Dr. Dobb's Journal - November 2007 - (Page 42)

d11vaugh_p5db 9/17/07 8:34 AM Page 42 State of the Art by Richard Vaughan Transparency On Demand Mandating, implementing, and using Aspect-Oriented Programming in JavaScript The “on-demand” approach to deploying JavaScript applications incrementally confers great Richard is an independent software developer, t rainer, and c onsultant based in London. He can be contacted via www.dodeca.co.uk. benefits, but nontrivial use can pollute functions with expressions that are unrelated to their essential nature. Aspect-Oriented Programming (AOP) can resolve this problem completely; yet AOP in JavaScript is an unexplored area of web development. In this article, I show the need for such techniques in web development, and explain the principles of function interception in JavaScript. In short, I present an elegant technique for loading JavaScript libraries transparently at the point of need. JavaScript Library Inclusion JavaScript programs are associated with HTML documents by means of script elements where, in the simplest case, opening and closing tags delimit the JavaScript syntax embedded within a page. However, many pages may require the same functionality, and this mandates placing the relevant code in discrete files that bear a .JS extension. Such libraries can then be pulled into the execution environment by specifying the file name as the value for the src attribute in an empty element; see Listing One. JavaScript’s interpreted nature aside, this is identical to the #include mechanism supported by C/C++, but the Pareto principle (en.wikipedia.org/wiki/ Pareto_principle) also indicates the seeds of trouble. Generally, 90 percent of a program’s runtime is spent executing only 10 percent of its code, implying that functionality should be loaded into the execution environment only when needed lest resources be consumed redundantly. Compiled applications accom42 Dr. Dobb’s Journal l www.ddj.com l November 2007 modate this by using DLLs, but without similar intervention a client-side JavaScript application can download large tracts of code that are never actually executed. Such inefficiency should be avoided assiduously in web deployment; it consumes bandwidth unnecessarily, increases download times, and stresses both server and client needlessly. The poor performance that results can render an otherwise competent application unpopular if not unusable, and this concern can only grow as web developers become ever more adventurous in their use of JavaScript. JavaScript On Demand The only solution to this is to adopt a “lazy” approach to loading, otherwise known as “on-demand JavaScript,” where code is retrieved from the server as the need arises. This can circumvent the redundancy problem, as only a fraction of an application need be downloaded initially, with additional functionality http://www.dodeca.co.uk http://en.wikipedia.org/wiki/Pareto_principle http://en.wikipedia.org/wiki/Pareto_principle http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - November 2007

Contents Hmmmm Alia Vox Developer Diaries Developer’s Notebook Smart Compilers - But Smart Enough? Conversations Grid-Enabling Resource-Intensive Applications Distributed Computing: Windows and Linux Adobe AIR: Desktop/Web Convergence Transparency on Demand Reusable Associations Effective Concurrency The Agile Edge Swaine’s Flames

Dr. Dobb's Journal - November 2007

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