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BOOK: Professional JavaScript for Web Developers 2nd edition ISBN: 9780470227800
This is the forum to discuss the Wrox book Professional JavaScript for Web Developers, 2nd Edition by Nicholas C Zakas; ISBN: 9780470227800
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Old September 14th, 2010, 12:50 PM
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Default Chapter 15 Page 524 Question

Page 524 has the following code:

function createDocument(){
if (typeof arguments.callee.activeXString != “string”){
var versions = [“MSXML2.DOMDocument.6.0”, “MSXML2.DOMDocument.3.0”,
for (var i=0,len=versions.length; i < len; i++){
try {
var xmldom = new ActiveXObject(versions[i]);
arguments.callee.activeXString = versions[i];
return xmldom;
} catch (ex){
return new ActiveXObject(arguments.callee.activeXString);


Is activeXString a custom property on the function?
If so, as it is defined within the function, is it not destroyed once the function exits? And if that is the case then shouldn't the for loop run each time the function is called; this would make the last line of code superfluous would it not?

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Old September 14th, 2010, 02:49 PM
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Yes, activeXString is a custom property on the function. Remember, in JavaScript functions are also objects. The function remains in memory until the page is unloaded, so you can add properties to it and retrieve their value later. So calling this function the first time causes the loop to be used in order to determine the correct ActiveX object to use. Once determined, this is cached in the function's activeXString property so all subsequent calls to the function simply return the already-identified correct string.
Nicholas C. Zakas
Author, Professional JavaScript for Web Developers (ISBN 0764579088)

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