IntersactionObserver() only observes the first element of a row instead of all

Question

I'm experimenting with IntersectionObserver(), and it's behaving weirdly: I'm using a very simple layout, just 8 images in the same div with flexbox, using wrap, so basically the 8 images are positioning themself in different rows, according to the size of the viewport. I'm applying a filter class (which adds a blur filter) to each element first and then removing it when they are displayed on the screen:

HTML:

<div class="flex--cont">
            <div class="box box-2">
                <img src="img/1.jpg" alt="" class="img img-1">
            </div>
            <div class="box box-1">
                <img src="img/2.jpg" alt="" class="img img-2">
            </div>
            <div class="box box-3">
                <img src="img/3.jpg" alt="" class="img img-3">
            </div>
            <div class="box box-4">
                <img src="img/4.jpg" alt="" class="img img-4">
            </div>
            <div class="box box-5">
                <img src="img/5.jpg" alt="" class="img img-5">
            </div>
            <div class="box box-6">
                <img src="img/6.jpg" alt="" class="img img-6">
            </div>
            <div class="box box-7">
                <img src="img/7.jpg" alt="" class="img img-7">
            </div>
            <div class="box box-8">
                <img src="img/8.jpg" alt="" class="img img-8">
            </div>
        </div>

JAVASCRIPT

const allImage = Array.from(document.querySelectorAll(".img"));
allImage.forEach((img) => img.classList.add("filter"));

const removeFilter = function (entries, observer) {
  const [entry] = entries;
  const image = entry.target;
  image.classList.remove("filter");
};

const ImageObserver = new IntersectionObserver(removeFilter, {
  root: null,
  threshold: 0.15,
});

allImage.forEach((img) => ImageObserver.observe(img));

The thing is that the observer actually only observes the very first element of each row, so if I have 2 rows, it only gets the 1st and the 5th image, if I have 3 rows it gets the 1rst, the 4th and the 7th image and so on. I do have applied it to all of the images. Why is it doing that? Thanks for your answers!

Answer 1

Only the first one was changing because that was all you were targeting in the change color function by destructuring only the first array element:

const [entry] = entries;

However, the InteractionObserver callback is not called per entry but for all entries that are triggered simultaneously; hence the entries array contains all of the items being observed and you need to check the isIntersecting property like this:

const changeColor = function(entries) {
  entries.forEach(entry => {
    if(entry.isIntersecting) {
      entry.target.style.background = 'blue';
    } else {
      entry.target.style.background = 'red';
    }
  })
}

From the MDN docs

let callback = (entries, observer) => {
  entries.forEach(entry => {
    // Each entry describes an intersection change for one observed
    // target element:
    //   entry.boundingClientRect
    //   entry.intersectionRatio
    //   entry.intersectionRect
    //   entry.isIntersecting
    //   entry.rootBounds
    //   entry.target
    //   entry.time
  });
};