dxSelectBox("instance") is undefined

I have two DevExtreme selection boxes A and B. If the selected option of Box A changes, I'd like to select the first option in box B in certain cases. The javascript of "onvaluechanged" ob box A looks as follows:

function selectBoxA_onValueChanged(e) {
    document.cookie = "selectedA_value=" + e.value;
    if ("Option2" !== e.value)
    {
        var selectBoxB = $("#selectBoxB").dxSelectBox('instance');
        var currentValue = selectBoxB.option('value');
        if (currentValue === "OptionB4") {
            var dataSource = selectBoxB.option("dataSource");
            if (dataSource.length > 0) {
                selectBoxB.option("value", dataSource[0]);
            }
            else {
                selectBoxB.option("value", null);
            }
        }
    }
    window.location.reload();
}

Unfortunately dxSelectBox('instance') returns 'undefined'. In the devTools console, when trying to access it I get an object (using $("#selectBoxB ").dxSelectBox("instance")). The razor setup looks as follows:

<div class="selectBox">
    <h6 class="title">SelectionBox B</h6>
    @(Html.DevExtreme().SelectBox()
        .ID("selectBoxB")
        .DataSource(Model.SelectBoxBContent)
        .Value(Model.SelectedBOption)
        .OnValueChanged("selectBoxB_onValueChanged")
    )
</div>

Additional explanation of the code:

• Option2 of box A adds more options in box B
• hence when "moving away" from that option, I have to change box B selection if an option is selected there that is not available in the other options of box A (OptionB4 in this case)
• this is code of an existing project where I'm new in, trying to adapt to the existing style/"how it's done"
• any suggestions to do this differently/better are welcome
• I'm new to DevExtreme

To resolve the issue where dxSelectBox('instance') returns undefined and improve the implementation, consider the following approach:

Key Issues Identified:

1. Instance Undefined: The page reloads immediately after changing the selection, potentially causing the DevExtreme instance to be unavailable.

2. Redundant Client-Side Logic: The client-side attempt to adjust the selection is overridden by the server reload.

3. Server-Side Control: The existing code uses a cookie to reload the page, indicating reliance on server-side rendering.

Solution:

Move the logic to the server-side where the server handles validating and setting the correct value for SelectionBox B based on the cookie value. This avoids client-side instance access issues and ensures consistency.

Step-by-Step Implementation:

1. Remove Client-Side Adjustment:
   Eliminate the code that tries to modify SelectionBox B's value client-side before reloading.

   function selectBoxA_onValueChanged(e) {
       document.cookie = "selectedA_value=" + e.value;
       window.location.reload();
   }
   

   Server-Side Validation:
   In your server code (e.g., Razor Page or Controller), after retrieving the selectedA_value cookie:

   • If Selection A is not "Option2", check if the current Selection B value is "OptionB4".

   • If so, set Selection B's value to the first available option in its data source.

   Example Razor/Server-Side Code:

   public ActionResult Index() {
       var model = new MyViewModel();
       var selectedA = Request.Cookies["selectedA_value"]?.Value;
   
       // Determine the correct data source for SelectionBox B
       model.SelectBoxBContent = selectedA == "Option2" ? GetOption2Data() : GetDefaultData();
   
       // Check if current SelectedBOption is valid
       if (selectedA != "Option2" && model.SelectedBOption == "OptionB4") {
           // Set to the first item if available
           model.SelectedBOption = model.SelectBoxBContent.FirstOrDefault()?.Value;
       }
   
       return View(model);
   }
   

2. Update Client-Side Reload (Optional):
   If you want to avoid a full page reload, consider using client-side data source updates instead. However, this requires adjusting how data is fetched (e.g., via AJAX).

Explanation:

• Server-Side Handling: By validating the Selection B value on the server, you ensure the correct value is rendered before the page is sent to the client, eliminating the need for client-side manipulation.

• Eliminates Instance Issues: Removing client-side access to the DevExtreme instance circumvents the undefined problem entirely.

• Consistency: The server and client states remain synchronized through cookie-based reloading, adhering to the existing project structure.

This approach aligns with the existing pattern of using cookies and server-side rendering, ensuring robustness and simplicity.