Creating a scroll position indicator in React

Question

I'm trying to code a scroll indicator progress bar in React. I have it working with Jquery but would like to know how to do it with pure Javascript.

  componentDidMount() {
    window.addEventListener('scroll', this.handleScroll);
  }

  handleScroll() {
    var winHeight = $(window).height(),
      docHeight = $(document).height(),
      value = $(window).scrollTop(),
      max, percent;

    max = docHeight - winHeight;
    percent = (value / max) * 100;
    this.props.updatePercent(percent);
  }

Also, should I bother doing this in pure Javascript? I've been told that Jquery should not be used used in React.

Solution 1

Is this the only place you used JQuery? If so, I'd recommend ditching it for pure javascript. Everything you can do with JQuery you can also do with React and pure JavaScript, and it's not worth the overhead here.

Here's a pure JavaScript version of your handleScroll function. Note that document height is notoriously annoying to compute, but I've taken the approach of this question (which just reproduces JQuery's implementation).

handleScroll() {
   var winHeight = window.innerHeight;

   // Annoying to compute doc height due to browser inconsistency
   var body = document.body;
   var html = document.documentElement;
   var docHeight = Math.max( body.scrollHeight, body.offsetHeight, 
                   html.clientHeight, html.scrollHeight, html.offsetHeight );

   var value = document.body.scrollTop;
   ...
}

Update

If you want to get the scroll position within an element, you'll need something like

var el = document.getElementById('story_body');
var minPixel = el.offsetTop;
var maxPixel = minPixel + el.scrollHeight;
var value = document.body.scrollTop;

// respect bounds of element
var percent = (value - minPixel)/(maxPixel - minPixel);
percent = Math.min(1,Math.max(percent, 0))*100;

Solution 2

To answer your second question: In this particular case, you could just stick to jQuery (although I prefer the vanilla javascript version).

With react, it is perfectly OK to use jQuery for:

• reading info from the real DOM, which are unknown to react (such as component height in the DOM, or scroll position in your case)
• ajax stuff

With React, you should NOT use jQuery for:

• Manipulating the DOM directly: only manipulate the DOM through react. (manipulating DOM with jQuery in react is a guarantee for big trouble)
• Reading DOM info that can and should be known to react, such as value of an input field. (things do not really break, but it makes your react code harder to debug if you use jQuery to circumvent react's strict design guidelines)

Comments

[domi91c]

Thanks, that works. I'm also wondering if instead of using the body's height I could use an element with an ID #article. Can you tell me how I might do that? I tried replacing var body = document.body with var body = document.getElementById('story_body'), but the scroll indicator doesn't appear when I scroll to the article.

[James Evans]

I'll edit my answer to show how that's done. The logic is a little different if you want to get the scroll position within an element rather than the whole document.