javascript - setTimeout(fun) 和 setTimeout(fun,4) 之间有区别吗

Question

setTimeout 的行为有点令人困惑， 我读过的所有资源都表明，当没有指定延迟时，任务将被附加到队列末尾，并在解释器无事可做时执行(空队列？)。 但是，请看以下示例:

setTimeout(function() { console.log('without delay!'); })
setTimeout(function () { console.log('with delay'); }, 1000);
var start = Date.now();
while (Date.now() < start + 3000) {} // block for 3 seconds
console.log('After wait');

根据上述前提，输出将是:

After wait
with delay
without delay

但是，输出是(在 Chrome 和 Firefox 中测试):

After wait
without delay
with delay

(注意，当调用setTimeout(fn,1000)时，当解释器在1秒后空闲时，fn就会被执行，这可以是1秒，5秒，如果口译员仍然忙碌，甚至永远。)

前面的示例让我推断出 setTimeout(fn)、setTimeout(fn,0) 和 setTimeout(fn, 4)(4ms是HTML5中的最小延迟)，因为没有延迟的函数先于延迟1秒的函数执行完毕。(两者都已准备好执行)

所以我的问题是它们是否等效？ (在 HTML5 中)。

Answer 1

没有超时相当于0，0相当于4，根据W3 specification for timers :

http://www.w3.org/TR/2011/WD-html5-20110525/timers.html#dom-windowtimers-settimeout :

...
4. Get the timeout, and let timeout be the result.
5. If the currently running task is a task that was created by the setTimeout() method, and timeout is less than 4, then increase timeout to 4.

http://www.w3.org/TR/2011/WD-html5-20110525/timers.html#get-the-timeout :

When the above methods are to get the timeout, they must run the following steps:

1. Let timeout be the second argument to the method, or zero if the argument was omitted.

2. Apply the ToString() abstract operation to timeout, and let timeout be the result. [ECMA262]

3. Apply the ToNumber() abstract operation to timeout, and let timeout be the result. [ECMA262]

4. If timeout is an Infinity value, a Not-a-Number (NaN) value, or negative, let timeout be zero.

5. Round timeout down to the nearest integer, and let timeout be the result.

6. Return timeout.