current process path, which will be server.js.

With just a few lines of code, we've created a way to remotely control Node processes. It's the first step towards multi-node analytics tools, remote memory management, automatic server administration, and more.