SuperTest: Listen at Random Port

In the example section of SuperTest (v0.15.0), it mentions something about ephemeral port:

You may pass an http.Server, or a Function to request() - if the server is not already listening for connections then it is bound to an ephemeral port for you so there is no need to keep track of ports. - https://github.com/visionmedia/supertest#example

Let’s dissect this sentence. There are a few key concepts to grasp:

  1. The request function will accept either a function or an http.Server object.
  2. When does the server not listening for connections?
  3. What is an ephemeral port?

Taking a peek at the index.js file in SuperTest source code, it is easy to see that it accepts both function and http.Server object, but prefer latter:

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// See `index.js` in [SuperTest] source code.
if ('function' == typeof app) app = http.createServer(app);
// See `lib/test.js` in [SuperTest] source code.
Test.prototype.serverAddress = function (app, path) {
  var addr = app.address();
  if (!addr) this._server = app.listen(0);
  var port = app.address().port;
  var protocol = app instanceof htts.Server ? 'https' : 'http';
  return protocol + '://1270.0.0.1:' + port + path;
});

Why it does this? This has to do with Express:

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> typeof require('http').createServer()
'object'
> typeof require('express')()
'function'
> typeof require('koa')()
'object'
> require('http').createServer(require('express')()).address()
null

Only when initiating an Express app, it returns a function, not an object. And follows up on lib/test.js, when SuperTest detects the created server is yet to bind to any port number, it will invoke app.listen(0), so called ephemeral port. In fact, it is just a random port.

When something is ephemeral, it last for a very short time. When allowing a server to accept connections, we usually do is setting the server to listen on a specific port:

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app.listen(3001);

What if setting this to 0 like above or omit this port number?