JavaScript: Is element in viewport?

isElementInViewport.js

/*
No jQuery necessary.
Thanks to Dan's StackOverflow answer for this:
http://stackoverflow.com/questions/123999/how-to-tell-if-a-dom-element-is-visible-in-the-current-viewport
*/

function isElementInViewport(el) {
  var rect = el.getBoundingClientRect();
  return (
    rect.top >= 0 &&
    rect.left >= 0 &&
    rect.bottom <= (window.innerHeight || document. documentElement.clientHeight) &&
    rect.right <= (window.innerWidth || document. documentElement.clientWidth)
  );
}

it won't work if the element is coming from top, if half coming by scroll up, in this case, the top is nagetive, but the bottom is positive.

width: 300 height: 300 top: -255.03125 right: 827.03125 bottom: 44.96875 left: 527.03125

hello guys, JS is new thing to me. Can someone please explain me what does (el) represent in this script? Because it is not mentioned anywhere else in code but on the beginning.

Hello Mistovic,
The (el) parameter is the DOM element that has been passed to the function to check if it is in the viewport or not yet.

Hello Mistovic,
The (el) parameter is the DOM element that has been passed to the function to check if it is in the viewport or not yet.
Hello, and thank you for a reply.

So basically, we created a new element (el) with boundingClientRect() and gave him coordinates (or size) with return (top, left, bottom, right).
is that understood well?