Tell the time

hostnames.txt

labnotes.org
example.com
mathforum.org
slack.com
github.com
facebook.com
twitter.com

time.js

const File = require('fs');
const HTTP = require('http');


const FILENAME = 'hostnames.txt';

// Tell the time.
getHostnames(FILENAME)
  .then(getDatesFromHostnames)
  .then(convertDatesToTime)
  .then(selectTime)
  .then(printTime)
  .catch(console.log);


// Returns a promise that resolves to an array of hostnames getting from file.
function getHostnames(filename) {
  return new Promise(function(resolve, reject) {
    File.readFile(filename, 'utf-8', function(error, data) {
      if (error)
        reject(error);
      else {
        const hostnames = data.trim().split('\n');
        resolve(hostnames);
      }
    });
  });
}


// Returns a promise that resolves to the Date header of a given host.
function getDateHeader(host) {
  return new Promise(function(resolve, reject) {
    // Make a HEAD request because we don't need the response body.
    const options = { method: 'HEAD', host };
    const request = HTTP.request(options, function(response) {
      resolve(response.headers.date);
    });
    request.on('error', reject);
    request.end();
  });
}


// Returns a promise that resolves to an array of date headers from the given
// list of hostnames.
function getDatesFromHostnames(hostnames) {
  return new Promise(function(resolve, reject) {
    const promises = hostnames.map(getDateHeader);
    Promise.all(promises).then(resolve).catch(reject);
  });
}


// Take an array of date strings, returns an array of valid dates in
// milliseconds.
function convertDatesToTime(dates) {
  return dates
    .filter(function(date) {
      return date;
    })
    .map(function(date) {
      return new Date(date).getTime();
    });
}


// Returns the current time from a list of times to guess.
function selectTime(times) {
  // Should have at least two values to be able to select the correct time.
  if (times.length <= 1)
    return null;

  // All of the time values should not have more than one second diff unless:
  // - The response time from one of the servers was too long.
  // - The date header from one of the servers was not correct.
  // When all values are good, we can assume the max value is the closest to the
  // current time.
  times.sort();
  const now = times[times.length - 1];
  return (now - times[0] <= 1000) ? now : null;
}


// Print the time in ISO 8601 format to the console.
function printTime(time) {
  if (time) {
    const output = (new Date(time)).toISOString();
    console.log(output);
  } else
    console.log('Could not tell the time.');
}