javascript - Javascript setTimeOut 真的是异步的吗？

Question

我正在开发测试nodejs应用程序，我想创建100个“线程”，每个线程使用setTimeOut在某个随机时间执行。

let count = 10;
let counter = 0;

for(let i = 0; i < count; i++) {

    // call the rest of the code and have it execute after 3 seconds
    setTimeout((async () => {
        counter++;

        console.log('executed thread',i, 'current counter is',counter);

        if(counter === count){
            console.log('all processed');
        }


    }), Math.random()*10);

    console.log('executed setTimeOut number ',i);

}

console.log('main thread done, awaiting async');

现在我不明白的是输出:

executed setTimeOut number  0
executed setTimeOut number  1
executed setTimeOut number  2
executed setTimeOut number  3
executed setTimeOut number  4
executed setTimeOut number  5
executed setTimeOut number  6
executed setTimeOut number  7
executed setTimeOut number  8
executed setTimeOut number  9
main thread done, awaiting async
executed thread 5 current counter is 1
executed thread 1 current counter is 2
executed thread 4 current counter is 3
executed thread 9 current counter is 4
executed thread 6 current counter is 5
executed thread 2 current counter is 6
executed thread 3 current counter is 7
executed thread 8 current counter is 8
executed thread 0 current counter is 9
executed thread 7 current counter is 10
all processed

我期望的是执行的线程X当前计数器是Y与执行的setTimeOut数字Z之间的混合，为什么它首先似乎将所有调用添加到setTimeOut中，然后才执行它们？即使我将计数设置为 1,000,000，这种情况仍然会发生。对我来说，这看起来不像是预期的行为。

Answer 1

对 setTimeout 的调用是同步发生的。然后，运行时会将一堆“任务”排队，以便稍后执行。当超时到期时，这些任务可以由运行时自由拾取并执行。因此，您的所有“执行的 setTimeOut number”消息首先出现，然后是您的“执行的线程...”。