jQuery Mobile, AJAX, and the DOM

I’ve been messing around with jQuery and jQuery Mobile lately, and ran into an issue that’s worth exploring. It might be a newbie issue–I’m sure seasoned JQM developers don’t fall into this trap much–but I figure that I might be able to save new developers some frustration.

Let me first describe the app I was working on. The app basically provides online utilities that I and other engineers might find useful from time to time in day to day work. The utilities are grouped into different categories (for example, Date Conversion tools, Data Encryption tools, etc). While I started out developing these tools for desktop browsers, I figured I also ought to port them to a mobile web app as well. While the desktop tools are all displayed on one single page, however, the mobile app would present each category of tools (e.g. Date Conversion) on its own page. The “homepage” would present a simple listview, allowing the user to select a category of tools to use.

The homepage is contained in its own HTML page (named, unsurprisingly, index.html). Each tool category also has its own page; for example, the Date Conversion tools are located on dc.html. It was with this page that I ran into problems. The primary date conversion tool is one that takes milliseconds and converts them to a formatted date, and vice-versa. Given this, I’d decided to allow users to choose the date format they want to work with. To implement this, I used a multi-page template in dc.html  The first page div consisted of the tool itself; the second div consisted of a group of radio buttons from which the user could select a date format.

Stripped of irrelevant code, dc.html looks like this:

<!DOCTYPE html>

function addDfChoices() {
        // This method populates the radio buttons. Its existence becomes important later
$(document).ready(function() {


<!– Start of main page –>
<div data-role=”page” id=”main”>

  <div data-role=”content” id=”main”>
    <span class=”section_content”>
      <label style=”text-align:baseline”>Millis since 1970</label>
      <a href=”#page_dateformat” style=”width:100px;float:right”>Format</a>
  </div><!– /main content –>
</div><!– /main page –>

<div data-role=”page” id=”page_dateformat”>
  <div data-role=”content”>
    <div data-role=”fieldcontain” id=”df_option_container”>
      <fieldset data-role=”controlgroup” id=”df_options”>
  </div><!– /dateformat content –>
</div><!– /dateformat page –>


I loaded dc.html into my browser (I did a lot of testing in Chrome, but also of course on mobile browsers such as mobile Safari on my iPhone). The page worked as expected. On page load, the main page div was displayed, while a small Javascript instantiated different date formats and inserted them into the df_options fieldset located within the hidden page_dateformat page div. Tapping the Format link invoked a slide animation which loaded the radio buttons. Additional Javascript code (not shown above) registered any radio button selections and accordingly modified the date format used in the main page.
Except… sometimes things didn’t work. Specifically, sometimes tapping on the Format link took me to the homepage instead.
Playing around a bit, I learned that if I manually loaded dc.html into my browser, things worked fine. If I started on index.html and then linked to dc.html, and then tapped the Format link, that’s when things went wrong.
This is the relevant code within index.html:
<ul data-role=”listview” data-inset=”true” data-theme=”d”>
  <li><a href=”./dc.html”>Date Conversion</a></li>
  <li><a href=”./de.html”>Data Encoding</a></li>
Nothing too special there.
Well, I decided to use Chrome’s DOM inspector to get a look at what was happening. I loaded dc.html and looked in the DOM inspector, and indeed I saw the two major page divs, main and page_dateformat. Then I loaded index.html and tapped on the Date Conversion link, and then took a look again at the DOM inspector after the date conversion tools page loaded. There was the main page div… but where was the page_dateformat div?
Here’s what jQuery was doing. When dc.html itself was loaded, the entire contents of the HTML file were loaded into the DOM. Since I was using a multi-page template, any div declaring data-role=”page” besides the first one (in my case, the page_dateformat div) was hidden… but it was still there in the DOM. The DOM looked roughly like this:
    div data-role=”page” id=”main”/
    div data-role=”page” id=”page_dateformat” display=”hidden”/
Now, when I started by loading index.html and then tapping the Date Conversion link, dc.html was retrieved but only the first data-role=”page” div was loaded into the DOM. Here, roughly, was the DOM at this point:

    div data-role=”page”/  

            ^– this was the content div for index.html; it’s hidden at this point

    div data-role=”page” id=”main”/  
            ^– this is the first data-role=”page” div in dc.html, loaded via AJAX then inserted into the DOM
Thus, when I tapped on the Format link (which, if you recall is marked up as <a href=”#page_dateformat”>Format</a>), the browser searched the DOM for a block with an ID of page_dateformat, couldn’t find one, and effectively gave up and reloaded the current page… which at this point was still index.html
At this point I reviewed the jQuery Mobile docs. The following little blurb provides a bit of a clue:

It’s important to note if you are linking from a mobile page that was loaded via Ajax to a page that contains multiple internal pages, you need to add a rel="external" or data-ajax="false" to the link. This tells the framework to do a full page reload to clear out the Ajax hash in the URL. This is critical because Ajax pages use the hash (#) to track the Ajax history, while multiple internal pages use the hash to indicate internal pages so there will be conflicts in the hash between these two modes.

I added data-ajax=”false” to the links on index.html  This solved the problem; dc.html was subsequently loaded completely into the browser’s DOM, allowing the page_dateformat div to be found. However, declaring data-ajax=”false” causes links to lose their animation effects. The pages are simply loaded the old-fashioned way.
I wasn’t too keen about that, so I removed the data-ajax=”false” declarations. I then changed the approach I took to the date format selection. Instead of a new “page” with radio buttons, I simply presented the user with a popup select list. For various reasons, I kept the dfChoices() Javascript function depicted above; the method was now invoked on page load to populate the select with date format options.
And immediately, I encountered the same success rate: when I started on dc.html, things worked great. When I started on index.html and linked to dc.html, the dfChoices() function was never invoked. And of course, the reason was the same as well. Much as the page_dateformat div hadn’t been loaded into the DOM, the dfChoices() function similarly was nowhere to be found.
At this point, I added data-ajax=”false”  back into the homepage’s links; for that matter, I also went back to using the multi-page template with radio buttons. There are certainly other solutions (separating out the radio buttons into their own html page and using some form of local storage to share state? putting Javascript functions into data-role=”page” divs instead of in the page header?) worth exploring, which I’ll do at some point. But just understanding the general issue is a good starting point.