Calculating an element node offset.

calcOffset.ts

function calcOffset(elm: HTMLElement) {
    const offset = {
        top: 0,
        left: 0,
    }

    do {
        offset.top += elm.offsetTop || 0;
        offset.left += elm.offsetLeft || 0;
    } while((elm = elm.offsetParent as HTMLElement));

    return offset;
}