setTimeout函数的顺序是0、0、0，但也有0,1,0，怎么办？

Question

I just tried executing below code

 setTimeout(function(){ console.log(1); }, 0);
 setTimeout(function(){ console.log(2); }, 0);
 setTimeout(function(){ console.log(3); }, 0);

Above code has logged 1, 2, 3 in sequence. But if I do the same thing with seconds values 0, 1, 1

setTimeout(function(){ console.log(1); }, 0);
setTimeout(function(){ console.log(2); }, 1);
setTimeout(function(){ console.log(3); }, 0);

Still the above code logs same 1, 2, 3 in sequence. Tested this in Chrome!

Would like to know whether this is happening due to console.log() . Any inputs on this?

Answer 1

The HTML standard does not require user agents to strictly order queued timer events by their intended wait time when said wait time has already elapsed. It is very likely that the third call to setTimeout , returning to native code, and running the first JS callback from the event loop takes more than 1 ms, so running the second callback immediately upon re-entering the event loop next is just as logical as running the third.

Answer 2

The resolution of context-switching in an operating system is typically greater than 1ms (expect 5-10ms or so); therefore, you cannot expect a difference of 1ms to be noticeable.

Notwithstanding the above, in terms of your browser's JavaScript engine, historically speaking and now officially as of HTML5, any difference below 4ms is likely to be ignored anyway:

Historically browsers implement setTimeout() "clamping": successive setTimeout() calls with delay smaller than the "minimum delay" limit are forced to use at least the minimum delay. The minimum delay, DOM_MIN_TIMEOUT_VALUE , is 4 ms (stored in a preference in Firefox: dom.min_timeout_value ), with a DOM_CLAMP_TIMEOUT_NESTING_LEVEL of 5ms.

In fact, 4ms is specified by the HTML5 spec and is consistent across browsers released in 2010 and onward. Prior to (Firefox 5.0 / Thunderbird 5.0 / SeaMonkey 2.2), the minimum timeout value for nested timeouts was 10 ms.

In addition to "clamping", the timeout can also fire later when the page (or the OS/browser itself) is busy with other tasks.

With all the timers firing during the same event tick, then, the only remaining ordering to go on is the order in which the timers were set, which results in 1, 2, 3 . Some browsers though, like Firefox, do honour the implicit ordering implied by the requested delays (even when those delays are rounded up to the same value) and in those browsers you will see 1, 3, 2 .

Answer 3

某些浏览器（例如，较旧版本的Internet Explorer）不支持10ms以下的计时器分辨率，因此它们会将1ms超时视为0ms。