window.onload = function() {
    var sl = new SocialHistory();
    alert(sl.doesVisit("www.jb51.net"));
}
如果用户曾经使用过www.jb51.net，那么该函数就会返回真，否则返回假。

其实原理并不复杂，它利用了链接的 a:visited 伪类的属性。首先在页面上生成一个iframe，并在这个iframe中设置 a 和 a:visited 为不同的样式。然后将网站的链接插入到 iframe 中。浏览器就会根据用户的访问历史，为访问过的链接设置 a:visited 的样式。最后再获得链接的最终样式，如果是 a:visited，就可以认为用户访问过该网站了。具体的实现方式可以参考源代码。

这个脚本主要用于显示社会性书签的图标，可以恰到好处地显示用户所使用的网站。但我担心，这样的做法是不是有盗取用户隐私之嫌？虽然这个方法只能判断用户有无访问特定的网站，并不能无限制地得到所有访问历史。



/*
* Social Limit - Only the social you care about.
*
* Enables your site to know which social bookmarking badges to display to your
* visitors. It tells you all social sites the user has gone to, or you can
* query for a specific one.
*
* For example:
*
* var sl = SocialHistory();
* alert( sl.doesVisit("Digg") ); // Returns true/false, -1 if unknown.
* var listOfVisitedSites = sl.visitedSites();
* var checkedSites = sl.checkedSites();
*
* If you want to add more sites to check, you can pass that in as a dictionary
* to History:
*
* var more = { "Humanized": "http://humanized.com",
* "Azarask.in": ["http://azarask.in", "http://azarask.in/blog"]
* };
* var sl = SocialHistory(more);
* alert( sl.doesVisit("Humanized") );
*
* For a list of built-in sites, see the sites variable below.
*
* Copyright (c) 2008 Aza Raskin (http://azarask.in/blog)
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
*
*/

var SocialHistory = function( moreSites ){

var sites = {
"Digg": ["http://digg.com", "http://digg.com/login"],
"Reddit": ["http://reddit.com", "http://reddit.com/new/", "http://reddit.com/controversial/", "http://reddit.com/top/", "http://reddit.com/r/reddit.com/", "http://reddit.com/r/programming/"],
"StumbleUpon": ["http://stumbleupon.com"],
"Yahoo Buzz": ["http://buzz.yahoo.com"],
"Facebook": ["http://facebook.com/home.php", "http://facebook.com", "https://login.facebook.com/login.php"],
"Del.icio.us": ["https://secure.del.icio.us/login", "http://del.icio.us/"],
"MySpace": ["http://www.myspace.com/"],
"Technorati": ["http://www.technorati.com"],
"Newsvine": ["https://www.newsvine.com", "https://www.newsvine.com/_tools/user/login"],
"Songza": ["http://songza.com"],
"Slashdot": ["http://slashdot.org/"],
"Ma.gnolia": ["http://ma.gnolia.com/"],
"Blinklist": ["http://www.blinklist.com"],
"Furl": ["http://furl.net", "http://furl.net/members/login"],
"Mister Wong": ["http://www.mister-wong.com"],
"Current": ["http://current.com", "http://current.com/login.html"],
"Menaeme": ["http://meneame.net", "http://meneame.net/login.php"],
"Oknotizie": ["http://oknotizie.alice.it", "http://oknotizie.alice.it/login.html.php"],
"Diigo": ["http://www.diigo.com/", "https://secure.diigo.com/sign-in"],
"Funp": ["http://funp.com", "http://funp.com/account/loginpage.php"],
"Blogmarks": ["http://blogmarks.net"],
"Yahoo Bookmarks": ["http://bookmarks.yahoo.com"],
"Xanga": ["http://xanga.com"],
"Blogger": ["http://blogger.com"],
"Last.fm": ["http://www.last.fm/", "https://www.last.fm/login/"],
"N4G": ["http://www.n4g.com"],
"Faves": ["http://faves.com", "http://faves.com/home", "https://secure.faves.com/signIn"],
"Simpy": ["http://www.simpy.com", "http://www.simpy.com/login"],
"Yigg": ["http://www.yigg.de"],
"Kirtsy": ["http://www.kirtsy.com", "http://www.kirtsy.com/login.php"],
"Fark": ["http://www.fark.com", "http://cgi.fark.com/cgi/fark/users.pl?self=1"],
"Mixx": ["https://www.mixx.com/login/dual", "http://www.mixx.com"],
"Google Bookmarks": ["http://www.google.com/bookmarks", "http://www.google.com/ig/add?moduleurl=bookmarks.xml&hl=en"],
"Subbmitt": ["http://subbmitt.com/"]
};

