Javascript inheritance

[NaReN]

I'm just reading up on OO Javascript but I just can't seem it to work

Here's what my code looks like :

function initXML() {
var myXML = new xmlDocument("<i tems><item>Appl e</item><item>oran ge</
item></items>");
myXML.getItems( );
}
function xmlDocument( aString ){
var parser = new DOMParser();
var xmlDoc = parser.parseFro mString(aString , "text/xml");
return xmlDoc ;
}

xmlDocument.pro totype.getItems (){
var items = xmlDoc.evaluate ("//item",xmlDoc, null,
XPathResult.ANY _TYPE,null);
var thisItem = items.iterateNe xt();
while (thisItem) {
thisItem = items.iterateNe xt();
}
}

However, when I try it out in FireFox 2 and call initXML I get :
Error: myXML.getItems is not a function

The above given way of inheritance works fine for user-defined objects
but gets stuck whenever the constructor returns a system-defined
object..

Any idea what I'm doing wrong ?

[RobG]

On Feb 21, 10:21 am, "NaReN" <kingc...@gmail .comwrote:

I'm just reading up on OO Javascript but I just can't seem it to work

Here's what my code looks like :

function initXML() {
var myXML = new xmlDocument("<i tems><item>Appl e</item><item>oran ge</
item></items>");
myXML.getItems( );
}
function xmlDocument( aString ){
var parser = new DOMParser();
var xmlDoc = parser.parseFro mString(aString , "text/xml");
return xmlDoc ;

}

xmlDocument.pro totype.getItems (){

That will (attempt to) call getItems(), I think you want to assign a
function reference to it:

xmlDocument.pro totype.getItems = function(){ ... }
You shouldn't assume that you can modify host objects or their
prototypes - test thoroughly.
--
Rob

[NaReN]

On Feb 20, 9:09 pm, "RobG" <[e-mail] .auwrote:

On Feb 21, 10:21 am, "NaReN" <kingc...@gmail .comwrote:

I'm just reading up on OO Javascript but I just can't seem it to work

Here's what my code looks like :

function initXML() {
var myXML = new xmlDocument("<i tems><item>Appl e</item><item>oran ge</
item></items>");
myXML.getItems( );
}
function xmlDocument( aString ){
var parser = new DOMParser();
var xmlDoc = parser.parseFro mString(aString , "text/xml");
return xmlDoc ;

}

xmlDocument.pro totype.getItems (){

That will (attempt to) call getItems(), I think you want to assign a
function reference to it:

xmlDocument.pro totype.getItems = function(){ ... }

You shouldn't assume that you can modify host objects or their
prototypes - test thoroughly.

--
Rob

Oops, I did use xmlDocument.pro totype.getItems = function(){ ... } ;
Must have accidentally deleted the 'function' keyword while copy
pasting, sorry !

Here's my code again :

function initXML() {
var myXML = new xmlDocument("<i tems><item>Appl e</
item><item>oran ge</
item></items>");
myXML.getItems( );
}
function xmlDocument( aString ){
var parser = new DOMParser();
var xmlDoc = parser.parseFro mString(aString , "text/xml");
return xmlDoc ;

}

xmlDocument.pro totype.getItems = function(){
var items = xmlDoc.evaluate ("//item",xmlDoc, null,
XPathResult.ANY _TYPE,null);
var thisItem = items.iterateNe xt();
while (thisItem) {
thisItem = items.iterateNe xt();
}
}
Thanks Rob ! Now, I wanted to extend default system objects , how
would I go about doing it ?

[RobG]

On Feb 21, 12:26 pm, "NaReN" <kingc...@gmail .comwrote:
[...]

Oops, I did use xmlDocument.pro totype.getItems = function(){ ... } ;
Must have accidentally deleted the 'function' keyword while copy
pasting, sorry !

Here's my code again :

function initXML() {
var myXML = new xmlDocument("<i tems><item>Appl e</
item><item>oran ge</
item></items>");

Manually wrap code at about 70 characters, auto-wrapping will nearly
always introduce errors. Use 2 spaces for indents too.

myXML.getItems( );
}
function xmlDocument( aString ){
var parser = new DOMParser();
var xmlDoc = parser.parseFro mString(aString , "text/xml");

When you call this function as a constructor, it will return the newly
created object. So that you can conveniently set properties of the
new object, it is set as the value of the function's this keyword.
You want to return an object that has an "xmlDoc" property, so:

this.xmlDoc = parser.parseFro mString(aString , "text/xml");

return xmlDoc ;

And since the new object is returned when called as a constructor,
there is no need for a return statement - just remove it.

>
}

xmlDocument.pro totype.getItems = function(){
var items = xmlDoc.evaluate ("//item",xmlDoc, null,

The variable 'xmlDoc' is a property of the object calling this
function along the prototype chain. To get that property, use the
this keyword:

var items = this.xmlDoc.eva luate("//item", this.xmlDoc, null,

XPathResult.ANY _TYPE,null);
var thisItem = items.iterateNe xt();
while (thisItem) {
thisItem = items.iterateNe xt();
}
}

Thanks Rob ! Now, I wanted to extend default system objects , how
would I go about doing it ?

You might be able to find documentation that tells you whether you can
or not, but you may find it difficult. If you can't find any, try it
and see. Then ask here (or wherever) to see if someone else knows.

Here is a working version:

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head><title>xm lDoc</title>
<style type="text/css"></style>
<script type="text/javascript">
//<![CDATA[
function initXML(s) {
var myXML = new xmlDocument(s);
myXML.getItems( );
}

function xmlDocument( aString ){
var parser = new DOMParser();
this.xmlDoc = parser.parseFro mString(aString , "text/xml");
}

xmlDocument.pro totype.getItems = function(){
var items = this.xmlDoc.eva luate("//item", this.xmlDoc, null,
XPathResult.ANY _TYPE, null);
var thisItem = items.iterateNe xt();
while (thisItem) {

// Debug...
alert(thisItem. textContent);

thisItem = items.iterateNe xt();
}
}

initXML('<items ><item>Apple</item><item>oran ge</item></items>');
//]]>
</script>
</head>
<body>
<div></div>
</body></html>
--
Rob

[NaReN]

Thanks a lot Rob, that helped a lot.