javascript - Trigger function only once on scroll - Stack Overflow

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I want to start a function when a div is scrolled into the viewport. My problem is, that the function is triggered/started again every time I continue to scroll.

HTML:

<div class="box"></div>

JS:

 $(document).ready(function() {

    function start() {
        alert("hello");
    }

    $(window).scroll(function() {
      if ( $(window).scrollTop() >= $('.box').offset().top - ($(window).height() / 2)) {
        $(".box").addClass("green");
        start();
      } else {
        $(".box").removeClass("green");
      }
    });
  });

To sum up: the function "start" should be started, when the div is scrolled into the viewport. But it should be not triggered again, after it was triggered once.

Fiddle

I want to start a function when a div is scrolled into the viewport. My problem is, that the function is triggered/started again every time I continue to scroll.

HTML:

<div class="box"></div>

JS:

 $(document).ready(function() {

    function start() {
        alert("hello");
    }

    $(window).scroll(function() {
      if ( $(window).scrollTop() >= $('.box').offset().top - ($(window).height() / 2)) {
        $(".box").addClass("green");
        start();
      } else {
        $(".box").removeClass("green");
      }
    });
  });

To sum up: the function "start" should be started, when the div is scrolled into the viewport. But it should be not triggered again, after it was triggered once.

Fiddle

• javascript
• jquery
• scroll
• triggers

Share Improve this question Follow asked Sep 16, 2019 at 17:16 Maximilian NeuseMaximilian Neuse 16311 gold badge55 silver badges1515 bronze badges 2

• 2 what about a variable as a flag, that is setted to true inside start(), then inside the scroll() you check if this flag is false before run start()? – Calvin Nunes Commented Sep 16, 2019 at 17:19
• my final fiddle: jsfiddle/eu8ab9Lq/1 – Maximilian Neuse Commented Sep 17, 2019 at 5:31

Add a ment  | 

5 Answers 5

Sorted by: Reset to default 6

You can setup a flag like:

var started = false;
function start() {
  if(!started) {
    alert("hello");
  }
  started = true;
}

Demo

  $(document).ready(function() {
    
    var started = 0;
function start() {
  if(started==0) {
    alert("Alert only once");
  }
  started = 1;
}
  
    $(window).scroll(function() {
      if ( $(window).scrollTop() >= $('.box').offset().top - ($(window).height() / 2)) {
        $(".box").addClass("green");
        start();
      } else {
        $(".box").removeClass("green");
      }
    });
  });

*{margin:0;}
.box { background: red; height: 200px; width: 100%; margin: 800px 0 800px 0; }
.green { background: green; }

<script src="https://cdnjs.cloudflare./ajax/libs/jquery/2.1.3/jquery.min.js"></script>
<br />

<center>
    
  <br />
  <h1>scroll down</h1>
  </center>

<div class="box"></div>

There's a bunch of ways to go about this. You could just remove the event listener (since you're using jQuery, I'll use the on and off methods):

 $(window).on('scroll', function() {
  if ( $(window).scrollTop() >= $('.box').offset().top - ($(window).height() / 2)) {
    $(".box").addClass("green");
    start();
  } else {
    $(".box").removeClass("green");
  }
  $(window).off('scroll');
});

if you want window scroll method to stop once the start method meet its requirement.. you can do it this way

  $(document).ready(function() {

  var toggleScroll = false;

    function start() {
        alert("hello");
    }

    $(window).one("scroll", checkToggleScroll);

    function checkToggleScroll(){

        if ( $(window).scrollTop() >= $('.box').offset().top - ($(window).height() / 2)) {
          $(".box").addClass("green");
          toggleScroll = true;
          start();
        } else {
          $(".box").removeClass("green");
        }

        if(!toggleScroll){
          $(window).one("scroll", checkToggleScroll);
        }
    }

  });

Just run the start() function when the $(".box) doesn't have the class "green" (that is added after a certain amount of scroll).

$(document).ready(function() {

  function start() {
    alert("hello");
  }

  $(window).scroll(function() {
    if ($(window).scrollTop() >= $('.box').offset().top - ($(window).height() / 2)) {
      if (!$(".box").hasClass("green")) {
        $(".box").addClass("green");
        start();
      }
    } else {
      $(".box").removeClass("green");
    }
  });
});

.box {
  background: red;
  height: 200px;
  width: 100%;
  margin: 800px 0 800px 0;
}

.green {
  background: green;
}

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

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