Dependency on `window`?

function delegateEvents(eventNames, document = window.document) {
  const e = document[$$EVENTS] || (document[$$EVENTS] = new Set());
  for (let i = 0, l = eventNames.length; i < l; i++) {
    const name = eventNames[i];
    if (!e.has(name)) {
      e.add(name);
      document.addEventListener(name, eventHandler);
    }
  }
}

function delegateEvents(eventNames, document = window.document) {
  const e = document[$$EVENTS] || (document[$$EVENTS] = new Set());
  for (let i = 0, l = eventNames.length; i < l; i++) {
    const name = eventNames[i];
    if (!e.has(name)) {
      e.add(name);
      document.addEventListener(name, eventHandler);
    }
  }
}