SOURCE

async function loadImgs(imgList, cb) {
  let proList = [];
  for (var i = 0; i < imgList.length; i++) {
    let pro = await imgLoader(imgList[i], i);
    proList.push(pro);
  }

  return Promise.all(proList)
    .then(() => {
    console.log("finish all");
    cb();
    return Promise.resolve();
  })
}
async function imgLoader(url, num) {
  return new Promise((resolve, reject) => {
    console.log("request" + num);
    setTimeout(resolve, 1000);
    console.log("return" + num);
  })
}

loadImgs(["xxx/a.png", "xxx/b.png"], function() {
  console.log("开始干活");
})