See the pink bar scrolling across the top as you scroll down this page?

Building a reading progress bar
For this post, I will do what I did with Typing animation with JavaScript: start from zero and experiment my way through the problem, to show it’s not that hard…

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This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar

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This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar

This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar

This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar

This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar

This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar

This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar This is a really long post to demonstrate a reading progress bar