getElementsByTagName (“div”).length returns zero for any webpage

Question

I am trying to develop a Firefox extension. The following code:

var divList = document.getElementsByTagName("div") ;
Components.utils.reportError("num of divs = " + divList.length) ;

Always says:- num of divs = 0.

I don't understand what is the mistake that I am making in this simple piece of code. When I replace "div" with "*", I always get a value around 1100 on any webpage.

Answer 1

Not sure what is the difference in the extension context, but it might have to do with namespaces - https://developer.mozilla.org/en-US/docs/Web/API/Element/getElementsByTagName. What is the tag of the first of 1100 elements you get when using "*"?

Answer 2

May be the code is run when the DOM is not ready.

Try to put the JS code inside a script tag at the very end of the body tag.
I'm not sure why * gets so many items though.