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How index.js file is able to communicate with index.html?

In React project both index.html and index.js file are inside different folder and there is no link in between these two file. Then how we are able target div whose id is root of HTML file by
using document.getElementById("root") inside index.js file.
please answer this in easy language because i have just start learning this , I am compeletly begineer.

React is one of the best JS libraries out there. It simplifies the development process without cluttering the implementation details explicitly. The index.html and index.js get linked during runtime when you start the script. React uses webpack under the hood where the entry for the webpack is index.js and when it is run(after all the conversions) it injects the javascript code in the HTML file which is index.html. And you can see this injected code in the index.html when you build the bundle inside the build folder.

Dynamically set the file path for static files in the index.html file in a React app via a JSON file

I'm trying to dynamically set the file paths to the static files (js & css) in the index.html file of my create-react-app such that they can point to different sub-directories depending on what I set in a settings.json file.
Example:
If I set the base_url in my settings.json file like this:
{
"BASE_PATH_URL": "/subdirec1"
}
I expect the file path in my index.html file to be like this:
<script src="/subdirec1/static/vendors/js/core/jquery-3.2.1.min.js" type="text/javascript"></script>
I'd be grateful if anyone could help me out here. Thanks!

If you're using webpack, you can use webpack variables that you can set within the webpack config object, which themselves come from a .json/.js file.
This is the example you can use if you're using webpack!
WARNING: Don't use the command below before reading up on it, because it will make a big mess of files you might not understand yet!
Since you're using create-react-app, I think it uses webpack under the hood but you need to npm run eject it to have more complete access to its configuration!

Node.js error including js file - file cannot be found

I'm running a node.js server. In the node I require and run the module by using
var h = require('h.js');
h.hello();
This prints Hello World to the console. However, I want to be able to run the code from h.js from a page(entrypage.html) in my browser as well. I try to import it by
<script type="text/javascript" src="/node_modules/h.js"></script>
However, this gives a 404 error when run on localhost.
GET http://localhost:8080/node_modules/h.js
How do I get access to this javascript file on the HTML page?
My file structure has a root with html/, js/, node_modules/ and server.js. The HTML page is inside html/, the js file in node_modules/
EDIT: I'm using the MEAN stack for this - MongoDB, Express.js, Angular.js and Node.js.

It should be possible to serve the file using express.static (Example: app.use('/node_modules/', express.static(__dirname + '/node_modules/'));), but I highly advise against serving node_modules and backend-specific files in the frontend!

first define path for your static resource which is your CSS,JS,Images etc.
app.use(express.static(path.join(__dirname, 'node_modules')));
then in html
<script type="text/javascript" src="h.js"></script>
NOTE :
Try to avoid to use node_modules folder for source path of your static resources

To be honest I'm not exactly sure what you're trying to achieve, I take it that you want to run the code in the browser just like running code in the node.js environment using require to include packages like h.js from npm.
If that's what you want to do I would recommend using a tool like browserify or webpack which collects your code and puts it into a file which can then be used in a web browser.

Proper way to require external js and css libraries in ember js?

I have been playing around with ember 1.13 and I can see that in some online tutorials they require js and css via index.html while some uses ember-cli-build.js or brocfile.js for older versions. I find it requiring properly when I use ember-cli-build.js but then I am not sure what exactly the use of index.html

It depends.
If you have a ember-cli-plugin it will add the files to the vendor files by itself normally. Like with ember-cli-materialize.
If you are installing a random bower package like Ladda, you would need to add the files you need manually to ember-cli-build.js:
module.exports = function(defaults) {
var app = new EmberApp(defaults, {
});
app.import('bower_components/ladda/dist/ladda-themeless.min.css');
app.import('bower_components/ladda/dist/spin.min.js');
app.import('bower_components/ladda/dist/ladda.min.js');
return app.toTree();
};
This will then be merged into your vendor.css and vendor.js which are linked to from index.html.
Also when you build the app the bower_components won't be available unless you've explicitly included something, so you cannot just link to them from index.html. It would also be a waste of network resources to include files separately. You shouldn't have to include anything in index.html unless it's an external resource.
brocfile.js is the old name for ember-cli-build.js since they've stopped using broccoli. Just use the newer one.

Including JS files in Derby.js

I am trying to learn Derby.js and I am having a lot of trouble. I know I can include packages such as jQuery through npm and add it to the node_modules folder, but this isn't quite what I want to do. I want to be able to include these files like I do in normal HTML.
So I want to do something like <Head:> <script src="js/jquery.js"></script>. This does not work though because it cannot find the js directory. I expect this has something to do with the way node.js runs an app and that the app itself will not hold the js directory.
Any help would be appreciated!

Derby offers the Script: tag:
<Scripts:>
<script type="text/javascript" src="/components/jquery/jquery.js"></script>
The components directory is because of the usage of bower. Put the components directory into the public directory. According to the express FAQ, the static routes search below the given directory (which is public in derby's example application). Configure bower to put the files under public/components (Choose bower install directory).
The public directory is configured at lib/server/index.js: .use(gzippo.staticGzip(publicPath, {maxAge: ONE_YEAR})), where publicPath is configured above to path.join(root, 'public').
Be aware that the "idea behind the inline script is that it runs immediately, before any of the external scripts are loaded. This should only be used in rare cases where the script should run before the page is displayed in the browser, such as sizing something to the window or autofuocusing an element in browsers that don't support the "autofocus" attribute." Nate Smith in the derby google group.
Inline scripts should be placed in inline.js, located in the same directory as the application's index.js.
If you require jQuery to do something on the loaded page, following code snipped worked at my side (Firefox, Chrome) in inline.js:
window.onload = function() {
alert($(this));
}