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Symfony 3.3 - where should I put CSS and JS files?

THIS link suggests that /app directory contains all configuration, templates (.html.twig files) and translations, and the /src directory contains PHP code.
However, HERE we can see that .css and .js files are loaded from /src/AppBundle/Resources/public directory and not /app/Resources/public directory. What is the best practice? How should I professionaly approach this problem, and where should I keep all the templates, .js and .css files?

I like to put css and javascript that serve for general purposes in /app/Resources/public and the ones specific to the concerned bundle in /src/YourBundleNameBundle/Resources/public. Like that when I want to see a code specific for a bundle I will just go to that bundle , when it's a general purpose code I would be sure that it's in the root. I have seen expert people doing it and it makes sense for me.
It's a matter of choice and there is no right and wrong answer.

the best practice is to put your css and js files in your bundle particularly in folder that you called "public" (yourBundle/Ressources/public/css/yourStyle.css)
and make sure to run this command before
php bin/console assets:install
and To avoid typing this command each time, I recommend you make a link (a kind of shortcut), by adding the active parameter:
assets:install --symlink
So, the web /bundles/folderBundle actually points to src/yourBundle/Resources/public
On Windows, you must run the command prompt in administrator mode

Webpack 2: Watch external files

I have a project that uses source files external to the project. Effectively, there is the actual project source code (an Typescript/Angular 2 application, lets call it the 'core' stuff), and this is a generic web application that is meant to be the base code that consumes these external source files.
The external files include additional stuff-- that could be SCSS files, images, evn additional JS. The way I want this to work is that webpack copies these external files from any source directory (this is critical, it is not part of the core project) to a local .tmp directory. The files in the .tmp directory are worked on along with the core src files to generate the prod output.
I can't figure out how to add these additional external source files to the watch list. Effectively what I'm looking to do is watch that directory and as things change, it re-copies the affected files to the local .tmp directory and triggers a recompile.
Presently I have to restart webpack and have a very very ugly solution using Grunt to watch the additional files. It's nasty but these kinds of workarounds have historically been what I've had to do with webpack.
Does anyone have a better solution? Ideally I'd like to not have to mix grunt with webpack. Webpack should be able to do this, but its hard to know whether there's an existing plugin for this or what the best approach would be.
Also, please spare the "look for it on google" or "read the docs" comments. I've combed through it all, hard, and have not found anything.
Thanks in advance.

As of now Webpack doesn't watch external files out-of-the-box . You need a plugin for that.
Basically idea is to have a file watcher module chokidar / watch , listening to the file change , and when there is a change, restart the webpack compilation phase . Webpack plugins can access the compilation object and we you need to hook it to a compiler phase i.e. 'emit' , 'after-emit' etc.
This Webpack plugin exactly solves your problem - https://www.npmjs.com/package/filewatcher-webpack-plugin .

What is "npm run build" in create-react-app?

I could not find any explanation regarding the work of "npm run build",
It is simple and easy to use and i get the "build" folder that works great,
But, in create-react-app, what happens exactly behind the scene?
Is it a complete different use of a build tool?
If not, is it utilizing other build tools?

Developers often break JavaScript and CSS out into separate files. Separate files let you focus on writing more modular chunks of code that do one single thing. Files that do one thing decrease your cognitive load as maintaining them is a quite cumbersome task.
What happens exactly behind the scene?
When it’s time to move your app to production, having multiple JavaScript or CSS files isn’t ideal. When a user visits your site, each of your files will require an additional HTTP request, making your site slower to load.
So to remedy this, you can create a “build” of our app, which merges all your CSS files into one file, and does the same with your JavaScript. This way, you minimize the number and size of files the user gets. To create this “build”, you use a “build tool”. Hence the use of npm run build.
As you have rightly mentioned that running the command (npm run build) creates you a build directory. Now suppose you have a bunch of CSS and JS files in your app:
css/
mpp.css
design.css
visuals.css
...
js/
service.js
validator.js
container.js
...
After you run npm run build your build directory will be:
build/
static/
css/
main.css
js/
main.js
Now your app has very few files. The app is still the same but got compacted to a small package called build.
Final Verdict:
You might wonder why a build is even worth it, if all it does is save your users a few milliseconds of load time. Well, if you’re making a site just for yourself or a few other people, you don’t have to bother with this. Generating a build of your project is only necessary for high traffic sites (or sites that you hope will be high traffic soon).
If you’re just learning development, or only making sites with very low traffic, generating a build might not be worth your time.

