JavaScript Document createEvent() Method

const elem = document.createElement("div");

elem.addEventListener("build", function () {
  console.log("build fired");
});

const event = document.createEvent("Event");
event.initEvent("build", true, true);
elem.dispatchEvent(event);
// "build fired"

const elem = document.createElement("div");

elem.addEventListener("build", function (e) {
  console.log(e.type, e.bubbles, e.cancelable);
});

const event = new Event("build", { bubbles: true, cancelable: true });
elem.dispatchEvent(event);
// "build" true true

const btn = document.createElement("button");
let legacyType = "";
let modernType = "";

btn.addEventListener("click", function (e) {
  if (!legacyType) legacyType = e.constructor.name;
  else modernType = e.constructor.name;
});

const legacy = document.createEvent("MouseEvents");
legacy.initEvent("click", true, true);
btn.dispatchEvent(legacy);

const modern = new MouseEvent("click", { bubbles: true, cancelable: true });
btn.dispatchEvent(modern);

console.log(legacyType);  // "MouseEvent"
console.log(modernType);  // "MouseEvent"