ArticleTemplates/assets/js/modules/services/impl/viewport-io.js

const observers = Object.create(null);
const callbacks = Object.create(null);
const elements = Object.create(null);

function observe(element, threshold, callback) {
  if (!observers[threshold]) {
    callbacks[threshold] = [callback];
    elements[threshold] = [element];
    observers[threshold] = new IntersectionObserver(entries => {
      entries.forEach(entry => {
        if (entry.isIntersecting) {
          callbacks[threshold].forEach((c, index) => {
            if (elements[threshold][index] === entry.target) {
              c(entry.intersectionRatio);
            }
          });
        }
      });
    }, { threshold });
  } else {
    callbacks[threshold].push(callback);
    elements[threshold].push(element);
  }
  observers[threshold].observe(element);
}

function unobserve(element, threshold, callback) {
  if (!observers[threshold]) return;

  observers[threshold].unobserve(element);

  const idx = callbacks[threshold].indexOf(callback);
  if (idx !== -1) {
    callbacks[threshold].splice(idx, 1);
    elements[threshold].splice(idx, 1);
  }

  if (callbacks[threshold].length === 0) {
    observers[threshold] = null;
  }
}

export default { observe, unobserve };