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jsp implements the back method in ie

高洛峰Original: 2016-12-29 16:15:201435browse

Usually we need to optimize the function. The general approach is to get the time at the beginning, and get the time again at the end. The time spent can be found by subtracting the two times. The function runs so fast, basically in milliseconds.

The functions given below are prepared for this purpose.

// 时间转为时间戳（毫秒）
function time2stamp(){
  var d = new Date();
  return Date.parse(d)+d.getMilliseconds();
}

Usage:

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");

New method:

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");

Test the efficiency of various loops, for...in such a slow loop will not be tested!

<!doctype html>
<html dir="ltr">
 <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>顺序</span></h2>
  <p id="time-1"></p>
  
  <h2>循环2：var n = divs.length; while (--n >= 0) { }<span>倒序</span></h2>
  <p id="time-2"></p>
  
  <h2>循环3：for (var i=0, n=divs.length; i<n; i++)<span>顺序</span></h2>
  <p id="time-3"></p>
  
  <h2>循环4：for (var n = divs.length - 1; n >= 0; n--) { }<span>倒序</span></h2>
  <p id="time-4"></p>
  
  <h2>循环5：for (var n = divs.length - 1; n--;) { }<span>倒序</span></h2>
  <p id="time-5"></p>
  
  <h2>循环6： var n = divs.length-1; do { } while (n--)<span>倒序</span></h2>
  <p id="time-6"></p>
 
  <h2>循环7： for(var i=0,el;el = divs[i++];){ } <span>顺序</span></h2>
  <p id="time-7"></p>
 </body>
</html>

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