javascript - setTimeout 不会立即启动

Question

当我将以下函数放在文档的头部时:

(function(){
    var d = new Date().getTime();
    console.log(new Date().getTime() - d);

    window.setTimeout(function(){
        console.log(new Date().getTime() - d)
    }, 1);

    window.setTimeout(function(){
        console.log(new Date().getTime() - d)
    }, 10);

    window.setTimeout(function(){
        console.log(new Date().getTime() - d)
    }, 100);

    window.setTimeout(function(){
        console.log(new Date().getTime() - d)
    }, 1000);
}());

我有这些日志:

0
401
401
402
1397

在 setTimeout 开始之前似乎有 400 毫秒的延迟。为什么？

Answer 1

Javascript 不是多线程的 - setTimeout 不会立即启动。当您调用 setTimeout 时发生的所有事情就是将该函数放入队列中，以便在当前执行堆栈展开时执行。在您的情况下，到达该点大约需要 400 毫秒。

参见 How Timers Work一个很好的教程。特别是，这句话是相关的:

In order to understand how the timers work internally there’s one important concept that needs to be explored: timer delay is not guaranteed. Since all JavaScript in a browser executes on a single thread asynchronous events (such as mouse clicks and timers) are only run when there’s been an opening in the execution.