Develop Reference

Run business logic using scripting language

We have a web application in which the web pages are built dynamically. Dynamically meaning, the whole UI is stored as JSON in the back-end and the front-end draws the UI accordingly. There are use-cases where the data collection happens across multiple forms. The order in which the forms are displayed depends on the data entered in previous forms.
e.g. Assume the user is viewing form1 and there could be a business logic like if age > 25 show form2 else form3
This is a simple example but it could get much much more complicated. And the logic differs across the clients. This business logic could also be developed by third party developers.
javascript is one of the options to store this business logic. But, the application is being built in Java7 and deployed in Google App Engine. Performance and Security are the important criteria to decide the approach. Following are the approaches that we have considered as of now:
Have a separate NodeJS application and make an API call to execute the business logic
Run Javascript in JavaScript Script Engine which is part of JDK 7
Could you suggest what are the best approaches to executing javascript in a Java application?

Rhino is an open-source implementation of JavaScript written entirely in Java. It is typically embedded into Java applications to provide scripting to end users.
Here is a link - http://www.mozilla.org/rhino/

Given the global nature of Javascript and the fact that the specific logic you're referring to, I tend to agree with Alex about JS. That said there are complaints about Rhino's performance so I wouldn't recommend that right off. Unless the performance proves horrible for your use case, I'd recommend using the built-in to start out. Then as you know more about your use-case, and specific performance profile, you can shift to a better solution if needed.
Here are some additional thoughts to help you frame the question:
You should run any scripting interpreter open to 3rd parties with a babysitter thread. (i.e., kill them after certain amount of resources consumed--that's not what babysitters should do, but alas...). Any complete scripting language can do a simple "while(true): wait()"
Unless the complexity of the logic is massive (i.e., you said one or two if-then's in your example; this can be handled by any language) language isn't a function of security or performance, but one of comfort and familiarity with those using it--i.e., JavaScript should be easy. If your customization team knows python, then go ahead and use an embedded jython, etc.
I wouldn't spin up separate services for simple customization. Again unless the complexity is massive, an embedded engine would be preferred. Spinning up a separate NodeJS server to run 2-lines of script is a pretty big waste of overhead, devops complexity and maintenance (think of spinning up a dev instance every time to run an if-then) -- it's not that hard, but it could be the reason for a 2-day 24-hour marathon debugging exercise in 2 years when you forget about this, or when you hire a junior dev who doesn't know about this and butchers it...

Client and server model in a web game

Currently, I am developing a 2-player tetris game. Originally, I was planning to implement a client-server model in a mobile environment.
However, now I change into web game, which is accessible by a browser. Since I am not used to a web development, I want to know if client-server model is necessary in a web game.
To be honest, I am a little bit skeptical writing both client and server code since a browser does a job to connect to the server.
Is it possible to build a network-based game by using only one server without a client part?

Is it possible to build a network-based game by using only one server without a client part?
Nope. You need a client.
It doesn't matter if you're in the browser or writing a native application. Yes, the browser solves a lot of client problems for you - it implements HTTP for you, and it has display (CSS), content (HTML), and evented logic (JavaScript) that you can use. But you still need to write your client using those technologies.
Even if the browser exposed a call in javascript like this:
window.startPeterHwangsTetrisGame();
You still have a client. The client is 1 line of code, and it is exactly the above script*.
Realistically, you should look for a game-oriented web development tutorial and start there.
*(aside from the necessary HTML within which that javascript exists)

Well, in one sorta correct way, no, because you're not redesigning the browser, but nobody would say it like that. Web design includes a lot of code / markup used on the client (browser), and while your server is sending the browser the data, and that data is a mix of CSS, HTML, and JavaScript (as well as media like graphics and perhaps music), the browser is the piece of software doing the lifting on those pieces once it receives them. That data is selected by the server, so the logic for that is on the server, but this is what people would think of as "client design" in the context of a web application.
There are also a number of models (ways of arranging your programming designs) which would lead to cleaner code, and a lot of these would send a grouping of information (called a model) to the client (the browser) and then the browser would do some interpretation of how to use that data (called a view). This is a simplification since I don't think you're ready for a big dive into specific theories, but you could start with a JavaScript and HTML tutorial and then learn a server language:
Python is my favorite, but PHP is big as is Java (NOT to be confused with JavaScript), and I make my living doing C#. Really there are plenty of great choices.
One commentator recommended Node.js because you'd use JavaScript on the server with Node--just like for the client part (the response data you're sending to the browser). That's double duty, but I am partial to Python (as I said). Just pick something, and as you learn, you'll get to know what you like. If you change languages, it'll be easier to learn your second one.

JavaScript on the server-side like PHP

I'm now thinking to establish my server-side code in JavaScript, and begin to do all on it, but I want to know about its security and flexibility compared to PHP.
I want to know too, if it can be successfully used to develop things like forum boards, full web-sites and things like this, as PHP does.

Javascript is just now starting to get some presence on the server, with things like ServerJS and nodeJS, but right now, you would probably be best off using PHP for your server side code, and javascript for client-side beautification.

The question is very, very broad. Interpreting it as "can I use Javascript on the server":
Fundamentally, sure, Javascript is a very powerful language and so you can do development in it server-side just like you can client-side (and if you do client-side scripting as well, you get some definite reuse benefits using Javascript on the server).
For Apache systems, there's the v8cgi project (a FastCGI Javascript plug-in with connectors, using Google's freaky-fast V8 engine).
On Microsoft-based systems, IIS supports Javascript (JScript) on the server out of the box (I use that all the time), which has access to all of the ActiveX stuff (e.g., for talking to databases, dealing with the file system, etc.).
If your server framework is JVM-based, there's Rhino, which is Javascript for the Java platform and has access to all (or nearly all) of the libraries available for Java — e.g., a huge ecosystem of libraries and plug-ins.
Aside from v8cgi, there are a couple of other projects built on Google's V8 engine.
There's a place that does a full stack for you called chromeserver (I don't know what their backend is; I'm not going to infer from the name).
Paul mentioned ServerJS and NodeJS.
There's the whole CommonJS project.
Etc. etc. etc. There's quite a list on Wikipedia.
Arguing against, there's a very rich ecosystem built around PHP. Unless you're using something like Rhino for the Java platform or JScript on IIS (because of the ecosystems they leverage), you may find that you don't have nearly that ecosystem available to you when developing in Javascript for the server. I mean, if you're looking for pre-built forum or wiki software (for example), let's just say you can't swing a dead cat without finding one based on PHP, and the same cannot be said of Javascript on the server.

The way they are usually used, PHP and JavaScript run in entirely different worlds, and are not really comparable. (There is a server-side version of JavaScript but it's fair to say it's not especially widespread yet, and doesn't run on standard web hosting.)
The security issues you are going to encounter in JavaScript (on the browser) side are very different from what you have to look out for in PHP.
I want to know too, if it can be sucessfully used to develop things like forum boards, full web-sites and things like this, as PHP does.
No, not with client-side Javascript. For dynamic applications, you will always need some server-side language backing it, be it PHP or some other language like ASP, Python, Ruby, Perl....

To replace PHP with Javascript, you need server-side Javascript and there is a lot happening on that front. Mozilla’s Rhino runs Javascript atop the JVM and it seems Google is also working on its own server side Javascript framework. The most popular in-production implementations are:
Helma: Several active projects are using it, runs on Jetty & Rhino and lets developers leverage the power of JVM, has its own object-oriented MVC framework
Project Phobos: runs on Glassfish & Rhino and lets developers leverage the power of JVM, includes plug-ins for NetBeans and integrates with jMaki Web UI framework
JSSP: A very simple server side framework, a lot like classic ASP, JSP and PHP
Aptana’s Jaxer showed a lot of promise, especially by bringing the DOM to the server side, but the project seems dead now. From what I understand, node.js is not a server-side Javascript framework in the same sense as Helma and Phobos. Instead it can be used for writing event-driven servers in Javascript (for example: writing your own web server).

Yes, my site is written by node.js
Using websvr, it's Java style have filter and handlers, hosting on debian OS.

This is slightly off-topic, but it may actually get to the core of your question:
if you want to use only one language for web applications, you may wanna have a look at Haxe.
It is a cross-platform language, that (among other targets) compiles to JavaScript and PHP source as well as NekoVM bytecode. For server-side JavaScript, there are NodeJS bindings.
This way you are not bound to a specific platform. The neko and PHP APIs are largely compatible, so you can deploy on both platforms, having the option to choose neko's speed and persistency or PHP's ease of deployment. Please note however, the PHP output has a little overhead although common optimizers as eaccelerator will make this barely noticeable.
Haxe is significantly less forgiving than both JavaScript and PHP. This makes it harder to learn, but a much safer, robust and in the end more productive tool.

In a word: no. Javascript is a client-side language. In order to do the things that you are describing, you need a server-side language such as PHP.
EDIT: OK, technically it is possible to implement Javascript in other areas besides the browser, but this is not very common.
5 YEAR EDIT: Well, 5 years later, this answer obviously is not accurate, with the popularity of things like node.js. Let that be a testament to how quickly things can change!

