https://github.com/stutrek/scrollMonitor

스크롤 모니터를 사용하면 요소가 뷰포트에 들어가거나 나올 때 이벤트를 수신 할 수 있습니다.

var scrollMonitor = require("scrollmonitor"); // if you're old school you can use the scrollMonitor global.
var myElement = document.getElementById("itemToWatch");

var elementWatcher = scrollMonitor.create( myElement );

elementWatcher.enterViewport(function() {
    console.log( 'I have entered the viewport' );
});
elementWatcher.exitViewport(function() {
    console.log( 'I have left the viewport' );
});