Check if an element is visible in the viewport using JavaScript

It's fairly common to need to check if an element is visible in the viewport. Surprisingly, there's no built-in method to do this, at least not in a straightforward way. Luckily, JavaScript provides all the necessary tools to roll up our own solution.
The first step requires us to get the coordinates of the element. We can do this, using the Element.getBoundingClientRect() method and object destructuring. This method returns an object with the coordinates of the element's top, right, bottom, and left edges, as well as the width and height of the element.
We also need to know the dimensions of the viewport. We can get these, using the Window.innerWidth and Window.innerHeight properties. Then, we can use these values to determine if the element is visible in the viewport.
If we want to check if the element is entirely visible, we can simply compare the element's coordinates with the viewport's dimensions. If we want to check if the element is partially visible, we need to compare the element's coordinates with the viewport's dimensions and the element's dimensions.
Wrapping everything up, we can use an optional argument to determine if we want to check if the element is partially visible or not.

const elementIsVisibleInViewport = (el, partiallyVisible = false) => {
  const { top, left, bottom, right } = el.getBoundingClientRect();
  const { innerHeight, innerWidth } = window;
  return partiallyVisible
    ? ((top > 0 && top < innerHeight) ||
        (bottom > 0 && bottom < innerHeight)) &&
        ((left > 0 && left < innerWidth) || (right > 0 && right < innerWidth))
    : top >= 0 && left >= 0 && bottom <= innerHeight && right <= innerWidth;
};

// e.g. 100x100 viewport and a 10x10px element at position
// {top: -1, left: 0, bottom: 9, right: 10}
elementIsVisibleInViewport(el); // false - (not fully visible)
elementIsVisibleInViewport(el, true); // true - (partially visible)

We also need to know the dimensions of the viewport. We can get these, using the Window.innerWidth and Window.innerHeight properties. Then, we can use these values to determine if the element is visible in the viewport.
If we want to check if the element is entirely visible, we can simply compare the element's coordinates with the viewport's dimensions. If we want to check if the element is partially visible, we need to compare the element's coordinates with the viewport's dimensions and the element's dimensions.
Wrapping everything up, we can use an optional argument to determine if we want to check if the element is partially visible or not.

From 30-seconds-of-code: element-is-visible-in-viewport