JavaScript setTimeOut是否真正異步？

Question

我正在開發測試nodejs應用程序，我想創建100個“線程”，每個線程都使用setTimeOut在某個隨機時間執行。

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');

現在我不明白的是輸出：

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

我希望在executed setTimeOut number Z之間混合executed thread X current counter is Y ，為什么它似乎首先似乎將所有調用添加到setTimeOut中，然后才執行它們？ 即使當我將計數設置為1,000,000時，這種情況仍在發生。 在我看來，這似乎不是預期的行為。

Answer 1

對setTimeout的調用是同步發生的。 然后，運行時就會排隊等待一堆“任務”，以便以后可以執行。 超時到期后，這些任務可以由運行時自由選擇並執行。 因此，所有“已執行setTimeOut號”消息首先出現，然后是“已執行線程...”。