This is a light-hearted introduction to using Streams in Node.js. I will walk you through building a stream capable of piping your terminal output to a browser. It is meant to be fun, and hopefully you can learn a few things along the way.

Streams are a powerful abstraction in computing. They offer a way of easily directing the I/O of one process to another. By using multiple processes, you gain parallelism managed by the operating system’s scheduler without having to worry about protecting shared memory. As a result, you can utilize multiple CPU cores quite easily.

Example of using pipes

Feel free to skip this section if you are comfortable with the underlying mechanism of Unix pipes

One common example of a stream is piping the output (STDOUT) of one process into the input (STDIN) of another:

$ while sleep 1; do echo 'Hi world!'; done | sed s/Hi/Hello/g
#=> Hello world!
#=> Hello world!
#=> Hello world!
...

You can think of this as a stream of data coming from the left side of the pipe ( | ) to the right side. The way this works under the hood in Linux is the pipe system call. A ‘pseudofile’ is created, and file descriptors are returned for both writing to and reading from said ‘pseudofile.’ In the above example, echo writes to the shared file, and sed reads from it. The beauty of this setup is that sed is not blocking echo, and this mechanism becomes a simple method for parallel processing of data.

Creating a stream in Node.js

Node.js offers an API for dealing with streams. I am going to go over a simple example of creating a stream which will allow us to pipe the output of a terminal (STDOUT) into the browser. What we need to create is a ‘Writable’ stream. The reason for this is that we are expecting to receive input from STDIN, meaning that we’ll need the input to actually be written to our stream.

The way to do this is to create a Writable() object and implement the _write() function. In doing so, we gain all of the behavior of the built-in streams in Node.js, such as as process.stdout, with our own custom write behavior.

var Writable = require("stream").Writable;
var BrowserStream = new Writable();

BrowserStream._write = function(chunk, enc, next) {
  // This just logs the output to our console for now, we change this later.
  console.log(chunk.toString(enc || 'utf8'));
  next();
};

Piping STDOUT to a browser

Now, instead of simply logging the output to the console, let’s actually send the data to the browser. Sockets are very similar to pipes in the sense that they allow for interprocess-communication (IPC) by creating byte streams. The main difference is that sockets will turn the data into packets, and send the packets over the network, while pipes can only use shared files over the local file-system.

To do this, we will use socket.io to set up a socket between our Node app and the client.

On the server we now have this:

// server.js

"use strict";

var app = require("express")();
var server = require("http").Server(app);
var io = require("socket.io")(server);
var Writable = require("stream").Writable;
var BrowserStream = new Writable();

// Create a server for the client to connect to
server.listen(7777);

BrowserStream._write = function(chunk, enc, next) {
  var string = chunk.toString("utf8");

  // Instead of logging to the console, write the data to a socket
  io.sockets.emit("news", string);
  next();
};

// We now can pipe stdin into BrowserStream
process.stdin.pipe(BrowserStream);

// We also pipe stdin to stdout so we can see it in the terminal
process.stdin.pipe(process.stdout);

On the client side, we have this:

<!-- index.html -->

<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <script src="https://cdn.socket.io/socket.io-1.3.2.js"></script>

    <script>
      var socket = io.connect("http://localhost:7777");

      socket.on("news", function(data) {
        console.log(data);
      });
    </script>

  </head>
</html>

Now we can reap the fruits of our labor. In your terminal, run these commands:
1. npm install -g socket.io express
2. Open index.html in a browser, and open up Developer Tools in the browser
3. Now, we can pipe all of our bash output to the browser: - bash | node server.js

All of your STDOUT should now be showing up in your browser console. For a more complete example, have a look at the one I made. It’s essentially the same, but it styles all of the output to make it look more terminal-like.

Conclusion

Although this example is a bit contrived, there are better uses for this type of concept. For example, you can create a simple command line tool which pipes in JSON, and then filters it however you see fit. Pipelines are a simple, parallel, and robust mechanism for manipulating data.

More Resources

  1. Stream Handbook
  2. Stream API