Is In Viewport Checker With Pure JavaScript – white-in-view
white-in-view is a very small JavaScript library that checks if an element is in the viewport and adds matched CSS classes to it based on the current in or out state.
JavaScript & CSS viewport
Detecting An Element Appears Or Disappears From The Viewport – in-view.js
Just another JavaScript library which detects when an element is in the viewport and triggers custom events when it is scrolled into or out of the view.
JavaScript Library To Detect If An Element Is In View – inView.js
inView.js is a pure JavaScript library that watches for the scroll event and executes callbacks when an element is scrolled in or out of the viewport.