JavaScript控制台陷阱？控制台中的setTimeout返回的值是什么？ [英] javascript console gotchas ? what are those values returned by setTimeout in console?

问题描述

在控制台中运行以下代码

When running the following code in console

var ajout1 = function(number) {
  number = number || 100;
  console.log(number + 1);
}

setTimeout(ajout1, 1000);
setTimeout(ajout1(500), 2000); // evaluates immediately and returns nothing but this is "by design" to show how NOT to write your timeout
setTimeout(function() {
  ajout1(1000)
}, 3000);

可能会预期以下结果

t+1: 101
t+2: 501
t+3: 1001

如果您仔细观察，您会发现t2将被立即求值，因此不会等2秒再返回。 （这是设计使然。它教（我）如何不写超时）。因此，预期输出变为：

If you look more closely you'll see that t2 will be immediately evaluated and so will not wait 2 second before returning. (This is "by design". It teaches (me) how not to write a timeout). So the expected output becomes:

t+0: 501
t+1: 101
t+3: 1001

事实上，我真正得到的是

In fact what I really get is

t+0: 501
t+0: <X
t+1: 101
t+3: 1001

X的价值不断增长。
我认为X是timerid，所以我将代码重写为

With X a ever growing value. I think X is the timerid so I rewrote my code as

var t1=setTimeout(ajout1,1000);
var t2=setTimeout(ajout1(500),2000);// no good
var t3=setTimeout(function(){ajout1(1000)},3000);

现在输出为

t+0: 501
t+0: <undefined
t+1: 101
t+3: 1001

如果我评估t1，t2，t3，则其值为X + 1，X + 2，X + 3。所以我猜我的下注X是计时器ID是正确的。
我的问题是为什么控制台只输出1个计数器id而不是3个计数器id的值？为什么我会返回未定义的值。

If i evaluate t1,t2,t3 their values are X+1, X+2, X+3. So i guess my bet X was the timer id is right. My question is why do the console outputs the value of only 1 counter id instead of the three ? Why did I get a "undefined" value returned.

我在Chrome和Edge控制台中进行了测试，结果相同

I tested in Chrome and Edge console with the same results

推荐答案

第二个 setTimeout（）中发生了什么？

What's happening in your second setTimeout()?

setTimeout(ajout1(500)/*evaluates immediately and returns nothing*/,2000);

然后，该代码如下所示：

And then, that code looks like this:

setTimeout(undefined,2000); //undefined, because ajout1(500) didn't return anything

然后

setTimeout("undefined",2000); //Coerced to string

...以及2秒后...

...and 2 seconds later...

eval("undefined") //Altough not really eval, but similar to it.

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