Why does this XUL code works and the Javascript equivalent doesn't?

xul way:

<toolbar id="PersonalToolbar">
  <toolbarbutton 
    id="Testing-Doit-Button2"
    class="bookmark-item pagerank"
    tooltiptext="Do it!"
    oncommand="testing_doit();" 
  />
</toolbar>

javascript way:

function createBookmarkItem() {
    const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
    var item = document.createElementNS(XUL_NS, "toolbarbutton");
    item.setAttribute("id", "Testing-Doit-Button2");
    item.setAttribute("class", "bookmark-item pagerank");
    item.setAttribute("tooltiptext", "Do it!");
    item.setAttribute("oncommand", "testing_doit();");
    return item;
}
function placeBookmarkItem() {
 var toolbar = document.getElementById("PersonalToolbar");
 var button = createBookmarkItem();
 toolbar.appendChild(button);
}
placeBookmarkItem();

The xul way shows a button. The Javascript way shows a button when I go to the Javascript Shell of the Extension Developer's Extension, then click enumerateWindows(), then click chrome://browser/content/browser.xul, then type the code, then hit enter. The Javascri开发者_如何学编程pt way doesn't show a button when I include button.js in button.xul. Why?

You need to place the placeBookmarkItem(); inside the documents onload event. The javascript gets executed before the document is in place. Otherwise place the <script/> element just before the </window> element in the end of the document.

A part of the code that I didn't quote here and that had an error made not work the part of the code that I quoted here and that didn't have an error. Take a look at "I add 10 functions to a code, I don’t even call any of them, but the code stops working!" for more information.