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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?

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

    function fall(rate) {

      circle.style.top += rate;

    function execute() {

   setInterval(execute, 500);

1 Answer

Steven Parker
Steven Parker
229,921 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";

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

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
229,921 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!