Simple trick for making sure that anything that you want to listen to window.onscroll doesn’t eat up too many cycles while it’s doing its thing. It’s called “throtting”.

Throttling basically means, if you’re receiving a steady stream of input from something, you don’t really want to be firing stuff off based on that steady stream. This is a performance suck. Let’s say you have this ---

window.addEventListener('scroll', function() {
  // Stuff that's actually kinda CPU intensive like
  // taking measurements, waiting for some element
  // to show up on the screen, for example. 

This function is going to be firing as many times a second as your computer can handle. If you’re on a beefy laptop in Chrome, this will probably not be noticeable, but make no mistake — none of your users are on as good a laptop as you are. You will definitely drop frames and your perceived performance will suck wind.

What’s the answer? Throttle that code. Like this.

// timeNow is the current time in milliseconds, created by
// casting new Date() to a number with +
var timeNow = +new Date();
window.addEventListener('scroll', function() {
  // if the current time in milliseconds - timeNow is
  // less than 250, abort.
  if((+new Date() - timeNow) < 250) return;
  // Else, reset timeNow to now.
  timeNow = +new Date();

This is hack-y looking because it’s kind of a hack. Underscore and Lodash have this built in, but it might be a little heavier than what you need. If you find yourself using this more than once in a file, please either bring in Lodash, or rip off their implementation into your project.