Determine the length of this Node-set?

Question

Javascript for Mozilla:

  while (result)
    {
    alert(result.childNodes[0].nodeValue);
    //txt=txt+result.childNodes[0].nodeValue+"<br/>";
    result=nodes.iterateNext();
    }

This is intended to return a series of strings from an xml file. When I use the alert line, it alerts as expected with the proper strings in a series. The "txt" variable on the commented line is supposed to go to an innerHTML further down in the function. But Firebug keeps telling me that the result.childNodes[0] is undefined. Why would it be defined in the alert but not the next line?

I hope that's enough code to determine the problem...if not I will post more.

Thanks for help

[edit]

this is the definition of result:

  var x=xmlDoc.responseXML;
  var nodes=x.evaluate(path, x, null, XPathResult.ANY_TYPE, null);
  var result=nodes.iterateNext();

I am retrieving XML

[edit]

okay I put the iterator in a for loop like this:

  for (var i=0; i<2; i++)
    {
    var res=(xmlNodes.childNodes[0].nodeValue);
    txt=txt+res+"<br/>";
    xmlNodes=xmlQuery.iterateNext();
    }

because I had a theory that once the iteration was null the loop would fail. So that's what happened--it worked until I set the loop one instance higher than the amount of nodes available. now I am trying to figure out how to get the "length" of the node-set. How do I do that?

Answer 1

I don't see any problems in the code you have shown so far. If childNodes[0] is undefined, then it has to be a text node or an empty node, and you should see an exception when trying to access a property such as nodeValue of childNodes[0] which is undefined. The exception will show up on alert or concatenation, or any other type of access.

This is the answer to your updated question.

now I am trying to figure out how to get the "length" of the node-set. How do I do that?

You can have the following types of node-sets returned from the evaluate function:

• Iterators
• Snapshots
• First Nodes

I'll skip "first nodes" as that doesn't apply in this situation.

Iterators

With iterators, you only get an iterateNext() method for traversing nodes sequentially. Iterators refer to live nodes, meaning if the nodes in the document were to change while you are traversing the resultset, the iterator will become invalid.

Here's an example with using an iterator to go over each resulting node:

var results = doc.evaluate("expr", doc, null, ORDERED_SNAPSHOT, null);
var node;

while(node = results.iterateNext()) {
    console.log(node);
}

If you want to use iterators, and find the number of matching results, a wrapper function that iterates through all nodes and returns them in an array might be useful.

function evaluateXPath(document, expression) {
    var ANY_TYPE = XPathResult.ANY_TYPE;
    var nodes = document.evaluate(expression, document, null, ANY_TYPE, null);
    var results = [], node;

    while(node = nodes.iterateNext()) {
        results.push(node);
    }

    return results;
}

Get nodes as an array and loop the array:

var results = evaluateXPath(doc, "expr");

for(var i = 0; i < results.length; i++) {
    console.log(results[i]);
}

Snapshots

Snapshots provide a static result of the nodes at the time of querying. Changes to the document will not affect this snapshot. Useful interfaces here will be the snapshotLength property, and snapshotItem(index) function.

Here's an example using a snapshot result:

var ORDERED_SNAPSHOT = XPathResult.ORDERED_NODE_SNAPSHOT_TYPE;
var results = doc.evaluate("expr", doc, null, ORDERED_SNAPSHOT, null);

for(var i = 0; i < results.snapshotLength; i++) {
    var node = results.snapshotItem(i);
    console.log(node);
}

See a working example.

It seems you are developing this for Firefox. Did you consider using E4X for this purpose? It provides a really easy interface for dealing with XML documents - for creating, manipulating, and querying.

Answer 2

now I am trying to figure out how to get the "length" of the node-set. How do I do that?

In XPath this is achieved using the standard XPath count() function.

count(someExpression)

evaluates to the number of nodes selected by someExpression.