<h1>这里是占位内容</h1>
<button>修改上面的内容</button>
function delay(duration) {
  const start = Date.now();
  while (Date.now() - start < duration) {}
}
const h1 = document.querySelector("h1");
const btn = document.querySelector("button");
btn.onclick = function () {
  h1.textContent = "可以看到占位内容3秒后才被修改";
  delay(3000);
};

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