JavaScript 30- Day 13 Study Notes

Jeffrey Amen
2 min readNov 29, 2020

Day 13 of JavaScript 30, a course by Wes Bos, showed the interaction between CSS and JavaScript. It involved getting images to slide into place as you scrolled down the page.

First, we collected our images from HTML.

const sliderImages = document.querySelectorAll('.slide-in');

Now, Bos used a single function with several calculations in it to produce the effect.

function checkslide(e){sliderImages.forEach(sliderImage => {//slide in when cursor is halfway point of image location.   const slideInAt = (window.scrollY + window.innerHeight) sliderImage.height / 2;// scrollY is px from top of view. innerHeight is px of view.// subtract 50% of image height so slide in there// calculate location of bottom of image   const imageBottom = sliderImage.offsetTop + sliderImage.height;//offsetTop is px from top of image to top of page//add image height to get px to bottom   const isHalfShown = slideInAt > sliderImage.offsetTop;//slide in when scrolled past 50%//toggle image in or out of view. 'active' affects CSS transition settings   const isNotScrolledPast = window.scrollY < imageBottom;   if(isHalfShown && isNotScrolledPast){      sliderImage.classList.add('active');     }else {      sliderImage.classList.remove('active');   }});

This program produced a cool effect on the screen. The math was not complicated, yet required some thinking and visualization in my head.

Follow my study notes as I progress through JavaScript 30.



Jeffrey Amen

Retired physician, writer, aspiring website designer and developer