Element.clientWidth 属性表示元素的内部宽度，以像素计。该属性包括内边距，但不包括垂直滚动条（如果有）、边框和外边距。
获取画布宽度的方式如下
canvas.width = window.innerWidth || document.documentElement.clientWidth || document.body.clientWidt
获取窗口的高度与宽度(不包含工具条与滚动条):
var w=window.innerWidth;
var h=window.innerHeight;
clientWidth 是对象可见的宽度，不包滚动条等边线，会随窗口的显示大小改变。 offsetWidth 是对象的可见宽度，包滚动条等边线，会随窗口的显示大小改变
1 <html>
2 <head>
3 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
4 <title>test</title>
5 <style type="text/css">
6 #All{ width: 100%; height: 3000px;}
7 </style>
8 </head>
9 <body>
10 <div id="All">
11 <div id="img" style="position: absolute; z-index:99;">
12 <a href="#" target="_blank"><img src="Images/OPVTI.jpg" border="0" style="cursor: pointer;" style="cursor: pointer;" /></a>
13 <script language="JavaScript">
14 <!--Begin
15 var xPos = document.body.clientWidth - 20;
16 var yPos = document.body.clientHeight / 2;
17 var step = 1;
18 var delay = 10;
19 var height = 0;
20 var Hoffset = 0;
21 var Woffset = 0;
22 var yon = 0;
23 var xon = 0;
24 var pause = true;
25 var interval;
26 img.style.top = yPos;
27 function changePos() {
28 width = document.body.clientWidth;
29 height = document.body.clientHeight;
30 Hoffset = img.offsetHeight;
31 Woffset = img.offsetWidth;
32 img.style.left = xPos + document.body.scrollLeft;
33 img.style.top = yPos + document.body.scrollTop;
34 if (yon) {
35 yPos = yPos + step;
36 } else {
37 yPos = yPos - step;
38 }
39 if (yPos < 0) {
40 yon = 1;
41 yPos = 0;
42 }
43 if (yPos >= (height - Hoffset)) {
44 yon = 0;
45 yPos = (height - Hoffset);
46 }
47 if (xon) {
48 xPos = xPos + step;
49 } else {
50 xPos = xPos - step;
51 }
52 if (xPos < 0) {
53 xon = 1;
54 xPos = 0;
55 }
56 if (xPos >= (width - Woffset)) {
57 xon = 0;
58 xPos = (width - Woffset);
59 }
60 }
61 function start() {
62 img.visibility = "visible";
63 interval = setInterval('changePos()', delay);
64 }
65 function pause_resume() {
66 if (pause) {
67 clearInterval(interval);
68 pause = false;
69 } else {
70 interval = setInterval('changePos()', delay);
71 pause = true;
72 }
73 }
74 start();
75 // End -->
76 </script>
77 </div>
78 </div>
79 </body>
80 </html>