for内使用await，是可以满足同步调用的
forEach内使用await，刚开始会把await上面的代码都给执行了，这在实战中是不被允许的，这种做法很容易出错
for await 使用，效果是并发调用，等较大者完成后，后面会连贯执行

function getDatas(times) {
  times = times || 0;
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(times);
    }, times * 1000);
  });
}
let arrays = [5, 10, 15];
async function execute() {
  for (let i = 0; i < arrays.length; i++) {
    console.log(i);
    let datas = await getDatas(arrays[i]);
    console.log(datas);
  }
}
execute();

//.....or 下面的execute，结果一样
async function execute() {
  let index = 0;
  for (let item of arrays) {
    console.log(index);
    let datas = await getDatas(item);
    console.log(datas);
    index++;
  }
}

function getDatas(times) {
  times = times || 0;
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(times);
    }, times * 1000);
  });
}
let arrays = [5, 10, 15];
arrays.forEach(async (item, index) => {
  console.log(index);
  let datas = await getDatas(item);
  console.log(datas);
});

function getDatas(times) {
  times = times || 0;
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(times);
    }, times * 1000);
  });
}
let arrays = [getDatas(6), getDatas(3), getDatas(9), getDatas(3)];
async function execute() {
  for await (let item of arrays) {
    console.log(item);
  }
}
execute();