Get element offset within viewport

getViewportOffset.js

/**
 * Get element offset
 * @param  {Element}  obj
 * @return {Boolean}
 */
export function getElementViewportOffset (element) {
  let left = element.offsetLeft,
    top = element.offsetTop,
    elementParent = element.parentNode

  do {
    let styles = getComputedStyle(elementParent)

    if (styles) {
      let position = styles.getPropertyValue('position')

      left -= elementParent.scrollLeft
      top -= elementParent.scrollTop

      if (/relative|absolute|fixed/.test(position)) {
        left += parseInt(styles.getPropertyValue('border-left-width'), 10)
        top += parseInt(styles.getPropertyValue('border-top-width'), 10)

        left += elementParent.offsetLeft
        top += elementParent.offsetTop
      }

      elementParent = position === 'fixed' ? null : elementParent.parentNode
    } else {
      elementParent = elementParent.parentNode
    }

  } while (elementParent && isElement(elementParent))

  return {
    left: left,
    top: top
  }
}



/**
 * Check if variable is element
 * @param  {Anything}
 * @return {Boolean}
 */
export function isElement (obj) {
  try {
    return obj instanceof HTMLElement
  }
  catch(e) {
    return
      (typeof obj === 'object') &&
      (obj.nodeType === 1) &&
      (typeof obj.style === 'object') &&
      (typeof obj.ownerDocument ==='object')
  }
}