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what main.js should contained in vuejs project?

I'm learning nodejs and vuejs to modify an already created web site.
I installed nodejs, vue and vue-cli and I launched "npm run serve" which apparently start "vue-cli-serve serve"
The problem is that I don't understand what this web server do on files, in this documentation : https://cli.vuejs.org/guide/prototyping.html , it's told:
It automatically infers the entry file in the current directory - the entry can be one of main.js, index.js, App.vue or app.vue. You can also explicitly specify the entry file:
vue serve MyComponent.vue
Ok, but does it run main.js, does it include it into a js file which is the loaded by the index.html on the client broswer.
I see in the browser that the page load a js file named like that: app.23d...js
My question is how this js file is created?
For instance, when the content of main.js is this one:
import './css/icon.css'
Vue.use(VueResource)
Vue.use(VueScrollTo)
what is the output in the app....js file?
It seems it doesn't work at all like php which I usually use on web server
Thank you

Vue uses Webpack to convert your potentially-numerous distinct .js files into bundles with names like app.23d92ab88708...js
From the Webpack documentation:
Concepts
At its core, webpack is a static module bundler for modern JavaScript applications. When webpack processes your application, it internally builds a dependency graph from one or more entry points and then combines every module your project needs into one or more bundles, which are static assets to serve your content from.
I don't suggest trying to get into the details of how Webpack works
This will take a lot of time. If you have an existing Vue project, you are much better off spending your time interpreting that as a Vue project, and accepting that the conversion into the actual app.23d.....js file or files is an automatic process that you do not need to involve yourself in.
It will avoid a colossal waste of time
It won't advance your understanding of how the Vue project works
Whatever you learn about the exact workings of today's Webpack, may be completely wrong about tomorrow's Webpack.
Nevertheless the interface that Webpack provides to you as a Vue programmer will remain constant over future versions.
This is the concept of software abstraction. It is highly advantageous to not have to know how every step of every process works, as long as you know how it is designed to respond to actions you take at a high level.
Horrific thought
I have just re-read your opening sentence:
I'm learning nodejs and vuejs to modify an already created web site.
Please tell me that you are not trying to modify an already created web site where you only have the compiled website available, without the Vue source code? That would be a stupendously painful enterprise.

How to Import non npm dependency in angular

I am working on angular 5 project want to create a visualization of an application system internal connection (something like a flowchart where systems internal components like server, mq, db can be dragged upon and connection be made) component in angular. Started with the jsplumb community edition (link) to do the same. Was able to successfully integrate jsplumb into angular 5 application. The only drawback is we cannot create the new element at runtime (That is drag and drop items and create connection). This is possible in the toolkit licenced version. Which I am not considering to work with.
So was looking into js-graph-it (link) The js-graph-it is a old project works nicely with simple html page. I am having problem injecting js-graph-it.js file into the angular project. As it is non npm file it is not defined and the system does not recognize the same if syntax like import { jsgt } from '../../assets/jsgraphit/js is used. If you observer I have placed the files in my asset folder. The first question is this the right place for 3rd party js file if not present in npm? How to import or the correct way to import such files not present in npm ?

You can import 3rd party JS files into your project and be bundled by the CLI by putting them in the scripts array in your angular-cli.json config file.
This will make them part of the global scope (e.g. available on the window object) as if you imported it via a <script> tag.
You can additionally include any CSS files or other assets part of the libraries in the config file as well via the styles and assets properties.
I think the proper way for you to manage these 3rd party libs is to store them in your assets directory as it looks like you are already doing.

laravel app.js very large file size

I have deployed a Laravel 5.3 application to Heroku. However, when loading /login, I noticed a very slow page load time. The problem seems to be a very large app.js file: /js/app.js. Here is a screenshot of the Network resource panel in DevTools: screenshot- Network panel. The 3rd resource from the top is the offending file.
I am not sure why this file has gotten so large. here is a link to the repository: https://github.com/AshMenhennett/Salon-Pricing.
I wasn't able to post anymore links, so do let me know if you would like direct links to specific files.
What should I be doing to mitigate this issue?

The most obvious thing you can do is to run npm run prod. This will compile the assets for production use. But in most cases, you must be looking at other solutions beyond running npm run prod. If your production file is too large, you must check your dependencies. Remove unnecessary dependencies and ensure that you don't use a lot of external libraries. For example, if you are using bootstrap, you should rely on Bootstrap's alerts in order to show alerts rather than using a Vue package to show alerts. I admit that sometimes you will need to use an external library to make your website interactive but to achieve that, you will have to sacrifice the performance. So your best bet in order to reduce the app.js file is to use the minimal external dependencies in your package.json.
The second thing you can do is use minimum HTML in your components' templates. A lot of components with heavy HTML/CSS will contribute to a larger app.js file. This is yet another approach that will result in a smaller app.js file.
Lastly, you should consider using Vue's component slots to pass HTML contents to your components. This will leave the HTML in your static files and only javascript data (API calls, props, etc.) will be compiled in the app.js file. This is an effective approach to build a smaller app.js file.
Edit: You can remove JQuery and Bootstrap scripts from the bootstrap.js file and can include these dependencies separately. It is always a good idea to have a few more scripts rather than having a very large script. i.e. browsers do parallel downloading and thus using JQuery and Bootstrap dependencies separately is a good idea.

