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                // Please open your Chrome console to watch output

const sleep = wait => new Promise(resolve => setTimeout(resolve, wait));

const __main = async function() {
  // 你的需求其实是在一组 task 中,循环执行,每个都 sleep,并返回结果
  const tasks = [1, 2, 3];

  let results = await tasks.reduce(async (previousValue, currentValue) => {
    // 这里是关键,需要 await 前一个 task 执行的结果
    // 实际上每个 reduce 每个循环执行都相当于 new Promise
    // 但第二次执行可以拿到上一次执行的结果,也就是上一个 Promise
    // 每次执行只要 await 上一个 Promise,即可实现依次执行
    let results = await previousValue
    console.log(`task ${currentValue} start`)
    await sleep(1000 * currentValue);
    console.log(`task ${currentValue} end`);
    return results
  }, []);