HTMLProgressElement：value 属性

确定的进度条：<progress id="pBar"></progress> <span>0</span>%
<br />
不确定的进度条：<progress></progress>

const pBar = document.getElementById("pBar");
const span = document.getElementsByTagName("span")[0];

pBar.max = 100;
pBar.value = 0;

setInterval(() => {
  pBar.value = pBar.value < pBar.max ? pBar.value + 1 : 0;

  span.textContent = Math.trunc(pBar.position * 100);
}, 100);