IntersectionObserver callback firing immediately on page load

Question

I'm very new to the IntersectionObserver API, and I've been experimenting with this code:

let target = document.querySelector('.lazy-load');

let options = {
    root: null,
    rootMargin: '0px',
    threshold: 0
}

let observer = new IntersectionObserver(callback, options);

observer.observe(target);

function callback() {
    console.log('observer triggered.');
}

This seems to work as it should, and callback() is called whenever .lazy-load element enters the viewport, but callback() also fires once when the page is initially loaded, which triggers `console.log('observer triggered.');

Is there a reason for this callback to be triggered when the page loads? Or is there a mistake in how I'm implementing this?

Edit: Altering the code to the below still fires the callback at page load.

let target = document.querySelector('.lazy-load');

let options = {
    root: null,
    rootMargin: '0px',
    threshold: 0
}

let callback = function(entries, observer) {
    entries.forEach(entry => {

        console.log('observer triggered.');

    });
};

let observer = new IntersectionObserver(callback, options);

observer.observe(target);

Answer 1

That is the default behaviour. When you instantiate an instance of the IntersectionObserver, the callback will be fired.

It is recommended to guard against this case.

entries.forEach(entry => {
  if (entry.intersectionRatio > 0) {
    entry.target.classList.add('in-viewport');
  } else {
    entry.target.classList.remove('in-viewport');
  }
});

Also I found this article as well as the docs to be very helpful, specifically about the intersectionRatio or isIntersecting properties on the IntersectionObserverEntry.

· https://www.smashingmagazine.com/2018/01/deferring-lazy-loading-intersection-observer-api/

· https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserver

· https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserverEntry