Develop Reference

Is there a way to host HTML website themes/templates (Themeforest, etc) on Node.js?

I was just wondering if there's a way to implement Node.JS and its main file as a sort of container for a fully implemented HTML template/theme like those on Themeforest.
I looked around and saw JSDOM but I don't think that will fit my needs.
In case it isn't clear, Themeforest hosts HTML5 themes that are fully functioning HTML/CSS/JS files that you can just upload to your host file server and it will serve up your front end. I'm just getting tired of making React-styled apps, and just want to try these nice looking themes for a change.

As you are looking to host static HTML/CSS/JS contents, you just have to run a basic server, as described by answers on this question. If you are looking for a production appropriate solution, you should also consider cloud storage/CDNs.

React.js server side rendering with PHP

I would like to develop themes/plugins for WordPress based on React. To make it search engine friendly, I need it to be rendered initially on the server (serverside-rendering).
The only way to do this, as far as I know, is to use react-php-v8js, which requires the PECL V8js extension. This is a problem since I have no control over the platform on which these themes/plugins will be run.
Is there a way to make React and WordPress work together without having to install additional extensions? Perhaps by building/compiling React files into PHP?

There's an article that describes how to do this:
https://sebastiandedeyne.com/server-side-rendering-javascript-from-php/
But it's a fairly complex setup and it requires using composer. That can be difficult in Wordpress projects since Wordpress tends to completely eschew the modern php architecture.
If you're looking for a library to help with SSR in PHP:
https://github.com/spatie/server-side-rendering
Best of luck on it.

If you want your content to be indexed by search engine without js, you can print your minimal content using Wordpress, just the bare minimum + crucial meta tags, maybe localize some initial state for your react app to boot. A bare bone theme such http://underscores.me/ would be sufficient. When js is available, you can replace your whole WordPress generated content with React ones.
The ideal one is to have React generate the content for you. But it's hard until we can see that nodejs / PECL V8js extension available everywhere.

If you can at least install nodejs and launch a node process then it should be ok, although not so simple.
You would need to generate a ssr version of your assets and use it in a node process that would listen on a socket to write the html result..
In your controller you can create a socket to your node process (something like stream_socket_client(...)) and then you can send a dummy function written as a javascript string to that socket (something like stream_socket_sendto($sock, "getResultForMyWidget(someParams){...}")). That function would be evaluated in the node process that would return the response to the controller (the html response as a ReactDOMServer.renderToString from the component you want to render).
That's it for the big picture.
There is a symfony plugin that illustates it very clearly (see this github) and comes with a dummy server node process to illustrate how it handles the socket listening and eval of the incoming function and returns the html result. See also the example in the sandbox for a bigger picture and in depth implementation. You should be able to adapt it to wordpress.

How to set up Jade for Cloud9 IDE?

I want to do some front-end designs in Cloud9, something like a portfolio page with some CSS animations etc, and rather than writing in vanilla HTML, I'm interested to learn/use Jade, since it will make the code look clearner.
How do I setup Jade in Cloud9? For example, set it up so if I change index.html to index.jade things will just work as normal.
p.s side question, is Jade a good choice for my purpose? I've heard Mustache, Handlebars, Slim, or even using React? I just want to write HTML like using Sass.

If you'd like to use Jade anywhere, you'd have to translate Jade to HTML before serving it. The following is the general way:
Store your Jade files in a separate folder
Use either the Jade command line or use Grunt plugin or Gulp plugin for compiling all your Jade templates to the destination directory where you're serving your HTML from.
[Optional step for Cloud9] If you've set that all up, you can use Cloud9's custom builders (Run > Build System > New Build System) to add that to a builder that you can invoke using the build menu.
I assume here you just want a simple Web page, and aren't using Express etc, which would have a somewhat different flow than the above.
Regarding your side question. I don't think StackOverflow is the best place to ask that, and in any case, the answer to any such question is eventually 'depends' and 'try each of them out', 'research', 'use what you know best', 'use what suits your needs best', and some combination / mutation of the above :).

Using Karma to test javascript in a play 2.2.x application

I've got a Play 2 application where my angular js dependencies (and possibly others) are declared using WebJars. When I serve up a page, I use the routing helper to fetch these javascript files.
If I want to test my javascript with Karma I need to specify the path to angular javascript files in order to run my angular dependent code. The examples I've seen on the web don't use web jars and just specify the path as ../app/assets/javascripts/lib/angular.js (or whatever). Is there a better way to do this?

I am not familiar with playframework and therefore I am not entirely understanding the restrictions it is bring to your JS Files but if this helps you can also do something like this in karma.cofig
files: [
'http://localhost:91/sample.js'
],
I performed some quick tests and it worked fine.

How to include external Javascript in GWT offline application

I have a GWT (well, GXT) application that uses an external JavaScript library to add functionality to my app. My application must work offline, too, and herein lies my problem.
I am aware that adding files to the public folder will make them accessible by my GWT app, but this will not work in case of offline use. GWT compiles my app to make it available offline without problem, but it doesn't include the external JavaScript library.
So, whenever I work within the application and reach the point where said library is needed, the browser will attempt a GET request because the library hasn't been loaded yet and doesn't remain in the cache of the browser reliably.
Is there a way to add the library to my app so that it will be cached together with my GWT app? The library consists of several folders, JS files, images, CSS, etc. My only idea is to dynamically create an Appcache Manifest that dumps ALL files in the browser cache.. in which case I'm scared of breaking the GWT offline functionality.

Yes you can generate a manifest at compile time. Just use a linker that extends com.google.gwt.core.ext.linker.AbstractLinker.
See for example this example manifest linker
or see Writing a GWT Linker
or see this stackoverflow thread
I do that to include google fonts and to produce a manifest that will only include files for that specific language permutation.