Why is while loop running before html DOM assignment in javascript?

Question

In Below, why does while loop run before document.body.style.backgroundColor assignment?

console.log("loadded");

const btn = document.getElementById("button");

btn.addEventListener("click", () => {
  document.body.style.backgroundColor = "red";
  const start = Date.now();
  const delaySeconds = 10;
  while (Date.now() < start + delaySeconds * 1000) {}
});

<button id="button">test</button>