get element position height relate viewport

getRelativePosition.js

  function getElementViewTop(element) {
    if (document.documentElement.getBoundingClientRect !== 'undefined') {
      return element.getBoundingClientRect().top
    } else {
      var actualTop = element.offsetTop
      var current = element.offsetParent
      while (current !== null) {
        actualTop += current.offsetTop
        current = current.offsetParent
      }
      var elementScrollTop = document.body.scrollTop
      return actualTop - elementScrollTop
    }
  }