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JavaScript

Posted by Jackson Monk
Jackson Monk
4,527 Points

How to change the value of circle.top?

I have made a circle with CSS, selected it in my circle variable, and used getBoundingClientRect() on it. I also have a fall() function that takes a rate and adds that rate to circle.top. Then I setInterval() to run the fall() function every half second. However, the circle.top does not seem to change. I am new to this method, so what am I missing?

<html>
<head>
  <style>
  .circle {
      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].getBoundingClientRect();

    function fall(rate) {

      circle.top += rate;
    }

    setInterval(fall(1), 500);
    </script>
  </body>
</html>

1 Answer

Alexander Davison
Alexander Davison
65,469 Points
Alexander Davison
Alexander Davison
65,469 Points

setInterval takes a function that takes no arguments. fall takes one argument. And fall(1) isn't a function, because JavaScript first executes fall(1), and it returns nothing, and the nothing (also called null) is passed into setInterval, which causes an error. Try to do something like this:

setInterval(function () {
    fall(1);
}, 500);

// Or, with the arrow function syntax:

setInterval(() => fall(1), 500);
Jackson Monk
Jackson Monk
4,527 Points

Hmm... It still seems to be doing the same thing. Here is the updated code:

<html>
<head>
  <style>
  .circle {
      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].getBoundingClientRect();

    function fall(rate) {

      circle.top += rate;
    }

    setInterval(function() {fall(1)}, 500);
    </script>
  </body>
</html>
Jackson Monk
Jackson Monk
4,527 Points

I have updated it to use the CSS top property and called the fall() function in another function. Now it works but it only does it once instead of every 500 milliseconds

<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>