1、js 中for循環和indexOf()性能對比
在js中提供了indexOf()函數以獲取某個字符在字符串中的index,可以通過它也判斷某個字符或字符串是否存在。
但同時在js中for循環也可以實現同樣的效果(判斷字符是否存在)。那么到底用哪個才好呢?
於是寫了兩個函數來比較：
function getValue() {
var str = "abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij"
var specialWord = new Array("'", "\\", "<", ">", "%", "?", "/", "+", "@", "&", "#", "$", "……", "^", "~", "!", "‘", "’", "！", "￥"); //可以繼續添加特殊字符 此 / 字符也不可輸入 輸出時會破壞JSON格式
for (var specialWordItem = 0; specialWordItem < specialWord.length; specialWordItem++) {
ar specialIndex = str.indexOf(specialWord[specialWordItem]);
if (specialIndex != (-1)) {
return false;
}
}
return true;
}
function getReValue() {
var str = "abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij"
var specialWord = new Array("'", "\\", "<", ">", "%", "?", "/", "+", "@", "&", "#", "$", "……", "^", "~", "!", "‘", "’", "！", "￥"); //可以繼續添加特殊字符 此 / 字符也不可輸入 輸出時會破壞JSON格式
for (var specialWordItem = 0; specialWordItem < specialWord.length; specialWordItem++) {
for (var j = 0; j < str.length; j++) {
if (specialWord[specialWordItem] == str.charAt(j)) {
return false;
}
}
}
return true;
}
unction bothFunction() {
getValue();
getReValue();
}
str字符串是1000個字符。判斷str中是否存在specialWord中的特殊字符。性能測試結果如下圖所示：
現在可以很明顯地看到用indexOf()性能要好很多。
2、js中的for循環寫法的效率對比
一共三種寫法 如下：
for (var i = 0; i < arr.length; i++)
for (var i in arr)
for (var i = 0, len = arr.length; i < len; i++)
先看下運行測試的界面：
再看下對應的三個結果，三種寫法執行的效率是有很大不同的：
如此看來，執行效率最快的是第三種：
for (var i = 0, len = arr.length; i < len; i++)
先聲明變量，再進行循環判斷，效率遠比遍歷數組高得多，也比循環取得數組長度的效率快得多！
下面是頁面源代碼：
<html>
<body>
<script type="text/網頁特效">
function makeArr(num) {
var arr = [];
for (var i = 0; i<num; i++){
arr.push('abc');
}
arr.join('');
return arr;
}
var num = 10000000;
function function1() {
var arr = makeArr(num);
var start = (new Date()).valueOf();
var count = 0;
for (var i = 0; i < arr.length; i++) {
count++;
}
var end = (new Date()).valueOf();
console.log('Count: '+count+' times');
console.log('Time spent: '+(end - start));
}
function function2() {
var arr = makeArr(num);
var start = (new Date()).valueOf();
var count = 0;
for (var i in arr) {
count++;
}
var end = (new Date()).valueOf();
console.log('Count: '+count+' times');
console.log('Time spent: '+(end - start));
}
function function3() {
var arr = makeArr(num);
var start = (new Date()).valueOf();
var count = 0;
for (var i = 0, len = arr.length; i < len; i++) {
count++;
}
var end = (new Date()).valueOf();
console.log('Count: '+count+' times');
console.log('Time spent: '+(end - start));
}
</script>
for (var i = 0; i < arr.length; i++) <br />
<button onClick="function1();">loop 1</button><br />
<p id="f1"></p>
for (var i in arr)<br />
<button onClick="function2();">loop 2</button><br />
<p id="f2"></p>
for (var i = 0, len = arr.length; i < len; i++)<br />
<button onClick="function3();">loop 3</button><br />
<p id="f3"></p>
</body>
</html>