How can I get the browser scrollbar height in javascript

There isn't a built-in method for this. However, the scrollbar's height is supposed to give an indication of how much of the available content fits within the available viewport. Going by that, we can determine it like:

var sbHeight = window.innerHeight * (window.innerHeight / document.body.offsetHeight);

Where window.innerHeight / document.body.offsetHeight is the percentage of content visible currently. We multiple that with the available viewport height to get the approximate scrollbar height.

Note: Different browsers may add/reduce some pixels to/from the scrolbar height.