How to alert when scroll page only first time using javascript?

问题

How to alert when scroll page only first time using javascript ?

I want to alert only first scroll page, How can i do that ?

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript">
  $(window).scroll(function() {
    var xxx;
    if (xxx === '') {
      var xxx = "test";
      var div = $("#myDiv");
      alert(div.height());
    }
  });
</script>
<DIV id="myDiv" style="height:auto; width:78;overflow:hidden"> Simple Test</DIV>
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回答1:

Use .one()

Attach a handler to an event for the elements. The handler is executed at most once per element per event type.

$(window).one("scroll", function() {
    console.log("foo");
});

Code snippet:

$(window).one("scroll", function() {
  console.log("foo");
});

div {
  height: 1000px;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div>foo</div>

回答2:

Put you variable as global and use boolean instead of string which is a better practice when using code flags.Besides, no need to re-define you variable when setting it within the scroll function.

var xxx; 

$(window).scroll(function () {

if(!xxx)
{
    xxx = true;
    var div = $("#myDiv");
    alert(div.height());
}
  });

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>

<DIV id="myDiv" style="height:auto; width:78;overflow:hidden"> Simple Test</DIV>
<br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br>
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回答3:

   onscroll = function () {
          var xxx = "test";
          var div = $("#myDiv");
          alert(div.height());
      
        onscroll = null; // remove function so wont get called next time
   };

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<DIV id="myDiv" style="height:auto; width:78;overflow:hidden"> Simple Test</DIV>
<br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br>
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来源：https://stackoverflow.com/questions/31637069/how-to-alert-when-scroll-page-only-first-time-using-javascript