javascript - js asynchronous problem, settimeout problem

Question

var i = 0, timer, j = 0;
while(i++ < 5) {
  timer = window.setTimeout(function(){
    j++;
    alert(j);
  }, 1000);
}
clearTimeout(timer);

The output is 1,2,3,4
5 why is it not output? And why is it output like this? Doesn't he assign values ​​and overwrite them every time? Why is it still being executed? What is the order? When is the statement clearTimeout executed?
If you change it slightly
var i = 0, timer, j = 0;
while(i < 5) {
timer = window.setTimeout( function(){
j ;
alert(j);
}, i*1000);
}
clearTimeout(timer);
This means output every second , if the above problem is solved, then there is nothing wrong
Continue to change

    var i = 0, timer, j = 0;
    while(i++ < 5) {
      timer = window.setTimeout(function(){
        j++;
        alert(j);
      }, j*1000);
    }

clearTimeout(timer);
这个时候，他是同时输出的，为什么跟上面用i的不一样？

















谢谢各位的回答哈。

Answer 1

First, 5 timers are created in the loop body. Each timer has its own ID. The ID starts executing after the call setTimeout时被返回，是一个数值。
其次，timer只是保存定时器的ID，并不会修改定时器的ID，所以当循环结束时，timer只是保存了最后一个定时器的ID。赋值覆盖的是什么应该清楚了吧。
定时器里的函数是在clearTimeoutis executed. So the last timer is cleared. Other timers are executed as usual.

The last one, while执行的时候setTimeout里的函数并没有被调用，因此j++并没有执行，所以在循环体内j一直是0。
注：匿名函数只是作为一个参数被传入setTimeoutin the function.

Answer 2

First question, when i=4, it should have alert(j==5) after 1 second, but then clearTimeout(timer) was executed immediately to cancel alert(j)

The second question is the same as above

The third question, when i=0, j is 0, you may think that the alert operation will be executed directly, but it is not the case. setTimeout(code, millisec) puts the code into a waiting queue and then executes it later, so When i=1, j is still 0. Similarly, i=2,3,4,5, so 1234 will be output continuously

Answer 3

Because all setTimeout是在clearTimeout(timer)we started executing after that, the fifth one has already been cleared.

The last one is executed after while执行的时候j是0，所有的setTimeout都是延迟0.

Answer 4

Take a good look at this while(i++ < 5) why it cannot output 5 because it is not larger than 5. Solution while(i++ <= 5)

Answer 5

Let’s answer the first one, because setTimeout is executed asynchronously, so clearTimeout(timer) will clear the first timer. That is to say, when i=0, the alert will not come out, j has not executed j++, and j is still equal to 0. After one second, the timer is generated again. At this time, i=1,j=0. Since the subsequent clearTimeout(timer) is executed synchronously, the timer is no longer cleared, so you can see your current results