javascript weirdness

input.js

"use strict";
class Input
{
  constructor(startButton)
  {
    this.startButton = startButton;
    startButton.onclick = this.start;
  }
  start()
  {
    //this button references is unset, I guess class "instances" works differently in js :/
    this.startButton.disabled = true;

    //this is what I have to do
    document.getElementById("startButton").disabled = true;
  }
}