random

Randomizing an Array with Sort

How to randomize an array? Use the sort command, with the option:

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-R, --random-sort
sort by random hash of keys

For example:

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$ seq 1 10 | sort -R
4
2
10
6
3
9
7
5
8
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$ seq 1 10 | sort --random-sort
9
6
1
3
2
8
7
5
4
10

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?

Locate HTTP Port Number from http.Server Instance

This is a bare minimal HTTP server:

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require('http').createServer().listen(port);

If the port number is evaluated to be falsy, such as 0, null, undefined or an empty string, a random port will be assigned.

Begin accepting connections on the specified port and hostname. If the hostname is omitted, the server will accept connections on any IPv6 address (::) when IPv6 is available, or any IPv4 address (0.0.0.0) otherwise. A port value of zero will assign a random port. - https://iojs.org/api/http.html#http_server_listen_port_hostname_backlog_callback

We can use command line tool such as netstat to find out the port number:

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$ netstat -ltpn | grep node
tcp6 0 0 :::44055 :::* LISTEN 5624/node

However, it will be very convenient without relying on external tools.

Luckily, we can find out the port number by using the address method:

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require('http').createServer().listen().address()

The result will be similar to:

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{ address: '::', family: 'IPv6', port: 44055 }

Furthermore, it works with all popular libraries that create an http.Server instance: