ASP.NET Core MVC with Razor - how can I repopulate a view with new model data?

You can implement a Javascript function that will send a request, like

function sendRequest(type, url, callback, async, params) {
    if (async !== false) async = true;
    var xhttp = new XMLHttpRequest();
    xhttp.onreadystatechange = callback;
    xhttp.open(type, url, async);
    xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xhttp.send(params);
}

type is the request type. It could be GET or POST, etc.

url is the path to the end point you want to send request to.

callback is a function to be executed when the request is responded to

async determines whether the request is to be asynchronous. If true, then Javascript will not wait for the response, but proceed with its work instead and when the response is received, then the callback will be executed.

params are the parameters to be passed.

The inner block of the function is an AJAX request to be sent. You can see it documented all over the place on the internet.

Now, you mentioned a form and a button. If this button happens to be a submit, which submits the form, then you can add a submit event, like

yourForm.addEventListener(
    "submit",
    function(event)
    {
        event.preventDefault();
        sendRequest("POST", yoururl, function() {
            //do something
        }, true, yourparams);
    },
    false
);

In the above you will need to find your form element, that is what yourForm represents, provide the URL, that's what yoururl represents and pass yourparams, whatever they may be.

I am not sure if this answers your question, but as I understand it:

• you have a model bound to your page, which has multiple inputs and possible return fields e.g.:

What you could do is:

assuming you have 2 models: User and FruitOrder

Create a ViewModel:

public class UserAndFruitVm
{
    public User Userdata {get; set; }
    public FruitOrder {get; set;}
}

In your controller change the Index method

namespace YourProject.Controllers
{
    public class FruitController(ApplicationDbContext context) : Controller
    { 
        private readonly ApplicationDbContext _context = context;

         public IActionResult Index(UserAndFruitVm modelData)
         { 
            if(modelData.UserData.Id != null)
            {
                modelData.UserData = _context.User.Find(id);  // gets all the userdata
            }
          }
          return View(modelData);
      }
 }  

And in your Index View change accordingly

@model YourProject.ViewModels.UserAndFruitVm

<form id="frmIndex" asp-action="Index" asp-controller="Fruit" enctype="multipart/form-data" method="post">

<div>
  <label> type user id </label>
  <input asp-for="User.Id"/>
</div>
<div>@html.displayfor(model => Model.User.Name)</div>
<div>@html.displayfor(model => Model.User.Description)</div>
<div>@html.displayfor(...)</div>
<div><input type=submit value="Get User"/>


<input asp-for="FruitOrder.Id" />
<input asp-for="FruitOrder.Quantity"
<input asp-for="FruitOrder.Type"
<input asp-for=(..other fruitorderfields.../)

</form>

<div>
<button type="submit" form="formIndex" asp-action="Create" asp-controller="Fruit">Save Order</button>
</div>