Elementが画面外に出たことを検知する

DisplayOutMonitor.js

class DisplayOutMonitor {
  constructor(option) {
    option = option || {}
    this.document = option.document || document
    this.interval = option.interval || 100
  }

  /**
   * @param {string}
   * @param {(isOut:boolean)=>void}
   */
  register(selector, cb) {
    var target = this.document.querySelector(selector);
    var isOut = false;
    setInterval(()=>{
      var p = target.getBoundingClientRect().top + target.getBoundingClientRect().height;
      if(p <= 0 && !isOut) {
        cb(isOut = true);
      }
      if(p > 0 && isOut) {
        cb(isOut = false);
      }
    }, this.interval);
  }
}

const displayOutMonitor = new DisplayOutMonitor()