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JavaScript

Posted by Jackson Monk
Jackson Monk
4,527 Points

Anyone know why setInterval() only runs once?

Trying to create a slow gravity effect on a CSS circle with JS. However, the circle only moves once instead of every 500 milliseconds like I tell it to in setInterval(). Why is this?

<html>
<head>
  <style>
  .circle {
      position: relative;
      padding: 15px;
      background-color: black;
      border-radius: 50%;
      width: 0.2%;
    }
  </style>
  </head>
  <body>
    <div class = circle></div>
  <script>
    let circle = document.getElementsByTagName('div')[0];

    function fall(rate) {

      circle.style.top += rate;
    }

    function execute() {
      fall(2);
    }

   setInterval(execute, 500);
    </script>
  </body>
</html>

1 Answer

Steven Parker
Steven Parker
224,924 Points
Steven Parker
Steven Parker
224,924 Points

The interval is running, but you don't see any effect because the property value is a string and you need to convert it to a number to do math with it:

function fall(rate) {
  var top = parseInt(circle.style.top || 0) + rate;
  circle.style.top = top + "px";
}
Jackson Monk
Jackson Monk
4,527 Points

I thought I tried that, but I thought it was weird because it ran once, and if it wasn't working it wouldn't run at all. I'll try this though

Jackson Monk
Jackson Monk
4,527 Points

Wow! Works perfectly! Why is it that when I tried it with parseInt(circle.style.top), it didn't work, but when I did parseInt(circle.style.top || 0), it worked perfectly?

Steven Parker
Steven Parker
224,924 Points
Steven Parker
Steven Parker
224,924 Points

That's because until you assign it, that property returns only an empty string. But the "|| 0" will substitute a zero when the string is empty.

Happy coding!