通常我们需要对函数进行优化，一般的做法是开始的时候获得时间，结束的时候再获得一次时间，两次时间相减就能到到花费的时间。而函数运行速度之快，基本上都是毫秒级的。
下面给出的函数就是就此准备的。
// 时间转为时间戳（毫秒）function time2stamp(){ var d = new Date(); return Date.parse(d)+d.getMilliseconds();}
用法：
var t1 = time2stamp();// 比较各游览器的DOM运行速度。var divs = document.getElementByTagName("div"); var t2 = time2stamp();alert("耗时：" + (t2 - t1) + " 毫秒");
新的方法:
var time1 = new Date// 比较各游览器的DOM运行速度。var divs = document.getElementByTagName("div"); alert("耗时：" + (new Date - time1) + " 毫秒");
测试各种循环的效率，for...in这样慢的循环就不测了！
<!doctype html><html dir="ltr" lang="zh-CN"> <head> <meta charset="utf-8"/> <title>类</title> <script type="text/javascript" charset="utf-8"> window.onload = function(){ for (var i=0, elm; i<4000; i++) { elm = document.createElement("div"); elm.innerHTML = "第一重div<div>第二重div<div>第三重div</div></div>"; document.body.appendChild(elm); } document.getElementById("start-test").onclick = startTest; var uuid = 0; function startTest() { var divs = document.getElementsByTagName("div"); var time1 = new Date(); var i = -1, n = divs.length;while (++i < n) { divs[i].uuid = uuid++; } document.getElementById("time-1").innerHTML = new Date() - time1; var time2 = new Date(); var i = divs.length; while (--i >= 0) { divs[i].uuid = uuid++; } document.getElementById("time-2").innerHTML = new Date() - time2; var time3 = new Date(); for (var i=0, n=divs.length; i<n; i++) { divs[i].uuid = uuid++; } document.getElementById("time-3").innerHTML = new Date() - time3; var time4 = new Date(); for (var n = divs.length - 1; n >= 0; n--) { divs[n].uuid = uuid++; } document.getElementById("time-4").innerHTML = new Date() - time4; var time5 = new Date(); for (var n = divs.length - 1; n--;) { divs[n].uuid = uuid++; } document.getElementById("time-5").innerHTML = new Date() - time5; var time6 = new Date(); var n = divs.length-1; do { divs[n].uuid = uuid++; } while (n--); document.getElementById("time-6").innerHTML = new Date() - time6; var time7 = new Date(); for(var i=0,el;el = divs[i++];){ el.uuid = uuid++; }; document.getElementById("time-7").innerHTML = new Date() - time7; } } </script> </head> <body> <style> h2 { margin:0; padding:10px; font-weight:500; font-size:13px; background:#8080C0; color:#fff; } p { margin:0; text-indent:2em; border-bottom:2px solid #a9ea00; background:#F2F1D7; } span.red{ color:red; } </style> <h1>JavaScript loop speed test</h1> <ul> <li>遍历页面上12000个DIV</li> </ul> <input id="start-test" type="button" value="Start test"> <h2>循环1： var i = -1, n = divs.length;while (++i < n) { }<span class="red">顺序</span></h2> <p id="time-1"></p> <h2>循环2：var n = divs.length; while (--n >= 0) { }<span class="red">倒序</span></h2> <p id="time-2"></p> <h2>循环3：for (var i=0, n=divs.length; i<n; i++)<span class="red">顺序</span></h2> <p id="time-3"></p> <h2>循环4：for (var n = divs.length - 1; n >= 0; n--) { }<span class="red">倒序</span></h2> <p id="time-4"></p> <h2>循环5：for (var n = divs.length - 1; n--;) { }<span class="red">倒序</span></h2> <p id="time-5"></p> <h2>循环6： var n = divs.length-1; do { } while (n--)<span class="red">倒序</span></h2> <p id="time-6"></p> <h2>循环7： for(var i=0,el;el = divs[i++];){ } <span class="red">顺序</span></h2> <p id="time-7"></p> </body></html>
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