首先先看實現代碼
html代碼部分
<!DOCTYPE html><html><head><title>Previewing Links</title><link rel="stylesheet"href="script05.css" rel="external nofollow" ><script src="script05.js"></script></head><body><h2>A Gentle Introduction to JavaScript</h2><ul><li><a href="jsintro/2000-08.html" rel="external nofollow" >August column</a></li><li><a href="jsintro/2000-09.html" rel="external nofollow" >September column</a></li><li><a href="jsintro/2000-10.html" rel="external nofollow" >October column</a></li><li><a href="jsintro/2000-11.html" rel="external nofollow" >November column</a></li></ul><div id="previewWin"> </div></body></html>
這個CSS設置預覽彈出窗口的樣式
#previewWin {background-color: #FF9;width: 400px;height: 100px;font: .8em arial, helvetica, sans-serif;padding: 5px;position: absolute;visibility: hidden;top: 10px;left: 10px;border: 1px #CC0 solid;clip: auto;overflow: hidden;}#previewWin h1, #previewWin h2 {font-size: 1.0em;}這個JavaScript進行服務器請求并且顯示彈出窗口
window.onload = initAll;var xhr = false;var xPos, yPos;function initAll() {var allLinks = document.getElementsByTagName("a");for (var i=0; i< allLinks.length; i++) {allLinks[i].onmouseover = getPreview;}}function getPreview(evt) {if (evt) {var url = evt.target;}else {evt = window.event;var url = evt.srcElement;}xPos = parseInt(evt.clientX);yPos = parseInt(evt.clientY);if (window.XMLHttpRequest) {xhr = new XMLHttpRequest();}else {if (window.ActiveXObject) {try {xhr = new ActiveXObject("Microsoft.XMLHTTP");}catch (e) { }}}if (xhr) {xhr.onreadystatechange = showContents;xhr.open("GET", url, true);xhr.send(null);}else {alert("Sorry, but I couldn't create an XMLHttpRequest");}}function hidePreview() {document.getElementById("previewWin").style.visibility = "hidden";}function showContents() {var prevWin = document.getElementById("previewWin");if (xhr.readyState == 4) {if (xhr.status == 200) {prevWin.innerHTML = xhr.responseText;}else {prevWin.innerHTML = "There was a problem with the request " + xhr.status;}prevWin.style.top = yPos+2 + "px";prevWin.style.left = xPos+2 + "px";prevWin.style.visibility = "visible";prevWin.onmouseout = hidePreview;}}分析：
1.
var allLinks = document.getElementsByTagName("a");for (var i=0; i< allLinks.length;i++) {allLinks[i].onmouseover = getPreview;}這是initAll()函數的內容，它遍歷頁面上的所有鏈接，并且在每個鏈接上添加onmouseover事件
處理程序。這個事件處理程序將（你下面將看到的）讀取目標頁面并且向（可能的）訪問者顯示預覽。
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