GeezerVison

The core of the solution is a function that adds more dates to the page (or loads more stuff in real-life) while we’re at the page end.

We can call it immediately and add as a window.onscroll handler.

The most important question is: “How do we detect that the page is scrolled to bottom?”

Let’s use window-relative coordinates.

The document is represented (and contained) within <html> tag, that is document.documentElement.

We can get window-relative coordinates of the whole document as document.documentElement.getBoundingClientRect(), the bottom property will be window-relative coordinate of the document bottom.

For instance, if the height of the whole HTML document is 2000px, then:

// when we're on the top of the page
// window-relative top = 0
document.documentElement.getBoundingClientRect().top = 0

// window-relative bottom = 2000
// the document is long, so that is probably far beyond the window bottom
document.documentElement.getBoundingClientRect().bottom = 2000

If we scroll 500px below, then:

// document top is above the window 500px
document.documentElement.getBoundingClientRect().top = -500
// document bottom is 500px closer
document.documentElement.getBoundingClientRect().bottom = 1500

When we scroll till the end, assuming that the window height is 600px:

// document top is above the window 1400px
document.documentElement.getBoundingClientRect().top = -1400
// document bottom is below the window 600px
document.documentElement.getBoundingClientRect().bottom = 600

Please note that the bottom can’t be 0, because it never reaches the window top. The lowest limit of the bottom coordinate is the window height (we assumed it to be 600), we can’t scroll it any more up.

We can obtain the window height as document.documentElement.clientHeight.

For our task, we need to know when the document bottom is not no more than 100px away from it (that is: 600-700px, if the height is 600).

So here’s the function:

function populate() {
  while(true) {
    // document bottom
    let windowRelativeBottom = document.documentElement.getBoundingClientRect().bottom;

    // if the user hasn't scrolled far enough (>100px to the end)
    if (windowRelativeBottom > document.documentElement.clientHeight + 100) break;

    // let's add more data
    document.body.insertAdjacentHTML("beforeend", `<p>Date: ${new Date()}</p>`);
  }
}

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