Javascript、Promises 和 setTimeout

Question

我一直在玩 Promises，但我无法理解以下代码发生了什么：

const promise = new Promise((resolve, reject) => {
  console.log('Promise started - Async code started')
  setTimeout(() => {
    resolve('Success')
  }, 10)
})

setTimeout(() => {
  console.log('Promise log inside first setTimeout')
}, 0)

promise.then(res => {
  console.log('Promise log after fulfilled')
})

console.log('Promise made - Sync code terminated')

setTimeout(() => {
  console.log('Promise log inside second setTimeout')
}, 0)

输出是：

Promise started - Async code started 
Promise made - Sync code terminated 
Promise log inside first setTimeout 
Promise log inside second setTimeout 
Promise log after fulfilled

正如预期的那样。

但是让我们检查以下代码的输出：

const promise = new Promise((resolve, reject) => {
  console.log('Promise started - Async code started')
  setTimeout(() => {
    resolve('Success')
  }, 1)
})

setTimeout(() => {
  console.log('Promise log inside first setTimeout')
}, 0)

promise.then(res => {
  console.log('Promise log after fulfilled')
})

console.log('Promise made - Sync code terminated')
setTimeout(() => {
  console.log('Promise log inside second setTimeout')
}, 0)

将要解决的承诺 setTimeout 计时器值从 10 毫秒更改为 1 毫秒

输出是：

Promise started - Async code started 
Promise made - Sync code terminated 
Promise log after fulfilled 
Promise log inside first setTimeout 
Promise log inside second setTimeout 

对此有何解释？

Answer 1

Chrome has an hardcoded minimum timeout of 1ms.

  base::TimeDelta interval_milliseconds =
      std::max(base::TimeDelta::FromMilliseconds(1), interval);

因此，对于 Chrome，您所有的 setTimeout( fn , 0 ) 都会转换为 setTimeout( fn , 1 )，因此会在您安排的第一个之后触发（记住 Promise 构造函数是同步调用的）。

所以我们实际上可以简化您的示例

setTimeout( () => console.log( '1ms delay' ), 1 );
setTimeout( () => console.log( '0ms delay' ), 0 );

在 Chrome 中，1ms 延迟总是会首先触发，这与常识相反，因为在内部它实际上是：

setTimeout( () => console.log( '1ms delay' ), Math.max(1, 1) );
setTimeout( () => console.log( '0ms delay' ), Math.max(1, 0) );

如果您将其设置为 1 和 2 而不是 0 和 1，您的期望将会实现。

const promise = new Promise((resolve, reject) => {
  console.log('Promise started - Async code started')
  setTimeout(() => {
    resolve('Success')
  }, 2)
})

setTimeout(() => {
  console.log('Promise log inside first setTimeout')
}, 1)

promise.then(res => {
  console.log('Promise log after fulfilled')
})

console.log('Promise made - Sync code terminated')
setTimeout(() => {
  console.log('Promise log inside second setTimeout')
}, 1)

Answer 2

我会用下面的例子来解释：

setTimeout(() => {
  console.log('1 ms timeout');
}, 1);                            // Moved to async queue at time = T0
setTimeout(() => {
  console.log('0 ms timeout')
}, 0);                            // Moved to async queue after 1 ms that synchronous call to setTimeout takes i.e. at T1
                                  // So at T1, queue will be [("1ms timeout", 0), ("0ms timeout", 0)]

因此会打印出来

1 ms timeout
0 ms timeout

上面的理解： 调用 setTimeouts 是同步的（即使它的回调放在异步队列中），即我们调用 setTimeout() 并移动到下一条语句 - 这个同步操作本身可能需要 1 毫秒。

换句话说，1ms 太短了，所以当 JS 引擎看到第二个异步语句时，第一个已经在队列中花费了 1ms。

我还建议您尝试以下方法

setTimeout(() => {
  console.log("First");
}, 2);                      // queue at T0 = [("First", 2)]

const forLoopLimit = 100;
for (var i = 0; i < forLoopLimit; i++){
    console.log(i * 10000);
}                           // Assume that it takes about 3 milliseconds
                            // queue at T3 = [("First", 0)]
setTimeout(() => {
  console.log("Second");
}, 0);                      // Assume it takes 0 milliseconds.
                            // queue at T4 = [("First", 0), ("Second", 0)]

这将在Second 之前打印First，即使前者的超时时间为2ms，而后者的超时时间为0ms。 现在把forLoopLimit改成1甚至10，你会看到同步任务现在不需要3毫秒，而且Second打印在First之前

另外值得一试的是：

console.log(Date.now());
console.log(Date.now());

多次尝试上述方法，您会发现有时控制台日志会有不同的时间戳。粗略地说，你可以说console.log() 和Date.now() 需要 0.5 毫秒。只是调用/执行同步内容的时间。

Answer 3

来自Concurrency model and the event loop

• setTimeout 在计时器到期后不会立即运行
• 零延迟实际上并不意味着回调将在零毫秒后触发。以 0（零）的延迟调用 setTimeout 毫秒后不执行回调函数 间隔。 基本上，setTimeout 需要等待队列消息的所有代码完成，即使您指定了特定的时间限制 为您的 setTimeout。

如果我们设置 2 和 1 毫秒会发生什么：

const promise = new Promise((resolve, reject) => {
  console.log('Promise started - Async code started')
  setTimeout(() => {
    resolve('Success')
  }, 2)
})

console.log('Promise log inside first setTimeout 1')
setTimeout(() => {
  console.log('Promise log inside first setTimeout 2')
}, 1)

promise.then(res => {
  console.log('Promise log after fulfilled ❌')

})

console.log('Promise log inside second setTimeout 1')
setTimeout(() => {
  console.log('Promise log inside second setTimeout 2')
}, 1)
});

输出总是：

Promise started - Async code started
Promise log inside first setTimeout 1
Promise log inside second setTimeout 1
Promise log inside first setTimeout 2
Promise log inside second setTimeout 2
Promise log after fulfilled ❌

结论

如果您想要正确的行为，值得摆脱零延迟。