获取用户视口大小(以 ems 为单位）

编辑
以下代码获取em的大小，但与我的css不对应。

function doOnOrientationChange() {
        // Orientation of device has changed, change the size of map and move the maphelp (if required)
        var eminpx = $("#1em").width();
        var largescreeninpx = eminpx*66.236;
        switch(window.orientation) {
            case -90:
            case 90:
                // Orientation is landscape
                console.log('66.236ems in px: ' + largescreeninpx + '. Screen width: ' + screen.height);
                $("#map").height(screen.width*0.7); // The screen width is the viewport height of the device because we're in landscape orientation
                // Will only work if we can actually get the em value of the width
                if (screen.height < largescreeninpx) {
                     // Small or medium screen, show map help below map
                     console.log('small screen');
                    $("#maphelp").css('margin-top', 0);
                } else {
                    // Larger screen, show map and map help side-by-side
                    $("#maphelp").css('margin-top', -screen.width*0.7);
                }
                break; 
            default:
                // Orientation is portrait
                alert('66.23ems in px: ' + largescreeninpx + '. Screen width: ' + screen.width);
                $("#map").height(screen.height*0.7);
                // Will only work if we can actually get the em value of the width
                if (screen.width < largescreeninpx) {
                     // Small or medium screen, show map help below map
                    $("#maphelp").css('margin-top', 0);
                } else {
                    // Larger screen, show map and map help side-by-side
                    $("#maphelp").css('margin-top', -screen.height*0.7);
                }
                break;
        }
    }

尽管在CSS中横向iPad不会触发@media screen and (min-width: 30em) and (max-width: 63.236em) {...}（这意味着它的宽度超过63.236ems），但在JS中它会触发console.log('small screen')，显示通过JS获取时屏幕宽度小于66.236ems？

原始问题（几乎回答）我正在尝试围绕"金发姑娘方法"创建一个网站。在电子表格中一切正常，但我正在尝试动态设置嵌入式地图的高度，使其始终是用户当前视口高度的 90%。但是，地图旁边也可能有一些"地图帮助"，允许浏览器视口足够大（在本例中为 66.23ems。纵向iPad超过66.23ems）。下面是我的代码，它被注释以显示不起作用的内容。

function doOnOrientationChange() {
        // Orientation of device has changed, change the size of map and move the maphelp (if required)
        switch(window.orientation) {
            case -90:
            case 90:
                // Orientation is landscape
                $("#map").height(screen.width*0.9); // The screen width is the viewport height of the device because we're in landscape orientation
                if ($(window).width() > 66.23) { // Need this in ems
                     // Small or medium screen, show map help below map
                    $("#maphelp").css('margin-top', 0);
                } else {
                    // Larger screen, show map and map help side-by-side
                    $("#maphelp").css('margin-top', -screen.width*0.9);
                }
                break; 
            default:
                // Orientation is portrait
                if ($(window).width() < 66.23) { // Need this in ems
                     // Small or medium screen, show map help below map
                    $("#maphelp").css('margin-top', 0);
                } else {
                    // Larger screen, show map and map help side-by-side
                    $("#maphelp").css('margin-top', -screen.height*0.9);
                }
                break;
        }
    }

所以，我的问题是，我如何通过检查 ems 来让它触发正确的响应？