Javascript setTimeOut真的是异步吗？

Question

我正在开发test nodejs app,我想创建100个"线程",每个线程使用setTimeOut在一些随机时间执行.

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');

现在我不理解输出:

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executed thread X current counter is Y在executed setTimeOut number Z它之间混合,为什么它首先似乎将所有调用添加到setTimeOut并且只在执行它们之后？即使我将数量设定为1,000,000,这仍然会发生.这对我来说看起来不像预期的行为.

Answer 1

呼叫setTimeout同步发生.然后运行时有一堆排队的'任务',它可以在以后执行.当超时到期时,运行时可以自由选择这些任务并执行.因此,所有'执行的setTimeOut数'消息首先出现,然后是'执行的线程......'.