Entry

How can I find the number of layers in a document using the DOM. NS6 doesn't support document.layers.length

Jan 23rd, 2001 08:43
Ian Grant, X X,

Since the new DOM doesn't support the document.all object nor the 
document.layers object, you are given the document.getElementsByTagName
() and document.getElementById() object methods instead.  Netscape 4's 
layers object gives you exactly how many layers there are through 
document.layers.length, whether you use the <LAYER> tag, the <SPAN> 
tag, or the <DIV> tag, or all three.  Unfortunately, in your case 
neither Internet Explorer nor Netscape 6 support the layers object.
Let's examine what is happening in each browser when you use their 
typical methods of accessing their respective DOMs:
    document.layers.myDiv
    document.layers['myDiv']
    document.layers[0]
    document.layers0
Might be synonymous in Navigator 4.  What you are accessing is an 
object array of layers elements, which is why you get the number of 
layers with document.layers.length.
    document.all.myDiv
    document.all['myDiv']
    document.all[14]
    myDiv
Might also be synonymous in Explorer 4+.  Once again you are accessing 
an array of elements, but not necessarily of <DIV>s and <SPAN>s.  
Rather, you are accessing every page element through the object 
array 'document.all'.
    document.getElementById('myDiv')
    document.getElementsByTagName('DIV')[0]
Might be synonymous in Netscape 6.  As in the previous examples, you 
are accessing an array, however getElementById() retrieves the page 
element for you by its ID, and getElementsByTagName() produces an array 
of whatever tag name you give it.  (By the way, these two methods will 
also work in Explorer 5.)
To determine how many layers (or in this case <DIV>s or <SPAN>s) there 
are, simply assign document.getElementsByTagName to an object and 
extract its length or simply do it all in one line of code:
    var obj = document.getElementsByTagName('div');
    alert(obj.length);
    // or in one line
    alert(document.getElementsByTagName('div').length);
Keep in mind that if you use both <DIV>s and <SPAN>s in your code, 
you'll have to check for both separately.
I hope this helps you.
Ian Grant
idgrant@pandi.20m.com