Old fashioned method of checking if an element is within the viewport.

inViewport.js

export default function inViewport(elem, options = {}) {
	const props = {
		offsetBefore: typeof options.offset === 'object' ? -Math.abs(options.offset.before) :  -Math.abs(options.offset) || 0,
		offsetAfter: typeof options.offset === 'object' ? options.offset.after : 0,
		threshold: options.threshold || 0
	}
    const bounding = elem.getBoundingClientRect();
    const bottom = () => {
    	if(!props.threshold) {
    		return (bounding.bottom + props.offsetBefore) - elem.offsetHeight;
    	}
    	else {
    		return ((bounding.bottom + props.offsetBefore) - elem.offsetHeight) + (elem.offsetHeight * (props.threshold / 100))
    	}
    };
    const top = () => {
    	let unit = options.threshold === 0 ? -Math.abs(elem.offsetHeight) : -Math.abs(elem.offsetHeight * (options.threshold / 100) + elem.offsetHeight);
    	return (bounding.top + props.offsetAfter) - elem.offsetHeight >= unit
    };

    return (
        top() &&
        bounding.left >= 0 &&
        bottom() <= (window.innerHeight || document.documentElement.clientHeight) &&
        bounding.right <= (window.innerWidth || document.documentElement.clientWidth)
    );
};

Nice one!