getElementCoordinates.js

/**
 * @param {HTMLElement} element - The element's coordinates to calulate
 * @param {HTMLElement} relativeElement - An optional relative element, defaults to `document.body`
 * @returns {{ offsetY: Number, offsetX: Number }}
 */
const getElementCoordinates = function(element, relativeElement = document.body) {
    const relativeRect = relativeElement.getBoundingClientRect();
    const elementRect  = element.getBoundingClientRect();
    const offsetY      = elementRect.top - relativeRect.top;
    const offsetX      = elementRect.left - relativeRect.left;
    return { offsetY, offsetX };
}