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I want to trigger my CSS transitions with Javascript when scrolling...how can i do this?

Here is my javascript so far...this loads my transitions when the website is loaded:

var elements  = document.getElementsByClassName('fade-out');

function fadingInElements() {
for (let i = 0; i<elements.length; i++) {
    elements[i].style.opacity = 1;
    elements[i].style.transition = 'opacity 1.5s';

window.onload = function() {

I want the function to fire when the user scrolls to the element itself, so they can actually see the transition. How can I do this?