Develop Reference

How do I write to an HTML file using expressjs

Ok, this sounds crazy, but let me explain. I want a user to be able to push a button, and then the html file is edited in such a way that it a NEW p element is created, and stays there PERMENENTLY (eg on reload and for EVERYONE to see). Any way on how I could do that.
Again I am using node.js/express, and html/js/css.

It really depends upon exactly what you're doing.
Conceptually, you would probably use some sort of template system for generating your HTML files (like Jade, Dust, EJS, Handlebars, Nunjucks, PUG, etc..) and then you store some state in a database and use a query from the database as input to the template whenever you render a particular page.
So, when you add an item to the database, it will show up in all future renders of the page.
So, your button would trigger a form post to your server which would add an item to the database and the response from the form post would be a rendering of the page after adding the item to the database. All future renders of the page from all users would also show those same items in the rendered page.

Django modify model instance from HTML

I want to know if it is possible to directly modify and save a model instance in the HTML Template and not via a view and extra URL.
My user has a Boolean Property, I want to display it as a toggle button on the website and the user should be able to toggle it on or off without leaving the website or reloading it.
class User(AbstractBaseUser, PermissionsMixin):
...
autoplay_enabled = models.BooleanField(default=True)
...
Is this possible without an extra view or form?
Basically I just need to set
request.user.autoplay_enabled = False (or True)
and then save() it
If I can't modify the object directly in the HTML template is it at least possible to just execute a function I have defined somewhere in my Python code, without having the need to create a new view?

What you're asking doesn't make any sense at all. HTML is just sent to the browser for display. You can't do anything "from HTML" without making some kind of request to the server, and the server won't do anything unless that request is received by some code - eg a view connected to a URL.
If you want something to happen without refreshing the page, you need to use Ajax. You can use a simple Ajax POST to a view that toggles the value and saves it - it only needs a dozen lines of code between front and back end.

HTML/Javascript: How to control refresh action without re-submit the form

We are trying to implement a web page that each time of page refreshing will not result in the form resubmit, how to achieve that? Is there any Javascript code or HTML can make it WITHOUT external javascript library(jquery, dojo or extJs)
The reason of such design is that the form is going to tie an unique relation to current data with means cannot do it twice but for security reason we have to use POST instead of GET, also after the action we still want to preserve user the right to do similar action on the same page to another relation. so how to avoid a consequence like that?
Thanks.

Suppose that the action to the form submits it to submit_form.php. That file can handle the data and do whatever it needs to do. Then in it's response, it can redirect the browser to a separate page (you'll have to look up the exact method of how to do this depending on what language you write your POST handler in). This separate page can show the results of the form submit using session variables or some other method.

How do you call a Django view with parameters from JavaScript?

I'm looking to capture and save the state of a page (Django template + backend) after the user makes some modifications (through JQuery) to the appearance of the page. Now that I've gotten hold of the innerHTML using a JS variable, I need to send it over to the Django view that will do the saving. How do I call this Django view and pass it the JS variable?
P.S: First ever question on stackoverflow, please let me know if the question isn't clear or is improperly formatted.

Handiest way to get started is to first make a proper form and a django view that reacts to it ("request.post"). The form should have fields for whatever you're changing in the page.
Next up, submit that form's variables from your page with jquery.ajax.
So the idea is to isolate the various problems:
What should be the form parameters?
Get a view running that makes the actual changes.
Get the javascript working.

Javascript function execution on link click?

I have a link, that when a user clicks on it, it loads a different page as normal but also executes a JS function that autofills a specific text-box on that different page. Is it better practice to use Jquery or Javascript to do this? How can I do this using either one of them?

You can't do this from the source page.
It's a security feature. Imagine if you wrote a JS function that went to an online banking page and auto-filled a bank transfer using the user's current cookie. That's why you can't.
If you control the other page then the sequence you can use is:
Save data to the server;
Go to the new page with a JS redirect;
The new page is loaded from the server;
While loading th epage the data that was saved from the server is retrieved and used to populate the text box.
So it can be done from the server but only if you save it there. The only way of doing that is using Ajax.
An alternative approach is:
Instead of a JS redirect, submit the page back to the server;
The server saves whatever data it needs to;
The server sends back an HTTP redirect to the new page;
The new page uses the saved data to construct the new page with the populated text box.

At the end of the script add return false;. This will make the page run the script without redirecting the page.
Edit: (after saw your edition).
Is it better practice to use Jquery or Javascript to do this? How can I do this using either one of them?
jQuery is a javascript library, this it doesn't matter if you use plain javascript or use jquery as long as you happy with the result.
And about what you say that you successfully manipulated a page fro the redirecter page... I don't see how it possible.