Section 18.3. DOM Inspection

18.3. DOM Inspection

DOM, Dynamic, Explore, Inspect, Interrogate, Investigate, Presentation, Render, Snapshot, State, View, Visualise, Visualize

Figure 18-6. DOM Inspection

18.3.1. Developer Story

Devi keeps clicking on the Clear button, but the document text remains stubbornly unaffected. She fires up a DOM Inspection tool and discovers that the button's onclick event handler isn't present, which explains why nothing was happening.

18.3.2. Problem

How can you inspect the DOM's state at any time, taking into account DOM manipulations that may have occurred since the page was loaded?

18.3.3. Solution

Use a DOM Inspection Tool to explore the dynamic DOM state. The initial HTML for an Ajax App is often minimal and, in any event, likely to change over time due to DOM Manipulation. A DOM Inspection tool traverses the browser's current DOM model, rather than the initial HTML source, to build up a presentation of the current DOM. Many times, the tool is interactive, allowing you to drill down the hierarchy, search for keywords, and so on. All of this is very useful for checking assumptions and diagnosing problems, since many Ajax bugs arise because the programmer misunderstood the DOM state at a particular time.

Incidentally, DOM Inspection is useful beyond the fact that the DOM is dynamic. Even for static applications, an interactive exploration tool is often a good complement to the mound of text that makes up a complex HTML document.

18.3.4. Tool Support Firefox DOM Inspector

Firefox ships with a DOM Inspector tool, accessible from the tools menu (Figure 18-7).[*] The inspector is a popup window showing the DOM hierarchy on one side and facing a summary of the selected node. The summary shows node attributes such as registered event handlers, as well as CSS style information.

[*] In Windows, you need to ensure that the Web Developer Tools option ( is selected during installation.

Figure 18-7. Firefox DOM Inspector Firefox Web Developer Extension

Chris Pederick's Web Developer extension ( is an invaluable Firefox toolbar for web development (Figure 18-8). Among its vast feature set are several commands especially relevant to DOM Inspection:

Figure 18-8. Web Developer toolbar


These commands augment the page with DOM information, which makes a great alternative to exploring the DOM in a completely separate view. For example, Display Topographic Information alters the page to reveal the layout of DOM elements and Display ID and Class Details augments each element with its id and class attributes.


There commandsfor example, Outline Block Level Elements or Outline Table Cellsdraw outlines around elements of a particular type. In fact, you can define custom outlines to outline any element you want.


These commands help expose form structure. Display Form Details augments the form with information about each control, while View Form Information opens up a summary page. View Rendered Source

Jennifer Madden's View Rendered Source ( is a Firefox extension that shows the entire DOM as an HTML document (Figure 18-9). Note that this is not the same thing as the good old View Source feature, because the rendered source reflects the DOM after any manipulation has taken place. The rendered source is shown in a pretty-formatted style, with colors and spacing to help convey the DOM structure. This is a free tool, and you can also purchase a slightly enhanced version for a small fee.

Figure 18-9. View Rendered Source IE Developer Toolbar

Microsoft's free IE Developer Toolbar for IE6+ ( has a number of DOM Inspection capabilities and is similar to the Firefox Web Developer Extension (Figure 18-10). It also includes a DOM Explorer similar to Firefox's DOM inspector (Figure 18-11).

Figure 18-10. IE Developer Toolbar

Figure 18-11. IE DOM Explorer

You can find a list of IE DOM Inspectors at IEDocMon

Tim Tabor's IEDocMon ( is an open source tool that extends IE to include an interactive view of the DOM. Mouseover DOM Inspector

Steven Chipman's Mouseover DOM Inspector ( is a convenient bookmarklet that lets you explore the DOM by mousing over page elements. It works on all recent browsers except Safari.

Ajax Design Patterns
Ajax Design Patterns
ISBN: 0596101805
EAN: 2147483647
Year: 2007
Pages: 169

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