Check If Elements Are In Viewport In Vanilla Javascript

With CSS you can do a lot of cool stuff like transform and transition. But you will still need JavaScript to add and remove classes to elements. In the code example you will see how to add classes to elements which are in the current viewport. It will not cover how to remove them later.

Let’s say you have some sections on your webpage and you want to show them with a nice transition when they become visible for the user.

You will need this SCSS with a visible class:

section.fly-in {
  transition: 1s ease-in-out;
  transform: translate(200px, 0);

  &.visible {
    transform: translate(0, 0);
  }
}

To add the class visible we use some JavaScript:

const sections = document.querySelectorAll('section.fly-in');

window.onscroll = function() {
  // Don't run the rest of the code if every section is already visible
  if (!document.querySelectorAll('section.fly-in:not(.visible)')) return;
  
  // Run this code for every section in sections
  for (const section of sections) {
    if (section.getBoundingClientRect().top <= window.innerHeight * 0.75 && section.getBoundingClientRect().top > 0) {
      section.classList.add('visible');
    }
  }
};

Note: this code is ES6, so maybe you may have to compile it to ES5 if you run it in a browser.

Happy coding! – Found a mistake or a typo? Please submit a PR to my GitHub-repo.

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