From the looks of your link you've not created a production version of your assets, and currently all the source maps are in your app.js file, which will be adding a lot of the file size, the css and js output are also not compress/minified either.
Assuming you're using laravel elixir, you just need to run gulp --production and this will remove the source maps, compress the js and css outputs, etc.

For people that are using Laravel Mix you just need to run npm run prod to compress and remove source maps from app.js itself.

You need to load the components asynchronously
Webpack has an awesome feature to create chunks of code. The key to this is to use async components. These components get loaded completely asynchronously whenever the component is present on the page you just loaded.
Let's do it.
In resources/js/app.js
I changed
Vue.component('jobs', require('./pages/employer/jobs/Index.vue').default);
To
Vue.component('jobs', () => import('./pages/employer/jobs/Index.vue'));
and in webpack.mix.js
mix.webpackConfig({
output:{
chunkFilename:'js/vuejs_code_split/[name].js',
}
});
Now by running npm run watch or prod each component file is saved public/js/vuejs_code_split/[name].js
And the main app.js is automatically calling those components when required.

Loading modules in qooxdoo desktop (browser environment)

I'm struggling how to integrate client-side modules like - just as an example - Apollo Client
into the qooxdoo-specific generate.py workflow so that they become available in the browser.
According to the installation notes:
To use this client in a web browser or mobile app, you'll need a build system capable of loading NPM packages on the client. Some common choices include Browserify, Webpack, and Meteor 1.3. [...]
Side note: I currently use Babel 6 to recursively transpile all my sources from a separate folder source.es6/ into the "official" source/ folder, which is then watched and processed by generate.py. Is it possible to use this somehow as a solution to my question?
OTOH, I would love to see at least some kind of integration with Webpack, Browserify or SystemJS.

I suggest you do the following. First, create a loadable package(s) from the Apollo Client and its dependencies, e.g. using Webpack. Then make sure these package(s) are loaded in your web page before you load your qooxdoo app. Then the Apollo API is available to your qooxdoo code.
If you choose to deploy the Apollo packages with <script> tags you can let generate.py do that by using the add-script config key.
I suggest you place the output of the Webpack run in your qooxdoo project's resource path and add #asset hints for those files in your main qooxdoo class. This will make sure they are copied into the build version of your app, and you can use the relative URI to these files, either in your index.html directly or in the add-script config settings.
I don't think your transpiling with Babel6 will help here. The Apollo code is already consumable and you woudn't want to disect it and make it part of your qooxdoo (es6) source tree, let alone its dependencies. I would rather treat it as a shrink-wrapped JS library as I described that is added like a resource.

Breaking a React-Native application into plugable modules

I am looking to write a React-Native application. I want to be able to download new modules at run-time on the device to extend functionality. There would be some core logic that knows how to request new modules based on some form input like a dbs. I do not want to bundle everything into a single monolithic bundle which is what I believe happens now with the built in packager.
This would something similar to how RequireJS works in browser. What I need to know is:
How do I build independent modules? react-native bundle doesn't seem to allow me to select which root modules to begin with and only works on root project
How can I at run-time request new functionality be injected into the current JavaScript environment?

React native starts by pointing at a JS bundle. This means that you would at least have to restart the app to reload the js bundle (assuming that you're reading it from a server and not from the ios device itself).
If you did have a way to update the js files on the server (through some sort of web service that updated based on things the user does) then restarting the app could theoretically reload the JS and provide new functionality to the app.

I was able to get something pretty close to this in JS only. First I had to expose a few more options in the current react-native bundler, mainly, the url (to change the "main" module) and blacklist (to keep from bundling react-native in second bundle) options to their packager (http://github.com/facebook/react-native/blob/master/packager/README.md).
I then had to write a custom fetcher that would download the second bundle and use eval() to evaluate it in the current environment.
The one rub is that I had to add __d('react-native',[],require('react-native')) in the first bundle which I think work like define() from require.js. This exports 'react-native' as an unmangled name that the modules which I plug in can access via normal require() statements. It's a bit confusing at first but from what I can tell they React-native packager works a little like r.js (see http://requirejs.org/docs/optimization.html).