It's briefly explained here: https://github.com/facebookincubator/create-react-app#npm-run-build-or-yarn-build.
It correctly bundles React in production mode and optimizes the build for the best performance.
The build is minified and the filenames include the hashes.
Behind the scenes, it uses babel to transpile your code and webpack as the build tool to bundle up your application.

webpack build to multiple files

I have web pack application and want to build output multiple files not a single javascript file. for example if I have such folder structure
components
1.js
2.js
actions
1.js.
2.js
my webpack build have to compile files in same folder structure. How i can achieve that?
I tried babel cli:
babel ./src --out-dir ./lib --source-maps --presets es2015,react --plugins babel-plugin-add-module-exports,babel-plugin-transform-decorators-legacy,babel-plugin-transform-class-properties --watch
It outputs files as I wanted but getting error
Cannot resolve module
Because it does not know anything about webpack resolve.
Any suggestions?

Getting webpack to output multiple files is possible but it does have limitations. First it's important to understand how this works. Webpack actually provides a runtime script that can load code "chunks" as they are needed, this is important for large applications so the user doesn't have to download the javascript for the entire app just to see the homepage. But webpack needs to keep track of these chunks and what they're named at build time to be able to load them correctly at run time. For that reason it has it's own file naming conventions to enable this functionality. See more here: https://webpack.js.org/guides/code-splitting/. So you can require.ensure all of your deps, but they won't be named and foldered they way you describe as webpack's runtime wouldn't be able to find them in that case.
The other thing that's important to consider is that webpack is a bundler, so it's meant to bundle files. Essentially your saying you don't want to bundle your files. So if that's the case, you should probably look into using require.js. Many people have moved from require to bundlers such as Wepback and Browserify as typically it's not efficient to download every little module seperately. But, every app is different so you may in fact have a good reason to not bundle.
If you do in fact want to do some bundling but want to optimize how it's done, then I can help with that as well, and webpack is certainly a great tool for that. But I'll need to understand your use case a little more to give the best advice.

How do front end devs bundle and minify files?

What's the best practice for minifying and bundling js/css in a pure front end app, and how do the tools work?
I know how this can be done with server side apps like .NET/Java/LAMP/etc. But what about pure front end projects, SPA projects or backendless projects that are built with say, ember or angular these days? Say your entire project consists of HTML/css/js, which interfaces with a RESTful service elsewhere.
What kind of process or tool do you use to minify and bundle the resources for that?
I've seen grunt plugins that exist for this, but I find the documentation to be pretty magical and it's still unclear to me how they work.
Specifically, does the tool:
1) Replace src="/js/a.js",src="/js/b.js" with src="/js/bundle-a+b.min.js"? (and likewise with css?) in the source html files?
2) have different modes for dev and release, or is the tool only run when the project is released?
Or are the resource requests entirely managed by a js tool and js/css files have to be requested via a library function? Wouldn't the lag be noticeable in this case?
Thanks.

Through the use of Build tools front end devs can have minified javascript, css, or even images and html files automatically minified as they develop. The most common is grunt, with gulp close behind.
You configure grunt tasks, like grunt-contrib-uglify and grunt-contrib-copy, and put those tasks under a grunt-contrib-watch task. Have the grunt watch task watch the files you modify, and every time a change is detected those .min files are automatically generated.
These build tools have no impact on your application, they are run before the files are servered. You were correct to assume there was an easy way to do this. I suggest you look at grunt getting started, a sample gruntfile, or a project that uses grunt - here's mine, it does minification like you requested. Clone my repo, run sudo npm install, then sudo grunt. I don't have watch set up in my project but grunt is very well documented.