In fact, looking at the source code of jQuery, I found that the code of each is very simple, but why is the performance different from the native for loop by dozens of times?
jQuery's core code of each
for (; i < length; i++) {
value = callback.call(obj[i], i, obj[i]);
if (value === false) {
break;
}
}Looks very simple, but why is it so much slower?
Write the test code as follows:
var length=300000;
function GetArr() {
var t = [];
for (var i = 0; i < length; i++) {
t[i] = i;
}
return t;
}
function each1(obj, callback) {
var i = 0;
var length = obj.length
for (; i < length; i++) {
value = callback(i, obj[i]);
/* if ( value === false ) {去掉了判断
break;
}*/
}
}
function each2(obj, callback) {
var i = 0;
var length = obj.length
for (; i < length; i++) {
value = callback(i, obj[i]);/*去掉了call*/
if (value === false) {
break;
}
}
}
function each3(obj, callback) {
var i = 0;
var length = obj.length
for (; i < length; i++) {
value = callback.call(obj[i], i, obj[i]);/*自己写的call*/
if (value === false) {
break;
}
}
}
function Test1() {
var t = GetArr();
var date1 = new Date().getTime();
var lengtharr = t.length;
var total = 0;
each1(t, function (i, n) {
total += n;
});
var date12 = new Date().getTime();
console.log("1Test" + ((date12 - date1)));
}
function Test2() {
var t = GetArr();
var date1 = new Date().getTime();
var total = 0;
each2(t, function (i, n) {
total += n;
});
var date12 = new Date().getTime();
console.log("2Test" + ((date12 - date1)));
}
function Test3() {
var t = GetArr();
var date1 = new Date().getTime();
var total = 0;
each3(t, function (i, n) {
total += n;
});
var date12 = new Date().getTime();
console.log("3Test" + ((date12 - date1)));
}
function Test4() {
var t = GetArr();
var date1 = new Date().getTime();
var total = 0;
$.each(t, function (i, n) {
total += n;
});
var date12 = new Date().getTime();
console.log("4Test" + ((date12 - date1)));
}Run the test and find that the difference between the first and second is not very big, which shows that the performance difference caused by the break judgment is very small, but the first The deviation between the second and third, and the fourth is more than double, and the only difference between the second and the third is that call is called. It seems that call will cause performance loss, because call will switch context. Of course, jQuery There are other reasons why each is slow. It also calls other methods in the loop, and call is just one reason.
Therefore, it can be said that call and apply are relatively performance-consuming methods in js. When performance requirements are strict, it is recommended to use them sparingly.
Let’s look at the performance comparison between jquery’s each and js native for loop through a piece of code
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>for与each性能比较</title>
<script src="../Cks/jquery-1.7.1.min.js" type="text/javascript"></script>
<script type="text/javascript" language="javascript">
function getSelectLength() {
var time1 = new Date().getTime();
var len = $("#select_test").find("option").length;
var selectObj = $("#select_test");
for (var i = 0; i < len; i++) {
if (selectObj.get(0).options[i].text == "111111") {
selectObj.get(0).options[i].selected = true;
break;
}
}
var time2 = new Date().getTime();
alert("for循环执行时间：" + (time2 - time1));
time1 = new Date().getTime();
$("#select_test").find("option").each(function () {
if ($(this).text() == "111111") {
$(this)[0].selected = true;
}
});
time2 = new Date().getTime();
alert("each循环执行时间：" + (time2 - time1));
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div><select id="select_test">
<option value='1'>111111</option>
<option value='2'>222222</option>
<option value='3'>333333</option>
<option value='4'>444444</option>
<option value='5'>5</option>
<option value='6'>6</option>
<option value='7'>7</option>
<option value='8'>8</option>
<option value='9'>9</option>
<option value='10'>10</option>
<option value='11'>11</option>
<option value='12'>12</option>
<option value='13'>13</option>
<option value='14'>14</option>
<option value='15'>15</option>
<option value='16'>16</option>
<option value='17'>17</option>
<option value='18'>18</option>
<option value='19'>19</option>
<option value='20'>20</option>
</select><input type="button" value="开始比较" onclick="getSelectLength();" /></div>
<div>
</form>
</body>
</html>Input discrepancy:
for loop execution time: 1
each loop execution time: 3
The two results directly illustrate the problem.
The above is the reason why the jQuery Each is much slower than the JS native for loop performance introduced by the editor. I hope it will be helpful to you. If you have any questions, please leave me a message and the editor will Reply to everyone promptly. I would also like to thank you all for your support of the PHP Chinese website!
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