javascript setTimeout(function(){console.log(3)}, 0); 它的值为 0
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setTimeout(function(){console.log(3)}, 0); it has value 0
提问by
When I execute the below code in the console I am getting as 1,4, undefined 3,2.
当我在控制台中执行以下代码时，我得到 1,4, undefined 3,2.
I would like to know why its not executing as 1,3,4and 2since in setTimeout(function(){console.log(3)}, 0);the milliseconds param is 0.
我想知道为什么它不执行1,3,4，2因为在setTimeout(function(){console.log(3)}, 0);毫秒参数中是0.
(function() {
console.log(1);
setTimeout(function(){console.log(2)}, 1000);
setTimeout(function(){console.log(3)}, 0);
console.log(4);
})();
回答by nril
Here's a great explanation by John Resig: http://ejohn.org/blog/how-javascript-timers-work/
But the bottom line is console.log(1) and (4) are executed 'in-line' and 2 and 3 are placed in the event queue, and don't get executed until the all of the in-line code is executed. So, even though the delay is 0 for (3), it still occurs after all statements are executed.
I'm not getting the undefined message when I test you code either.
这是 John Resig 的一个很好的解释：http: //ejohn.org/blog/how-javascript-timers-work/
但底线是 console.log(1) 和 (4) 是“内嵌”执行的，而 2和 3 被放置在事件队列中，并且在所有内嵌代码执行完毕之前不会被执行。因此，即使 (3) 的延迟为 0，它仍然会在所有语句执行后发生。
当我测试你的代码时，我也没有收到未定义的消息。
回答by ThiefMaster
The undefinedis, as mentioned in a comment, the return value of your function.
的undefined是，作为一个评论，你的函数的返回值提及。
The reason why 4comes before 3is simply that there is no parallelism in JavaScript so the timer callbacks cannot execute until your function has returned.
4出现在之前的原因3很简单，因为 JavaScript 中没有并行性，因此在您的函数返回之前，计时器回调无法执行。
回答by Hacketo
Short Answer : the undefinedcome from the result of the executed statement in your javascript console, the function return nothing, so undefined
简短回答：undefined来自 javascript 控制台中执行语句的结果，该函数不返回任何内容，因此未定义
Let try to decompose this:
让我们尝试分解它：
console.log(1);//1console.log(4);//4- the code you executed return
undefined setTimeout(function(){console.log(3)}, 0);//3Executed the next stacksetTimeout(function(){console.log(2)}, 1000);//21 sec after
console.log(1);//1console.log(4);//4- 你执行的代码返回
undefined setTimeout(function(){console.log(3)}, 0);//3执行下一个栈setTimeout(function(){console.log(2)}, 1000);//21 秒后
Edit : Test this
编辑：测试这个
var a = (function(){})(); // print undefined, a is also undefined, no returned value
