Mitm.js is a library for Node.js (and Io.js) to intercept and mock outgoing network TCP and HTTP connections. Mitm.js intercepts and gives you a Net.Socket to communicate as if you were the remote server. For HTTP requests it even gives you Http.IncomingMessage and Http.ServerResponse — just like you're used to when writing Node.js servers. Except there's no actual server running, it's all just In-Process Interception™.

Intercepting connections and requests is extremely useful to test and ensure your code does what you expect. Assert on request parameters and send back various responses to your code without ever having to hit the real network. Fast as hell and a lot easier to develop with than external test servers.

Mitm.js works on all Node versions: ancient v0.10, v0.11 and v0.12 versions, previous and current LTS versions like v4 to v12 and the newest v22 and beyond. For all it has automated tests to ensure it will stay that way.

I've developed Mitm.js on a need-to basis for testing Monday Calendar's syncing, so if you find a use-case I haven't come across, please fling me an email, a tweet or create an issue on GitHub.

• Intercept both TCP socket connections (Net.connect) and HTTP requests (Http.request and Https.request).

• Hooks to Node.js's network functions at a very low level with the goal of not having to patch existing classes and have everything behave as if bytes were arriving from the network.

• Does not have any kitchen sink features or yet another API to assert on intercepted connections.
  That's a different responsibility handled better by assertion libraries (you'll do no better than to pick Must.js for that ;-).

• Use an API you already know to assert or respond to requests — Mitm.js gives you access to a vanilla Net.Socket to respond with:

  mitm.on("connection", function(socket) { socket.write("Hello back!") })
  
  var socket = Net.connect(22, "example.org")
  socket.write("Hello!")
  socket.setEncoding("utf8")
  socket.on("data", console.log) // => "Hello back!"

• When you do HTTP or HTTPS requests, Mitm.js gives you both a Http.IncomingMessage and Http.ServerResponse to play the server with. That means you'll be using an API you're already familiar with rather than yet another idiosyncratic domain specific language.

  Mitm.js comes very handy to ensure your code makes requests with the appropriate parameters:

  mitm.on("request", function(req, res) {
    req.headers.authorization.must.equal("OAuth DEADBEEF")
  })
  
  Http.get("http://example.org")

  It's also useful to see if your code behaves as you'd expect if everything is not 200 OK:

  mitm.on("request", function(req, res) {
    res.statusCode = 402
    res.end("Pay up, sugar!")
  })
  
  Http.get("http://example.org", function(res) {
    res.setEncoding("utf8")
    res.statusCode // => 402
    res.on("data", console.log) // => "Pay up, sugar!"
  })

  Http.IncomingMessage and Http.ServerResponse are the same objects you get when you write Node.js HTTP servers with Net.Server or use a library like Express.js.

• Bypass interception selectively for some connections (such as your SQL server) and let them connect as usual.

  mitm.on("connect", function(socket, opts) {
    if (opts.host == "sql.example.org" && opts.port == 5432) socket.bypass()
  })

• Developed with automated tests. Yeah, I know, why should one list this a feature when writing tests is just a sign of professionalism and respect towards other developers? But in a world where so many libraries and "production" software are released without any tests, I like to point out that I even write tests for testing libraries. ;-)

npm install mitm

From v1.0.0 Mitm.js will follow semantic versioning, but until then, breaking changes may appear between minor versions (the middle number).

Require Mitm.js and invoke it as a function to both create an instance of Mitm and enable intercepting:

var Mitm = require("mitm")
var mitm = Mitm()

Mitm.js will then intercept all requests until you disable it:

mitm.disable()

In tests, it's best to use the before and after hooks to enable and disable intercepting for each test case:

beforeEach(function() { this.mitm = Mitm() })
afterEach(function() { this.mitm.disable() })

After you've called Mitm(), Mitm.js will intercept and emit connection on itself for each new connection.
The connection event will be given a server side Net.Socket for you to reply with:

mitm.on("connection", function(socket) { socket.write("Hello back!") })

var socket = Net.connect(22, "example.org")
socket.write("Hello!")
socket.setEncoding("utf8")
socket.on("data", console.log) // => "Hello back!"

After you've called Mitm(), Mitm.js will intercept and emit request on itself for each new HTTP or HTTPS request.
The request event will be given a server side Http.IncomingMessage and Http.ServerResponse.

For example, asserting on HTTP requests would look something like this:

mitm.on("request", function(req, res) {
  req.headers.authorization.must.equal("OAuth DEADBEEF")
})

Http.get("http://example.org")

Responding to requests is just as easy and exactly like you're used to from using Node.js HTTP servers (or from libraries like Express.js):

mitm.on("request", function(req, res) {
  res.statusCode = 402
  res.end("Pay up, sugar!")
})

Http.get("http://example.org", function(res) {
  res.statusCode // => 402
  res.setEncoding("utf8")
  res.on("data", console.log) // => "Pay up, sugar!"
})

Please note that HTTPS requests are currently "morphed" into HTTP requests. That's to save us from having to set up certificates and disable their verification. But if you do need to test this, please ping me and we'll see if we can get Mitm.js to support that.

Unfortunately because Node.js's web server doesn't seem to support custom HTTP methods (that is, ones beyond require("http").METHODS), Mitm.js doesn't support them out of the box either. The Node.js HTTP parser throws an error given a request with an unsupported method. However, as Mitm.js also supports intercepting at the TCP level, you could hook in your own HTTP parser. I've briefly alluded to it in issue #63.

You can bypass connections listening to the connect event on the Mitm instance and then calling bypass on the given socket. To help you do so selectively, connect is given the options object that was given to Net.connect:

mitm.on("connect", function(socket, opts) {
  if (opts.host == "sql.example.org" && opts.port == 5432) socket.bypass()
})

Bypassed connections do not emit connection or request events. They're ignored by Mitm.js.

In most cases you don't need to bypass because by the time you call Mitm in your tests to start intercepting, all of the long-running connections, such as database or cache connections, are already made.

You might need to bypass connections you make to localhost when you're running integration tests against the HTTP server you started in the test process, but still want to intercept some other connections that this request might invoke.
The following should suffice:

mitm.on("connect", function(socket, opts) {
  if (opts.host == "localhost") socket.bypass()
})

All events that Mitm will emit on an instance of itself (see Using Mitm.js for examples):

Mitm.js is released under a Lesser GNU Affero General Public License, which in summary means:

• You can use this program for no cost.
• You can use this program for both personal and commercial reasons.
• You do not have to share your own program's code which uses this program.
• You have to share modifications (e.g. bug-fixes) you've made to this program.

For more convoluted language, see the LICENSE file.

Andri Möll typed this and the code.
Monday Calendar supported the engineering work.

If you find Mitm.js needs improving, please don't hesitate to type to me now at andri@dot.ee or create an issue online.