check if element is in viewport - vanilla JS. Use by adding a “scroll” event listener to the window and then calling isInViewport().

in_viewport.js

// Determine if an element is in the visible viewport
function isInViewport(element) {
  var rect = element.getBoundingClientRect();
  var html = document.documentElement;
  return (
    rect.top >= 0 &&
    rect.left >= 0 &&
    rect.bottom <= (window.innerHeight || html.clientHeight) &&
    rect.right <= (window.innerWidth || html.clientWidth)
  );
}
The above function could be used by adding a “scroll” event listener to the window and then calling isInViewport().

The event listener on scroll will execute a function many timer as a user scrolls about while the element is in the viewport - madness with a slideshow! Also it takes at least 2 scroll events for element reach the viewport fully.

Need to hit the function only once...

let timerOn = false;

function isInViewport() {
  const ele = document.querySelector(".carousel");
  const rect = ele.getBoundingClientRect();
  if (rect.top >= 0 && rect.bottom <= window.innerHeight && rect.left >= 0 && rect.right <= window.innerWidth) {
	  if (!timerOn) timer();
  }
};

function timer() { 
  timerId = setInterval("plusSlides(1)", 3000); 
  timerOn = true;
}

window.addEventListener('scroll', function (event) {
  isInViewport();
}) ;