this_dom_event_error.js

function ScrollController(offset) {
  this.offsets = { offsetY: offset };
}

ScrollController.prototype.registerScrollHandler = function() {
  window.addEventListener('scroll', function(event) {
    if (window.scrollY === this.offsets.offsetY) {
      console.log(`${this.offsets.offsetY}px`);
    }
  }, false);
}

var controller = new ScrollController(100);
controller.registerScrollHandler();