Monitoring Visible Objects on Screen

i found this one when researching how to detect visible objects on screen, and then doing something with them. This one does not use any external scripts to change the box colour as it becomes visible and then goes out of the screen.

it does seem quite complex for what it does, i might try and make a simpler version of this. 

other options are:

JQuery Appear

The jQuery appear plugin if you are into JQuery it makes it very simple. 

$('#moo').appear(function() {
  $(this).text('Hello world');

Other Options: 


<div id="box">
  <div class="vertical">
    Welcome to <strong>The Box!</strong>


#box {
  background-color: rgba(40, 40, 190, 255);
  border: 4px solid rgb(20, 20, 120);
  transition: background-color 1s, border 1s;
  width: 350px;
  height: 350px;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 20px;

.vertical {
  color: white;
  font: 32px "Arial";

.extra {
  width: 350px;
  height: 350px;
  margin-top: 10px;
  border: 4px solid rgb(20, 20, 120);
  text-align: center;
  padding: 20px;


const numSteps = 20.0;

let boxElement;
let prevRatio = 0.0;
let increasingColor = "rgba(40, 40, 190, ratio)";
let decreasingColor = "rgba(190, 40, 40, ratio)";

// Set things up
window.addEventListener("load", (event) => {
  boxElement = document.querySelector("#box");

}, false);

function createObserver() {
  let observer;

  let options = {
    root: null,
    rootMargin: "0px",
    threshold: buildThresholdList()

  observer = new IntersectionObserver(handleIntersect, options);

function buildThresholdList() {
  let thresholds = [];
  let numSteps = 20;

  for (let i=1.0; i<=numSteps; i++) {
    let ratio = i/numSteps;

  return thresholds;

function handleIntersect(entries, observer) {
  entries.forEach((entry) => {
    if (entry.intersectionRatio > prevRatio) { = increasingColor.replace("ratio", entry.intersectionRatio);
    } else { = decreasingColor.replace("ratio", entry.intersectionRatio);

    prevRatio = entry.intersectionRatio;
Welcome to The Box!

External Link for Monitoring Visible Objects on Screen

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This is my test area for webdev. I keep a collection of code snippits here, mostly for my reference. Also if i find a good site, i usually add it here.


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