Angular5/Typescript can't set variable

Question

I want to check if a server is reachable on init of an angular element. I have a function like this:

 export class AppComponent implements OnInit {
  dataServerOffline: boolean = true;

  constructor() {
  }

  ngOnInit() {
    var img = document.body.appendChild(document.createElement("img"));
    img.style.display = "none";
    img.src = "http://myUrl.com/online.jpeg";
    img.onload = function () {
      // I know that this variable is not accessible form here.
      this.dataServerOffline = false;
    };
    img.onerror = function () {
      this.dataServerOffline = true;
    };    
  }

No matter what I try, at the end "dataServerOffline" is undefined. How would I go about changing "dataServerOffline" inside of the "onload" and "onerror" functions?

I have already tried something like this:

export class AppComponent implements OnInit {
  dataServerOffline: boolean = true;

  constructor() {
  }

  ngOnInit() {
    var temp;
    var img = document.body.appendChild(document.createElement("img"));
    img.style.display = "none";
    img.src = "http://myUrl.com/online.jpeg";
    img.onload = function () {
      temp = false;
    };
    img.onerror = function () {
      temp = true;
    };
    this.dataServerOffline = temp;
  }