denoland-deno/tests/specs/run/beforeunload_event/before_unload.js

let count = 0;

console.log("0");

globalThis.addEventListener("beforeunload", (e) => {
  console.log("GOT EVENT");
  if (count === 0 || count === 1) {
    e.preventDefault();
    setTimeout(() => {
      console.log("3");
    }, 100);
  }

  count++;
});

console.log("1");

setTimeout(() => {
  console.log("2");
}, 100);