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Questions about js counter

一个新手
一个新手Original
2017-09-13 10:29:541383browse


js implementation of counter

Implementation specific code

<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>计时器</title>
<script type="text/javascript">
  var num=0;  
  function startCount() {
    document.getElementById("count").value=num;
    num=num+1;
      setTimeout("startCount()",1000);     //1处
  }
  setTimeout("startCount()",1000);       //2处
  </script>
  </head>
  <body>
  <form>
  <input type="text" id="count" />
  </form>
  </body>
  </html>

As shown in the code, 1 is obviously an infinite loop, so it is indispensable. The stopwatch counts infinitely. Note here that the unit in js is milliseconds, so it is 1000 milliseconds (1min=60s 1s=1000ms)
But when the code at 2 is deleted, there is only a blank box.

Reason

Netizens understand this. If it is wrong, you can communicate directly
First of all, the program is compiled directly from top to bottom, so the function will be compiled. At this time, the following code That is to say, the HTML code has not been compiled yet, so the function cannot recognize the ID. If the function settimeout is added, it will stay for 1s. At this time, the program has been completed, so it can be passed.
But this statement goes against the process of C language compilation. Because the function is implemented by calling, there is no problem of unrecognizability. . . .

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