PHP and JavaScript are two different languages that do two different things. One cannot replace the other. You are most likely going to use a combination of the two. JavaScript for client-side stuff. PHP for server-side stuff.

Questions about capability of Javascript

Many years back, I was told that Javascript was harmful, and I remember being annoyed with endless popup when I right-clicked an image to download it.
Now it seems suddenly that Javascript is great, and you can do a lot of things with it to let users have native-like web application experience.
I admit I have missed 6-7 years of Javascript literature, so I hope to start anew with SO kickstarting me to understand the following:
Is Javascript mainly concerned about user interface i.e. smoothen interaction between application and users and not about logic processing, number crunching or form processing etc.?
Can Javascript write to local hard drive (besides cookies)?
Can Javascript web application run with Javascript capabilities in browsers turned off? (I would think outright no, but an article on Adaptive Path said 'maybe')
Is AJAX illegal to use due to Eolas patent claim? Is it worth it spending effort learning it when the future is not secure? (I know AJAX is not Javascript)
Thanks. Hoping for enlightenment.

Yes. JavaSscript is usually used to enhance the user's experience and make the site easier to use. It is also possible to delegate validation tasks and the like to JavaScript, however (though this should never absolve the server of its responsibility to check input).
No.
That depends on how the application is written. If it's done properly, then the JavaScript will merely enhance the interface, and the application will still work without it; this is called progressive enhancement.
Not at all. AJAX is used extensively on this very site!
One reason for the resurgence of popularity for JavaScript lately is the emergence of several frameworks. These make the process of writing JavaScript much, much easier, allowing tasks that would previously have been horribly complex to be implemented with minimal time and effort. The most popular of these is jQuery, which is a good place to start if you're intending to get in on the action.
Overall, JavaScript is a very powerful tool that allows you to create very rich interfaces. Well worth learning.

Yes, Javascript is all about client side processing, but also about AJAX where it calls back to the server asynchronously so that users do not see pages reloading.
No
No, but there are ways to gracefully degrade the experience for non javascript users. It requires carefult planning however.
No, that lawsuit was just about the browser technology that enables it. As a developer you dont have to worry about that.

Can Javascript write to local hard drive (besides cookies)?
Not really. However, as HTML5 support becomes more widespread you'll be able to use things like Web Storage and Web SQL. You won't be able to write arbitrary files on the user's hard drive, but using those two technologies you'll be able to persistently store and access data.
Can Javascript web application run
with Javascript capabilities in
browsers turned off? (I would think
outright no, but an article on
Adaptive Path said 'maybe')
It really depends on how you define "web application." You can write web apps without using Javascript for anything but UI candy, in which case you can degrade gracefully without without it. However, it's also possible to write web apps that rely heavily (entirely, even) on Javascript, which will utterly fail without it.
Is AJAX illegal to use due to Eolas
patent claim? Is it worth it spending
effort learning it when the future is
not secure?
I'm not a lawyer, but I'd agree with the other answers -- you shouldn't worry about it. I'm certainly still writing AJAX stuff :)
Is Javascript mainly concerned about
user interface i.e. smoothen
interaction between application and
users and not about logic processing,
number crunching or form processing
etc.?
It's about both. And more than that.
Javascript has really come into its own in the past few years. Browsers have gotten a lot faster at executing it quickly, and people have been figuring out new ways to use the language itself to its full potential. You can really start using Javascript like a full-out application programming language, and not just to write little scripts that animate something or validate input.
If you're just getting back into the language and haven't read Crockford yet, I would highly recommend it. It's a great starting point to realizing the full potential of Javascript.
Edit: Some good Crockford Links
Javascript: The World's Most Misunderstood Programming Language
Javascript: The Good Parts (This is a presentation. Crockford also wrote a book by the same name that I haven't read myself, but I hear it's quite excellent.)

It's mainly for UI, but it can be used to save server-time on some operations (for example, Mathoverflow uses it to render LaTeX) and it's becoming popular to do so. But when you do this, you need to be respectful of the end-users time, because JavaScript can hold up some browsers, while it runs. But in general, it's a good and interesting idea.
Not without permission
If it's written correctly, it can. It's called "Graceful degredation" (some other variant terms exist, but the idea is the same). The basic idea is that you have it such that the JavaScript fails 'gracefully', and links that would normally get handled via JavaScript (i.e. to do some inline next-paging) will navigate to a 'backup' page that shows the relevant content.
I don't know about that, but AJAX can be implemented in different ways, XMLHTTPRequest is just one of them :) (And the most common, and suitable). Generally you like a library do this for you anyway (jQuery, or otherwise) but you can do it yourself for fun.

Yes, in my experience JavaScript is generally used to create a streamlined interface and relays information from the client to a server application for processing.
Yes, if the browser is configured to allow this (most aren't by default since this can be very dangerous).
No, JavaScript will not run if the browser is configured to have JavaScript disabled.
I wouldn't forgo learning JavaScript for this reason - as for the legality of the whole thing I wouldn't feel comfortable advising you about this. Still I think JavaScript is worth learning in spite of this situation.

The Eolas patent covers the embedding of objects in a HTML document (see US patent 5,838,906 titled "Distributed hypermedia method for automatically invoking external application providing interaction and display of embedded objects within a hypermedia document") ... this scope would not seem to include AJAX as a suite of technologies (being essentially scripting in a document to load content elements).

Partial answers:
I think all the security vulnerabilities associated with javascript have been fixed? IIRC the problems weren't with javascript, they were with particular browser's implementation of javascript.
I wouldn't worry about any patent claims on the AJAX technology. Patent sueing and counter-sueing is common place in the software world and invariably ends up with the affected parties licencing each other's technology. AJAX is not going anywhere :)

GWT or DOJO or something else?

I come from the Microsoft world (and I come in peace). I want to rapidly prototype a web app and if it works out, take it live - and I don't want to use ASP.Net.
I am not sure which web application toolkit to use though. Should I use GWT, DOJO...other recommendations? I am open to any server-side language but am looking at RoR, Php or even Java (J2EE to be precise). I am not much of a Javascript/CSS guy so a toolkit that might make it relatively easy on me on those fronts would be preferable.
Also, I am a mac user at home. What IDEs go along with the framework you would recommend?
Any recommendations guys?

If you're open to doing Java, GWT is the way to go. It allows you to have a relatively uniform codebase across client-server, and to only use one language on both.
There are some limitations to doing very off-the-beaten-path AJAXy things (which GWT makes difficult, but not impossible), but it doesn't sound like that's your use case anyway.
GWT will allow you to scale up by using more of its features as your app gets more complex - and your prototype won't be throwaway code.

If you want to write the front and back end in JAVA, and want to do complex ajax type thing, then GWT is a great way to go.
The easiest way to think about it is that building a GWT app is kind of like building a JAVA swing application that hooks into a server. Just like a swing app that uses a server you can make it fat or thin. When you're done it all compiles down into HTML and javascript, and has very good modern browser support (ie6+ ff, opera, safari).
It does abstract all the javascript and HTML away, but if you want it to look good you'll still need to understand CSS.
I think anyone who says that that it ruins MVC or that it's a muddying of client vs server doesn't understand GWT. GWT is a CLIENT side framework. And it is only used on the CLIENT. GWT does provide an RPC mechanism to hook it into JAVA (and other) back ends, but that's just a communication protocol, it doesn't mean that your server code magically becomes your client code. Sure you can write a whole bunch of business rules into your UI if you really wanted to, but you can do this with any framework, so it would be silly to say that GWT is somehow different in that respect.

GWT is a good choice, while if you choose more powerful JavaScript framework based on GWT (e.g. SmartGWT), the compiled stuff is too heavyweight.
Choose direct JavaScript if you need a compact project.

I am a fan of GWT, however I am very familiar with Java. I found it to be intuitive, and surprisingly easy to get good results quickly. If you are to use GWT, then you'll definitely want to use the free, and immensely powerful Eclipse IDE.
One disadvantage of GWT is that it requires Javascript to be supported by the browser, there is no "graceful degradation".

We have evaluate a large list of frameworks and have decide us for Echo2.
You need only to code in Java. Javascript you need only if you want write your own components.
There are no startup performance problems with large projects like GWT.
You can use the full range of Java in your client code because it run on the server. In GWT you can use only very small set of Java classes.
The IDE for Java is Eclipse. This is independent of the used framework.

I'm a fan of jQuery, the chainability of actions, traversals, and commands is really powerful. A good friend of mine is crazy about Mootools, he works at a Java shop FWIW. He mentioned a cool feature of Mootools is that you can specify the functionality you want the framework to include and it will generate the entire library on a single line in a file that you can include on your page to minimize the weight of the framework (pretty cool feature). Really it just depends on what you are most comfortable with. jQuery has great tutorials, is super fast, and can be used along with other javascript frameworks.

Not related to GWT, but have you considered other backends that GWT could work nicely with?
Grails is one backend that ties quite nicely with GWT.

Personally, I would avoid server-side frameworks that try to embed or hide the client-side framework. I'm sure that GWT is great for getting something going quickly, and is probably fine for certain kinds of applications, but you'll probably run into lots of problems "on the edges" for more complex applications. Decoupling the client framework from the server-side framework avoids those problems.