Check out example codes for "How to execute a script after the c# function executed". It will help you in understanding the concepts better.

Code Example 1

BY LOVE,  At the place of swal , you can use alert() function also.

int NumberOfRows = oRSEntities.Menu_Item.Count();
string message = $" {NumberOfRows} Records saved successfully.";
string script = "window.onload = function(){ swal('";
script += message;
script += "')};";
ClientScript.RegisterStartupScript(this.GetType(), "SuccessMessage", script, true);

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