зеркало из https://github.com/mozilla/pjs.git
84 строки
3.4 KiB
HTML
84 строки
3.4 KiB
HTML
<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
|
|
<html>
|
|
<head>
|
|
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
|
|
<meta name="Author" content="Norris Boyd">
|
|
<meta name="GENERATOR" content="Mozilla/4.5 [en]C-NSCP (WinNT; I) [Netscape]">
|
|
<title>Performance Hints</title>
|
|
</head>
|
|
<body bgcolor="#FFFFFF">
|
|
|
|
<center>
|
|
<h1>
|
|
Performance Hints</h1></center>
|
|
<h3>
|
|
<tt>var</tt> Statements</h3>Use <tt>var</tt> statements when possible. Not only is it good
|
|
programming practice, it can speed up your code by allowing the compiler to
|
|
generate special code to access the variables. For example, you could rewrite
|
|
<p><tt>function sum(a) {</tt>
|
|
<br><tt> result = 0;</tt>
|
|
<br><tt> for (i=0; i <
|
|
a.length; i++)</tt>
|
|
<br><tt> result += a[i];</tt>
|
|
<br><tt> return result;</tt>
|
|
<br><tt>}</tt>
|
|
<p>as
|
|
<p><tt>function sum(a) {</tt>
|
|
<br><tt> var result = 0;</tt>
|
|
<br><tt> for (var i=0; i
|
|
< a.length; i++)</tt>
|
|
<br><tt> result += a[i];</tt>
|
|
<br><tt> return result;</tt>
|
|
<br><tt>}</tt>
|
|
<p>This is not equivalent code because the second version does
|
|
not modify global variables <tt>result</tt> and <tt>i</tt>. However, if you don't intend for any other function to
|
|
access these variables, then storing them globally is probably wrong anyway
|
|
(what if you called another function that had a loop like the one in <tt>sum</tt>!).
|
|
<br>
|
|
<h3>
|
|
Arrays</h3>Use the forms of the Array constructor that
|
|
specify a size or take a list of initial elements. For example, the code
|
|
<p><tt>var a = new Array();</tt>
|
|
<br><tt>for (var i=0; i < n; i++)</tt>
|
|
<br><tt> a[i] = i;</tt>
|
|
<p>could be sped up by changing the constructor call to <tt>new Array(n)</tt>. A constructor call like that indicates to
|
|
the runtime that a Java array should be used for the first <i>n</i> entries of the array. Similarly,
|
|
<tt>new
|
|
Array("a", "b", "c")</tt> or <tt>["a", "b", "c"]</tt> will cause a 3-element
|
|
Java array to be allocated to hold the contents of the JavaScript array.
|
|
<br>
|
|
<br>
|
|
<h3>
|
|
<tt>eval</tt> and <tt>new Function</tt></h3>Avoid calling <tt>eval</tt> when
|
|
possible. Calls to <tt>eval</tt> are slow because the script
|
|
being executed must be compiled. Constructing a new function object can be slow
|
|
for the same reason, while function expressions are more efficient because the
|
|
function can be compiled. For example, the code
|
|
<p><tt>function MyObject(a) {</tt>
|
|
<br><tt> this.s = a;</tt>
|
|
<br><tt> this.toString = new
|
|
Function("return this.s");</tt>
|
|
<br><tt>}</tt>
|
|
<p>could be written more efficiently as
|
|
<p><tt>function MyObject(a) {</tt>
|
|
<br><tt> this.s = a;</tt>
|
|
<br><tt> this.toString =
|
|
function () { return this.s }</tt>
|
|
<br><tt>}</tt>
|
|
<p>Beginning with Rhino 1.4 Release 2, code
|
|
passed to eval and new Function will be interpreted rather than compiled to
|
|
class files.
|
|
<br> </p>
|
|
<h3>
|
|
with</h3>Using the <tt>with</tt>
|
|
statement prevents the compiler from generating code for fast access to local
|
|
variables. You're probably better off explicitly accessing any properties of the
|
|
object.
|
|
<br>
|
|
<p>
|
|
<hr WIDTH="100%">
|
|
<br><a href="index.html">back to top</a>
|
|
<br>
|
|
</body>
|
|
</html>
|