How can I create a custom event for scroll up and down?

The MDN told me how to create a custom event (which I didn't get it well)

var event = new Event('build');

// Listen for the event.
elem.addEventListener('build', function (e) { ... }, false);

// Dispatch the event.
elem.dispatchEvent(event);

and I know how to detect mouse scroll as well...

var doScroll = function (e) {
    // cross-browser wheel delta
    e = window.event || e;
    var delta = Math.max(-1, Math.min(1, (e.wheelDelta || -e.detail)));

    // Do something with `delta`
    document.body.innerHTML = delta;

    e.preventDefault();
};

if (window.addEventListener) {
    window.addEventListener("mousewheel", doScroll, false);
    window.addEventListener("DOMMouseScroll", doScroll, false);
} else {
    window.attachEvent("onmousewheel", doScroll);
}

but I don't know how to mix these two with each other and create a custom event so I can have something like this:

window.addEventListener('scrollUp', sUpFunction);
window.addEventListener('scrollDown', sDownFunction);

Thank you guys.

First I would use the "wheel" event instead of the non-standardrized "mousewheel" event.
I created a simple implementation of a scrollUp and scrollDown custom events dispatch.

EDIT

I have a added the IE polyfill for the CustomEvent in order to support IE as well

// For IE support
(function () {
  if ( typeof window.CustomEvent === "function" ) return false; //If not IE

  function CustomEvent ( event, params ) {
    params = params || { bubbles: false, cancelable: false, detail: undefined };
    var evt = document.createEvent( 'CustomEvent' );
    evt.initCustomEvent( event, params.bubbles, params.cancelable, params.detail );
    return evt;
   }

  CustomEvent.prototype = window.Event.prototype;

  window.CustomEvent = CustomEvent;
})();

let element = document.getElementsByClassName("scroll-area")[0],
scrollUpEvent = new CustomEvent("scrollUp"),
scrollDownEvent = new CustomEvent("scrollDown");

function scrollDown(){
  console.log("scrolled down");
}
function scrollUP(){
  console.log("scrolled up");
}

function scrollHappened(e){
  if(e.deltaY < 0){
    element.dispatchEvent(scrollUpEvent);
  } else {
    element.dispatchEvent(scrollDownEvent);
  }
}
element.addEventListener("wheel", scrollHappened);
element.addEventListener("scrollUp", scrollUP);
element.addEventListener("scrollDown", scrollDown);

.scroll-area {
  border: solid 2px black;
  height: 30px;
}

<div class="scroll-area">Scroll on me</div>