为什么 Javascript "await" 没有等待正确的时间？

Question

此代码未按预期运行：

function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

function diff(expected) {
  let x = expected - Date.now();
  if (x > 0) {
    return `earlier ${x} ms`;
  } else if (x < 0) {
    return `late ${-x} ms`;
  } else {
    return `just in time`;
  }
}

start = Date.now();
async function demo() {
  let loop_count = 5;
  for (let i = 0; i < loop_count; i++) {
    console.log(diff(start + i * 1000) + `: Waited ${i} seconds...`);
    await sleep(i * 1000);
  }
  console.log(diff(start + loop_count * 1000) + ': Done');
}

demo();

输出为：

$ node test.js 
just in time: Waited 0 seconds...
earlier 993 ms: Waited 1 seconds...
earlier 993 ms: Waited 2 seconds...
late 10 ms: Waited 3 seconds...
late 2011 ms: Waited 4 seconds...
late 5013 ms: Done

这种差异从何而来？

Answer 1

Where does this difference come from?

因为你测错了。首先，您在 await sleep(i * 1000); 之前执行 console.log(diff(start + i * 1000) + `: Waited ${i} seconds...`);。此测试必须在 sleep.

之后

另一个问题是您没有更改 start 值，并且您始终使用相同的 start 值作为每次迭代的参考值。

所以在 i>1 之后你会得到错误的结果：你已经等了 1 秒，所以 i=2 从 start 开始的总时间是 (1 + 2) 秒，从 i=3 到 start 经过的总时间是 (1 + 2 + 3) 秒，...。

更新代码，使之前睡眠的秒数也包含在计算中，得到预期结果：

function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

function diff(expected) {
  let x = expected - Date.now();
  if (x > 0) {
    return `earlier ${x} ms`;
  } else if (x < 0) {
    return `late ${-x} ms`;
  } else {
    return `just in time`;
  }
}

function sumUp(num) {
  let res = 0;
  for(let i=0; i<=num ; i++) {
    res += i;
  }
  return res;
}

start = Date.now();
async function demo() {
  let loop_count = 5;
  for (let i = 0; i < loop_count; i++) {
    await sleep(i * 1000);
    console.log(diff(start + sumUp(i) * 1000) + `: Waited ${i} seconds...`);
  }
  console.log(diff(start + sumUp(loop_count) * 1000) + ': Done');
}

demo();

结果将始终是 late，因为 setTimeout 将等待至少给定的时间。对于您测量错误的方式，每个调用的计时器都会堆积起来。

为什么 Javascript "await" 没有等待正确的时间？

Why doesn't Javascript "await" wait for the correct amount of time?

javascript

async-await

for-await