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