How to pass a value to razor variable from javascript variable?

You can’t. and the reason is that they do not “live” in the same time.
The Razor variables are “Server side variables” and they don’t exist anymore after the page was sent to the “Client side”.

When the server get a request for a view, it creates the view with only HTML, CSS and Javascript code. No C# code is left, it’s all get “translated” to the client side languages.

The Javascript code DOES exist when the view is still on the server, but it’s meaningless and will be executed by the browser only (Client side again).

This is why you can use Razor variables to change the HTML and Javascript but not vice versa. Try to look at your page source code (CTRL+U in most browsers), there will be no sign of C# code there.

In short:

  1. The server gets a request.

  2. The server creates or “takes” the view, then computes and translates all the C# code that was embedded in the view to CSS, Javascript, and HTML.

  3. The server returns the client side version of the view to the browser as a response to the request. (there is no C# at this point anymore)

  4. the browser renders the page and executes all the Javascript

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