test_watchalert.gs

'use strict';

if ((typeof GasTap)==='undefined') {
  eval(UrlFetchApp.fetch('https://raw.githubusercontent.com/zixia/gast/master/src/gas-tap-lib.js').getContentText())
}

var test = new GasTap()

function testServerWatchAlert() {
  test('Got alive response code', function (t) {
    var response = checkHttpResponse('https://www.github.com')
    t.equal(response, 200, 'got 200')
  })

  test('Got dead response code', function (t) {
    var response = checkHttpResponse('http://xxx.xxx.com')
    t.equal(response, 999, 'got 999.')
  })

  test("Got dead server's URL.", function (t) {
       var url = ['https://www.github.com', 'https://gist.github.com', 'http://x.x.x.x']
       var url_expected = ['http://x.x.x.x']
       var url_dead = getDeadUrl(url)
       t.equal(JSON.stringify(url_dead), JSON.stringify(url_expected), "got dead URL")
  })

  test("Got no dead server's URL", function (t) {
    var url = ['https://www.github.com', 'https://gist.github.com']
    var url_expected = []
    var url_dead = getDeadUrl(url)
    t.equal(JSON.stringify(url_dead), JSON.stringify(url_expected), "got no URL")
  })
  
  test('Got messages from dead servers', function (t) {
    var url = ['http://xxx.xxx.xxx', 'http://x.x.x.x']
    var msgs_expected = ['This server is dead: ' + url[0], 'This server is dead: ' + url[1]]
    var msgs_dead = url.map(sendMessage)
    t.equal(JSON.stringify(msgs_dead), JSON.stringify(msgs_expected), 'These messages equal expected messages')
  })

  test.finish()
}

watchalert.gs

'use strict';

if ((typeof SlackApp)==='undefined') {
  eval(UrlFetchApp.fetch('https://raw.githubusercontent.com/soundTricker/SlackApp/master/bin/Code.gs').getContentText())
}

function serverWatchAlert() {
  var urls = ['http://www.google.com', 
              'http://www.yahoo.com'];
  var urls_dead = getDeadUrls(urls);
  if (urls_dead != []) { Logger.log(urls_dead.map(sendMessage)) };
}

function getDeadUrls(urls){
  var urls_dead = urls.filter( function(s) { if (checkHttpResponse(s)!=200) return s; });
  return urls_dead;
}

function checkHttpResponse(url){
  var options = {
    "muteHttpExceptions" : true,
    "validateHttpsCertificates" : false,
    "followRedirects" : false,
  }
  try{
    var response = UrlFetchApp.fetch(url, options);
    return response.getResponseCode();
  } catch(e){
    return 999;
  }
}

function sendMessage(url){
  var token = PropertiesService.getScriptProperties().getProperty('SLACK_ACCESS_TOKEN');
  var slackApp = SlackApp.create(token);
  var options = {
    channelId: "#general",
    userName: "bot",
    iconUrl: "https://www.gstatic.com/images/icons/material/product/2x/apps_script_64dp.png",
  };

  var message = 'this www server is dead' + url;
  slackApp.postMessage(options.channelId, message, {
  username: options.userName,
  icon_url: options.iconUrl,
  });
  return message;
}