对于已知元素，document.getElementById返回null - 为什么？

Question

I'm using the Google Chrome JavaScript console and I was just looking at the Gmail page and just practicing manipulating the DOM. However, when I do the following it just comes back as null:

document.getElementById('gbx3');

There is a div element in the page that has an id of 'gbx3' - so why is it returning null? What would/could be causing this? The same thing occurs using the Firefox web console.

If you try and access the 'gb' id (this is the main top toolbar) in the same Gmail page it comes back null, but if you access this element at google.com it will come back with the element.

Answer 1

GMail is composed of frames. This one works:

frames[3].document.getElementById('gbx3');

In general, if you know the ID of the iframe element, the contentDocument property can be used (provided that you don't have same-origin problems, and the document is loaded):

document.getElementById('hist_frame').contentDocument.getElementById('gbx3');

Answer 2

As per my comment:

My best bet is that gbx is inside the iframe that contained the search result, and document.getElementById works on the same scope as your console

You can search all the frames to find what you need:

for(var i = 0; i < frames.length; i++) {
   var curDoc = frames[i].document;
   if(curDoc.getElementById('gbx')) console.log(curDoc.getElementById('gbx'));
}