javascript - 为什么下面的代码总是在立即之前打印超时？

Question

为什么总是出现下面的:

0

...

999

超时内

立即内

setImmediate 不应该在回调队列中有优先级吗？

setImmediate(()=>{
    console.log('Inside the immediate');
});

setTimeout(()=> {
    console.log('Inside the timeout');
}, 1);

for (let i = 0; i < 1000; i++) {
    console.log(i);
}

Answer 1

GitHub 上有几个问题引用了此行为。 A comment在 these issues 之一上很好地解释了为什么会发生:

What happens with the code you mentioned is that the timer and the immediate are added, and then the libuv event loop starts. When it starts, the libuv event loop first check for timers, and if the time between when a timer was added and when the event loop starts is greater than the timer's timeout value, then that timer will fire before any immediate.

所以基本上，如果您还没有进入事件循环，您的 setTimeout 中的回调将在第一次计时时执行。否则，setImmediate 将首先执行。

作为演示，如果您将 setTimeout 和 setImmediate 包装在另一个 setTimeout 中，则 setImmediate 会被执行第一:

setTimeout(function() {
    setTimeout(function() {
        console.log('setTimeout executed!')
    }, 0);
    setImmediate(function() {
        console.log('setImmediate executed!')
    });
}, 20);