for( var site in moreSites ) {
// If we don't have the site, create the URL list.
if( typeof( sites[site] ) == "undefined" ) sites[site] = [];

// If the value is string, just push that onto the URL list.
if( typeof( moreSites[site] ) == "string" )
sites[site].push( moreSites[site] );
else
sites[site] = sites[site].concat( moreSites[site] );
}

var visited = {};

function getStyle(el, scopeDoc,styleProp) {
if (el.currentStyle)
var y = el.currentStyle[styleProp];
else if (window.getComputedStyle)
var y = scopeDoc.defaultView.getComputedStyle(el,null).getPropertyValue(styleProp);
return y;
}

function remove( el ) {
el.parentNode.removeChild( el );
}

// Code inspired by:
// bindzus.wordpress.com/2007/12/24/adding-dynamic-contents-to-iframes
function createIframe() {
var iframe = document.createElement("iframe");
iframe.style.position = "absolute";
iframe.style.visibility = "hidden";

document.body.appendChild(iframe);

// Firefox, Opera
if(iframe.contentDocument) iframe.doc = iframe.contentDocument;
// Internet Explorer
else if(iframe.contentWindow) iframe.doc = iframe.contentWindow.document;

// Magic: Force creation of the body (which is null by default in IE).
// Also force the styles of visited/not-visted links.
iframe.doc.open();
iframe.doc.write('<style>');
iframe.doc.write("a{color: #000000; display:none;}");
iframe.doc.write("a:visited {color: #FF0000; display:inline;}");
iframe.doc.write('</style>');
iframe.doc.close();

// Return the iframe: iframe.doc contains the iframe.
return iframe;
}

var iframe = createIframe();

function embedLinkInIframe( href, text ) {
var a = iframe.doc.createElement("a");
a.href = href;
a.innerHTML = site;
iframe.doc.body.appendChild( a );
}

for( var site in sites ) {
var urls = sites[site];
for( var i=0; i<urls.length; i++ ) {
// You have to create elements in the scope of the iframe for IE.
embedLinkInIframe( urls[i], site );

// Automatically try variations of the URLS with and without the "www"
if( urls[i].match(/www\./) ){
var sansWWW = urls[i].replace(/www\./, "");
embedLinkInIframe( sansWWW, site );
} else {
// 2 = 1 for length of string + 1 for slice offset
var httpLen = urls[i].indexOf("//") + 2;
var withWWW = urls[i].substring(0, httpLen ) + "www." + urls[i].substring( httpLen );
embedLinkInIframe( withWWW, site );
}

}
}

var links = iframe.doc.body.childNodes;
for( var i=0; i<links.length; i++) {
// Handle both Firefox/Safari, and IE (respectively)
var displayValue = getStyle(links[i], iframe.doc, "display");
var didVisit = displayValue != "none";

if( didVisit ){ visited[ links[i].innerHTML ] = true; } } remove( iframe );
return new (function(){ var usedSites = []; for( var site in visited ){ usedSites.push( site ); }
// Return an array of visited sites.
this.visitedSites = function() {
return usedSites;
}

// Return true/false. If we didn't check the site, return -1.
this.doesVisit = function( site ) {
if( typeof( sites[site] ) == "undefined" )
return -1;
return typeof( visited[site] ) != "undefined";
}

var checkedSites = [];
for( var site in sites ){
checkedSites.push( site );
}
// Return a list of the sites checked.
this.checkedSites = function(){
return checkedSites;
}
})();
}