[翻译]High Performance JavaScript(026)
Use the Fast Parts 使用速度快的部分
Even though JavaScript is often blamed for being slow, there are parts of the language that are incredibly fast. This should come as no surprise, since JavaScript engines are built in lower-level languages and are therefore compiled. Though it's easy to blame the engine when JavaScript appears slow, the engine is typically the fastest part of the process; it's your code that is actually running slowly. There are parts of the engine that are much faster than others because they allow you to bypass the slow parts.
虽然JavaScript经常被指责缓慢，然而此语言的某些部分具有难以置信的快速。这不足为奇因为JavaScript引擎由低级语言构建。虽然JavaScript速度慢很容易被归咎于引擎，然而引擎通常是处理过程中最快的部分，实际上速度慢的是你的代码。引擎的某些部分比其它部分快很多，因为它们允许你绕过速度慢的部分。
Bitwise Operators 位操作运算符
Bitwise operators are one of the most frequently misunderstood aspects of JavaScript. General opinion is that developers don't understand how to use these operators and frequently mistake them for their Boolean equivalents. As a result, bitwise operators are used infrequently in JavaScript development, despite their advantages.
位操作运算符是JavaScript中经常被误解的内容之一。一般的看法是，开发者不知道如何使用这些操作符，经常在布尔表达式中误用。结果导致JavaScript开发中不常用位操作运算符，尽管它们具有优势。
JavaScript numbers are all stored in IEEE-754 64-bit format. For bitwise operations, though, the number is converted into a signed 32-bit representation. Each operator then works directly on this 32-bit representation to achieve a result. Despite the conversion, this process is incredibly fast when compared to other mathematical and Boolean operations in JavaScript.
JavaScript中的数字按照IEEE-754标准64位格式存储。在位运算中，数字被转换为有符号32位格式。每种操作均直接操作在这个32位数上�
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隐藏成员变量
在函数体内定义的变量为局部变量，离开函数就挂掉了
在函数体内使用this.成员变量名，则为window对象级变量，即全局变量
故需要这样隐藏成员变量，向外只暴露get、set函数
function testClass(name){
var _firstname=name;
return {
getname : function() {
return _fir ......
function total(){
var i=0;
for(j=1;j<=20;j++)
{
var step="step"+j;
if(document.getElementById(step)){
if(document.getElementById(step).checked==true)
{
i=i+parseInt(document.getElementById(step).value);
}
}
}
document.getElementById("total").innerHTML = i;
}
function Resetvalue(){
......
Regular Expression Optimization 正则表达式优化
Incautiously crafted regexes can be a major performance bottleneck (the upcoming section, "Runaway Backtracking" on page 91, contains several examples showing how severe this can be), but there is a lot you can do to improve re ......
Splitting Up Tasks 分解任务
What we typically think of as one task can often be broken down into a series of subtasks. If a single function is taking too long to execute, check to see whether it can be broken down into a series of smaller functions that complete in smaller ......
Data Formats 数据格式
When considering data transmission techniques, you must take into account several factors: feature set, compatibility, performance, and direction (to or from the server). When considering data formats, the only scale you need for comparison is speed.
......
