代码示例:(标识:js_promise_3)
<!DOCTYPE html>
<html>
<body>

<h1>JavaScript Promise</h1>

<p>等待 3 秒(3000 毫秒)让此页面发生变化。</p>

<h1 id="demo"></h1>

<script>
const myPromise = new Promise(function(myResolve, myReject) {
  setTimeout(function(){ myResolve("I love You !!"); }, 3000);
});

myPromise.then(function(value) {
  document.getElementById("demo").innerHTML = value;
});
</script>

</body>
</html>
运行结果: