predator

Finds the exposed box, if there is one, of a child DOM element.

Useful if you want to check if an element is visible to the user, and how much of it.

Usage

npm install predator

Then:

var predator = require('predator');

var box = predator(someElement);

->

{
    top: offset from top of screen
    left: offset from left of screen
    right: right edge offset from left of screen
    bottom: bottom edge offset from top of screen
    height: height of the exposed region
    width: width of the exposed region
    hidden: true if the element is completely obscured from view
    original: the original result of child.getBoundingClientRect()
}

Performance

predator should be fast enough to use in a render loop, even on mobile devices.