At the Forge - Unobtrusive JavaScript

 in
Remove JavaScript event handlers from HTML files using Prototype and Lowpro.

Assigning events in this way has some advantages over using the onclick and related attribute-based event handlers. It lets us define all of our event handlers in a single place—typically at the end of the HTML file. Thus, we have some separation between our HTML and JavaScript.

But, what if we want to go one step further, putting all our JavaScript into a separate file? Listing 3 shows a new version of our HTML file, now called test-3.html. Instead of having the JavaScript at the bottom of the page, I put it in a separate file, called atf-events.js (Listing 4). However, if you try to load this file, you quickly will discover that it doesn't work. We get a JavaScript error upon loading the file (clearly evident and readable if you're using the wonderful Firebug debugger for Firefox), telling us that $('hyperlink') is null.

How can this be? If you look through Listing 3, you still will see an HTML element with an ID of hyperlink. And, we definitely have included the Prototype library, so $() should work. How can it be, then, that $('hyperlink') returns null?

The answer is subtle, but well known to JavaScript programmers: $('hyperlink') is available only after the HTML element with an ID of hyperlink has been loaded. Because our JavaScript file was loaded (in the <head> of the document) before the hyperlink element was defined, JavaScript threw us an error.

One solution to this problem is to load our JavaScript at the end of the file, right before the closing </body> tag. Another possibility is to define all of our event handlers in a function that itself is executed only after the entire document is loaded. In other words, we define a function (set_event_handlers) that defines all of our event handlers. Then, we attach this function to the window.onload event, which executes only after the entire document has been loaded. The code, shown in Listing 5, is exactly the same as Listing 4, except the functionality is wrapped in the set_event_handlers function, which is invoked based on an event.

______________________

Comments

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

So interresting...

Arimbourg's picture

Thanks for this article. Clear and so usefull...

White Paper
Linux Management with Red Hat Satellite: Measuring Business Impact and ROI

Linux has become a key foundation for supporting today's rapidly growing IT environments. Linux is being used to deploy business applications and databases, trading on its reputation as a low-cost operating environment. For many IT organizations, Linux is a mainstay for deploying Web servers and has evolved from handling basic file, print, and utility workloads to running mission-critical applications and databases, physically, virtually, and in the cloud. As Linux grows in importance in terms of value to the business, managing Linux environments to high standards of service quality — availability, security, and performance — becomes an essential requirement for business success.

Learn More

Sponsored by Red Hat

White Paper
Private PaaS for the Agile Enterprise

If you already use virtualized infrastructure, you are well on your way to leveraging the power of the cloud. Virtualization offers the promise of limitless resources, but how do you manage that scalability when your DevOps team doesn’t scale? In today’s hypercompetitive markets, fast results can make a difference between leading the pack vs. obsolescence. Organizations need more benefits from cloud computing than just raw resources. They need agility, flexibility, convenience, ROI, and control.

Stackato private Platform-as-a-Service technology from ActiveState extends your private cloud infrastructure by creating a private PaaS to provide on-demand availability, flexibility, control, and ultimately, faster time-to-market for your enterprise.

Learn More

Sponsored by ActiveState