You will be redirected in 30 seconds or close now.

ColdFusion Authors: Yakov Fain, Jeremy Geelan, Maureen O'Gara, Nancy Y. Nee, Tad Anderson

Related Topics: Silverlight, ColdFusion, Microsoft Cloud, Adobe Flex, Machine Learning

Silverlight: Blog Feed Post

Eval JavaScript in a Global Context

For more standards-respecting browsers, the way to do this should be to use the call function

Even though it's considered bad practice, it's often handy to eval code in JavaScript.  And in my case, it was simply necessary, since the JSF specification requires eval of scripts. And it's also necessary to execute those evaluated scripts in the global scope. It's not as easy as it first looks.

For our first naive implementation, we'd simply used eval(src) in our first pass at the implementation.

This is utterly wrong, and to understand why, you'll need to understand scopes. JavaScript has what you can think of as two different scopes - function scope, where you're executing something in the context of a function, and global scope, where you're executing something in a global context - for instance, if I say var j = 1; within a function's scope, then the variable j is set to 1 within that function. If I say the same expression, var j = 1 within the global scope, then j is set to 1 everywhere in the program - in every function, provided that that function doesn't define a j variable in its local scope. In browsers, the global context is window - this is the default object that everything gets hung off of if you don't specify any other object.

So, when we said eval(src), we were executing the src scripts within the local scope of the function where eval was called - that meant that I would be getting different results when variables were declared and set than would be expected - in fact, for some cases, it just seemed like the scripts weren't being executed at all.

So, what to do? Well, as is usual for the browser JavaScript, there's Internet Explorer, then there's everyone else. As is usual, IE, the crazy cousin Larry of the browser world, has a convenient, well intentioned, and utterly nonstandard way to do this: window.execScript(src) It works great - and the other ways I'll detail here break rather infamously, so use this non-standard function on IE.

For more standards-respecting browsers, the way to do this should be to use the call function, which is a standard function attached to every Function object. So, eval.call(window, src) should work. But to understand why, it's important to know about context, in addition to scope. Every function call has it's own context: this is the object that's represented by the special value this. When we use the call function, the first parameter is the context object we'll use for this. This is handy for all kinds of purposes, but for us, it's just nice to use to set the context to the window object - which, you'll recall, is the global.

Sadly, eval.call(window,src) breaks on Chrome - it complains about contexts not matching. Odd - and I was unable to Google up why this might be so. But a couple lucky guesses later, and I discovered that window.eval.call(window,src) works on all non-IE browsers. Now, when I say "var j = 1", the window[j] is the variable that's set... So, that's good. Why do we have to add the extra window. on Chrome? Not sure - I could guess, but it's too likely to be wrong.

At this point, I thought we'd licked the problem. No such luck. Sure, global variables are getting set, but it turns out that if you say: alert(this) - then you would correctly receive the global object back on Chrome and Safari, but not Firefox - there, you'd get back the object that was the enclosing object before the call function got called. Very odd, and likely a bug in their implementation.

With a little help from Werner Punz, we figured out that they best way to get around this issue is to wrap the calling function in an anonymous globally scoped function. Like the Chrome bug, I can guess why this might work, but it would only be a guess. Better not to clutter up the internets with more guesses - I'll just stick to what I know works.

Here's the code that I now use to do a global eval:

More Stories By Jim Driscoll

Jim Driscoll has worked at Sun Microsystems for 12 years, working on such projects as the first version of Servlets (in the Java Web Server), and the initial implementation of Java 2, Enterprise Edition. He is currently a Senior Engineer working on the implementation of Java Server Faces, helping to integrate technologies such as AJAX and Comet into the new release.

IoT & Smart Cities Stories
CloudEXPO | DevOpsSUMMIT | DXWorldEXPO are the world's most influential, independent events where Cloud Computing was coined and where technology buyers and vendors meet to experience and discuss the big picture of Digital Transformation and all of the strategies, tactics, and tools they need to realize their goals. Sponsors of DXWorldEXPO | CloudEXPO benefit from unmatched branding, profile building and lead generation opportunities.
DXWorldEXPO LLC announced today that Big Data Federation to Exhibit at the 22nd International CloudEXPO, colocated with DevOpsSUMMIT and DXWorldEXPO, November 12-13, 2018 in New York City. Big Data Federation, Inc. develops and applies artificial intelligence to predict financial and economic events that matter. The company uncovers patterns and precise drivers of performance and outcomes with the aid of machine-learning algorithms, big data, and fundamental analysis. Their products are deployed...
All in Mobile is a place where we continually maximize their impact by fostering understanding, empathy, insights, creativity and joy. They believe that a truly useful and desirable mobile app doesn't need the brightest idea or the most advanced technology. A great product begins with understanding people. It's easy to think that customers will love your app, but can you justify it? They make sure your final app is something that users truly want and need. The only way to do this is by ...
Digital Transformation and Disruption, Amazon Style - What You Can Learn. Chris Kocher is a co-founder of Grey Heron, a management and strategic marketing consulting firm. He has 25+ years in both strategic and hands-on operating experience helping executives and investors build revenues and shareholder value. He has consulted with over 130 companies on innovating with new business models, product strategies and monetization. Chris has held management positions at HP and Symantec in addition to ...
Dynatrace is an application performance management software company with products for the information technology departments and digital business owners of medium and large businesses. Building the Future of Monitoring with Artificial Intelligence. Today we can collect lots and lots of performance data. We build beautiful dashboards and even have fancy query languages to access and transform the data. Still performance data is a secret language only a couple of people understand. The more busine...
The challenges of aggregating data from consumer-oriented devices, such as wearable technologies and smart thermostats, are fairly well-understood. However, there are a new set of challenges for IoT devices that generate megabytes or gigabytes of data per second. Certainly, the infrastructure will have to change, as those volumes of data will likely overwhelm the available bandwidth for aggregating the data into a central repository. Ochandarena discusses a whole new way to think about your next...
Cell networks have the advantage of long-range communications, reaching an estimated 90% of the world. But cell networks such as 2G, 3G and LTE consume lots of power and were designed for connecting people. They are not optimized for low- or battery-powered devices or for IoT applications with infrequently transmitted data. Cell IoT modules that support narrow-band IoT and 4G cell networks will enable cell connectivity, device management, and app enablement for low-power wide-area network IoT. B...
The hierarchical architecture that distributes "compute" within the network specially at the edge can enable new services by harnessing emerging technologies. But Edge-Compute comes at increased cost that needs to be managed and potentially augmented by creative architecture solutions as there will always a catching-up with the capacity demands. Processing power in smartphones has enhanced YoY and there is increasingly spare compute capacity that can be potentially pooled. Uber has successfully ...
SYS-CON Events announced today that CrowdReviews.com has been named “Media Sponsor” of SYS-CON's 22nd International Cloud Expo, which will take place on June 5–7, 2018, at the Javits Center in New York City, NY. CrowdReviews.com is a transparent online platform for determining which products and services are the best based on the opinion of the crowd. The crowd consists of Internet users that have experienced products and services first-hand and have an interest in letting other potential buye...
When talking IoT we often focus on the devices, the sensors, the hardware itself. The new smart appliances, the new smart or self-driving cars (which are amalgamations of many ‘things'). When we are looking at the world of IoT, we should take a step back, look at the big picture. What value are these devices providing. IoT is not about the devices, its about the data consumed and generated. The devices are tools, mechanisms, conduits. This paper discusses the considerations when dealing with the...