How to use the Element.scrollHeight property

Element.scrollHeight

The Javascript element.scrollHeight property along with the scrollTop property are very useful when you want to control user input to a webpage where thw screen is scrolled.

I have used the scrollTop property for the setting of the banner at the top of some of my webpages. In general the behaviour of the page when it is scrolled was one of the motivators for this page. A search for functions that can be used for this purpose led me to the example on the MDN website.

The example on this page, taken from a demo on MDN,shows the use of the Element.scrollHeight property to enable a check that a block of text has been scrolled to the end.

The example has a textarea element containig text that could be part of an agreement or licence terms. There is also a checkbox element and button control that are diabled until the text in the box is scrolled to the end.

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The MDN Explaination of Element.scrollHeight:

The Element.scrollHeight read-only property is a measurement of the height of an element's content, including content not visible on the screen due to overflow.

The scrollHeight value is equal to the minimum height the element would require in order to fit all the content in the viewport without using a vertical scrollbar. The height is measured in the same way as clientHeight: it includes the element's padding, but not its border, margin or horizontal scrollbar (if present). It can also include the height of pseudo-elements such as ::before or ::after. If the element's content can fit without a need for vertical scrollbar, its scrollHeight is equal to clientHeight

The MDN Example - The license agreement

This is an example of a license agreement that you have to scoll to the end so that you can then agree to the conditions. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vestibulum at laoreet magna. Aliquam erat volutpat. Praesent molestie, dolor ut eleifend aliquam, mi ligula ultrices sapien, quis cursus neque dui nec risus. Duis tincidunt lobortis purus eu aliquet. Quisque in dignissim magna. Aenean ac lorem at velit ultrices consequat. Nulla luctus nisi ut libero cursus ultrices. Pellentesque nec dignissim enim. Phasellus ut quam lacus, sed ultricies diam. Vestibulum convallis rutrum dolor, sit amet egestas velit scelerisque id. Proin non dignissim nisl. Sed mi odio, ullamcorper eget mattis id, malesuada vitae libero. Integer dolor lorem, mattis sed dapibus a, faucibus id metus. Duis iaculis dictum pulvinar. In nisi nibh, dapibus ac blandit at, porta at arcu. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Praesent dictum ipsum aliquet erat eleifend sit amet sollicitudin felis tempus. Aliquam congue cursus venenatis. Maecenas luctus pellentesque placerat. Mauris nisl odio, condimentum sed fringilla a, consectetur id ligula. Praesent sem sem, aliquet non faucibus vitae, iaculis nec elit. Nullam volutpat, lectus et blandit bibendum, nulla lorem congue turpis, ac pretium tortor sem ut nibh. Donec vel mi in ligula hendrerit sagittis. Donec faucibus viverra fermentum. Fusce in arcu arcu. Nullam at dignissim massa. Cras nibh est, pretium sit amet faucibus eget, sollicitudin in ligula. Vivamus vitae urna mauris, eget euismod nunc. Aenean semper gravida enim non feugiat. In hac habitasse platea dictumst. Cras eleifend nisl volutpat ante condimentum convallis. Donec varius dolor malesuada erat consequat congue. Donec eu lacus ut sapien venenatis tincidunt. Quisque sit amet tellus et enim bibendum varius et a orci. Donec aliquet volutpat scelerisque. Proin et tortor dolor. Ut aliquet, dolor a mattis sodales, odio diam pulvinar sem, egestas pretium magna eros vitae felis. Nam vitae magna lectus, et ornare elit. Morbi feugiat, ipsum ac mattis congue, quam neque mollis tortor, nec mollis nisl dolor a tortor. Maecenas varius est sit amet elit interdum quis placerat metus posuere. Duis malesuada justo a diam vestibulum vel aliquam nisi ornare. Integer laoreet nisi a odio ornare non congue turpis eleifend. Cum sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Cras vulputate libero sed arcu iaculis nec lobortis orci fermentum. Please enter the following to confirm that you have read this text.

I agree

The code

The "notice" box and the "rules" are styled as below:

<style>
#notice {
  display: inline-block;
  margin-bottom: 12px;
  border-radius: 5px;
  width: 600px;
  padding: 5px;
  border: 2px #7FDF55 solid;
}

#rules {
  width: 600px;
  height: 130px;
  padding: 5px;
  border: #2A9F00 solid 2px;
  border-radius: 5px;
}
</style>

The following function is called when the page is loaded:

script>
function checkReading () {
  if (checkReading.read) {
    return; 
  }
  checkReading.read = this.scrollHeight - this.scrollTop === this.clientHeight;
  document.registration.accept.disabled = document.getElementById("nextstep").disabled = !checkReading.read;
  checkReading.noticeBox.innerHTML = checkReading.read ? "Thank you." : "Please, scroll and read the following text.";
}

onload = function () {
  var oToBeRead = document.getElementById("rules");
  checkReading.noticeBox = document.createElement("span");
  document.registration.accept.checked = false;
  checkReading.noticeBox.id = "notice";
  oToBeRead.parentNode.insertBefore(checkReading.noticeBox, oToBeRead);
  oToBeRead.parentNode.insertBefore(document.createElement("br"), oToBeRead);
  oToBeRead.onscroll = checkReading;
  checkReading.call(oToBeRead);
}
/script>

Enhancements

To confirm that a user has actually read the agreement, as opposed to just scrolling to the end, text is added that has to be entered for process to continue.

scrollTop

The operation of the example described on this page and the "agreement" page

The scrollTop property of the Element interface gets or sets the number of pixels by which an element's content is scrolled from its top edge. This value is subpixel precise in modern browsers, meaning that it isn't necessarily a whole number.

The example on the MDN page is worth reproducing here.

Element.scrollHeight

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