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Evaluating Your Needs
It is difficult to formulate a template to use for object detection. Each page's compatibility goals and object detection needs are somewhat different.
Perhaps the most important step in
implementing object detection is identifying your needs. For
example, if your scripts interact only with dynamic style attributes
of elements, the primary object detection task is identifying the
applicable syntax for referencing elements: using
If you don't want to repeat so much code in your functions, you can alternatively establish global variable flags for element reference categories at the top of your scripts:
Then use these global variables in your conditional expressions for execution branching, as follows:
Mold your decision branches so that the most likely true condition tests first. For example, with so many users now visiting with at least basic W3C DOM browsers, test for W3C DOM support first. Yes, it's true that IE5 supports both element reference styles, but the W3C DOM version will include NN6 users and future standards-compatible browsers, as well.
For functions that act as event handlers, multiple object detection routines are needed to handle both the diversity of event models and element reference syntaxes, as shown in the following example borrowed from the article Supporting Three Event Models at Once:
The more you work with the object models,
the more you discover ways to use clues about one feature as an
indicator of support for other features. For example, every browser
that supports the
As a result, some of your branching may need
to be many levels deep. Fear not. Except in huge scripts that must
process tons of data in numerous iterative loops, the performance
impact of nested
As you work on the code, you need to visualize execution paths that encounter your detection branches. Be sure to visualize not only how execution flows when browsers support your cool features, but also what happens when detection finds no support for your features. Will your scripts degrade gracefully in older browsers?
While the chief benefits of using object detection are freedom from browser version concerns and a hedge against the unknown (alternative browsers and future browser versions), you are not liberated from the job of testing your code on as many browsers as you can. Just because a browser indicates that it supports a particular object or property doesn't mean that the support is identical across all supporting browsers.
As intimated in the article Scripting for IE 5 Macintosh Edition, implementations of even the most modern browser standards can present differences in rendered results. Or, as occurred frequently in Netscape Navigator 6.0, object model features were present but inactive, until version 6.1. Test, test, test!
When Browser Sniffing is OK
You may occasionally encounter problems whose
solution requires the old-fashioned browser version sniffing. In
general these problems are limited to known bugs or "alternative
behavior" of an object in a browser version. For example, if
MegaBrowser 3.05 for Windows always reports that an otherwise
well-supported object property has a value of zero no matter what
the rendering shows and generates an error if you try to set the
property value, you'll have to code a detour for that browser
version. You'll be back to the old
Some page designers also prefer to use OS-specific style sheets or content, rather than trust one size to fit all. In such cases, it's perfectly normal and appropriate to branch according to OS version. The following script statements would be in the HEAD portion of a document to link a Mac-specific style sheet definition file for Macintosh clients (and hope for the best on Windows, Unix, and other OSes):
Worth the Effort?
Until you get comfortable with the planning and thought processes needed to implement object detection, you may wonder if so much extra up-front work will pay off. If you have been through revision cycles in the past as new browsers disrupt your existing scripts, you can begin to appreciate how one of the goals of this technique -- less maintenance going forward -- will ease your load. Of course, you probably won't know that your hard work early on saved you from headaches down the road. That makes it a thankless job in a way, but one that, rather than making you look good in the future, can prevent you from looking bad.
Using object detection also leads to good programming practice. You'll need to know the resources at your disposal and visualize execution paths for when things work and when they don't. You also grow to appreciate the importance of developing code that anticipates errors and handles them gracefully. Once you grow comfortable with object detection, you'll wonder why you bothered with convoluted sniffing matrices in the past.