问需要在javascript中计算offsetRight

EN

var offsetright = window.innerWidth - obj.offsetLeft - obj.offsetWidth

...

// Set helper defaults
dropdownElem.style.left = 'auto';
dropdownElem.style.zIndex = '10';

// Get the elem and its offsetParent
let elem = dropdownElemContainer;
let elemOffsetParent = elem.offsetParent;

// Cache widths
let elemWidth = elem.offsetWidth;
let elemOffsetParentWidth = 0;

// Set the initial offsets
let top = elem.offsetHeight; // Because I want to visually append the elem at the bottom of the referenced bottom
let right = 0;

// Loop up the DOM getting the offsetParent elements so you don't have to traverse the entire ancestor tree
while (elemOffsetParent) {
    top += elem.offsetTop;
    elemOffsetParentWidth = elemOffsetParent.offsetWidth;
    right += elemOffsetParentWidth - (elem.offsetLeft + elemWidth); // Most important line like your own example
    // Move up the DOM
    elem = elemOffsetParent;
    elemOffsetParent = elemOffsetParent.offsetParent;
    elemWidth = elemOffsetParentWidth;
}

// Set the position and show the elem
dropdownElem.style.top = top + 'px';
dropdownElem.style.right = right + 'px';
dropdownElem.style.display = 'block';

//Object references
function getObject(id) {
   var object = null;
   if (document.layers) {
    object = document.layers[id];
   } else if (document.all) {
    object = document.all[id];
   } else if (document.getElementById) {
    object = document.getElementById(id);
   }
   return object;
}
//Get pixel dimensions of screen
function getDimensions(){
    var winW = 630, winH = 460;
    if (document.body && document.body.offsetWidth) {
     winW = document.body.offsetWidth;
     winH = document.body.offsetHeight;
    }
    if (document.compatMode=='CSS1Compat' && document.documentElement && document.documentElement.offsetWidth ) {
     winW = document.documentElement.offsetWidth;
     winH = document.documentElement.offsetHeight;
    }
    if (window.innerWidth && window.innerHeight) {
     winW = window.innerWidth;
     winH = window.innerHeight;
    }
    return{"width":winW, "height":winH}
}
//Get the location of element
function getOffsetRight(elem){
    element=getObject(elem)
    var width = element.offsetWidth
    var right = 0;
    while (element.offsetParent) { 
        right += element.offsetLeft; 
        element = element.offsetParent;
    }
    right += element.offsetLeft;
    right = getDimensions()["width"]-right
    right -= width
    return right
}