Possible to access MVC ViewBag object from Javascript file?

Is it possible to do the following from a javascript file in an MVC application?

$(function(){
   alert(@ViewBag.someValue);
}

Currently it throws the error:

> reference to undefined XML name @ViewBag

I don't believe there's currently any way to do this. The Razor engine does not parse Javascript files, only Razor views. However, you can accomplish what you want by setting the variables inside your Razor view:

<script>
  var someStringValue = '@(ViewBag.someStringValue)';
  var someNumericValue = @(ViewBag.someNumericValue);
</script>
<!-- "someStringValue" and "someNumericValue" will be available in script -->
<script src="js/myscript.js"></script>

As Joe points out in the comments, the string value above will break if there's a single quote in it. If you want to make this completely iron-clad, you'll have to replace all single quotes with escaped single quotes. The problem there is that all of the sudden slashes become an issue. For example, if your string is "foo \' bar", and you replace the single quote, what will come out is "foo \\' bar", and you're right back to the same problem. (This is the age old difficulty of chained encoding.) The best way to handle this is to treat backslashes and quotes as special and make sure they're all escaped:

  @{
      var safeStringValue = ViewBag.someStringValue
          .Replace("\\", "\\\\")
          .Replace("'", "\\'");
  }
  var someStringValue = '@(safeStringValue)';

Not in a JavaScript file, no.
Your JavaScript file could contains a class and you could instantiate a new instance of that class in the View, then you can pass ViewBag values in the class constructor.

Or if it's not a class, your only other alternative, is to use data attributes in your HTML elements, assign them to properties in your View and retrieve them in the JS file.

Assuming you had this input:

<input type="text" id="myInput" data-myValue="@ViewBag.MyValue" />

Then in your JS file you could get it by using:

var myVal = $("#myInput").data("myValue");

in Html:

<input type="hidden" id="customInput" data-value = "@ViewBag.CustomValue" />

in Script:

var customVal = $("#customInput").data("value");

In order to do this your JavaScript file would need to be pre-processed on the server side. Essentially, it would have to become an ASP.NET View of some kind, and script tags which reference the file would essentially be referencing a controller action which responds with that view.

That sounds like a can of worms you don't want to open.

Since JavaScript is client-side, why not just set the value to some client-side element and have the JavaScript interact with that. It's perhaps an additional step of indirection, but it sounds like much less of a headache than creating a JavaScript view.

Something like this:

<script type="text/javascript">
    var someValue = @ViewBag.someValue
</script>

Then the external JavaScript file can reference the someValue JavaScript variable within the scope of that document.

Or even:

<input type="hidden" id="someValue" value="@ViewBag.someValue" />

Then you can access that hidden input.

Unless you come up with some really slick way to actually make your JavaScript file usable as a view. It's certainly doable, and I can't readily think of any problems you'd have (other than really ugly view code since the view engine will get very confused as to what's JavaScript and what's Razor... so expect a ton of <text> markup), so if you find a slick way to do it that would be pretty cool, albeit perhaps unintuitive to someone who needs to support the code later.

Use this code in your .cshtml file.

 @{
    var jss = new System.Web.Script.Serialization.JavaScriptSerializer();
    var val = jss.Serialize(ViewBag.somevalue); 
    }

    <script>
        $(function () {
            var val = '@Html.Raw(val)';
            var obj = $.parseJSON(val);
            console.log(0bj);
    });
    </script>

Create a view and return it as a partial view:

public class AssetsController : Controller
{
    protected void SetMIME(string mimeType)
    {
        this.Response.AddHeader("Content-Type", mimeType);
        this.Response.ContentType = mimeType;
    }

    // this will render a view as a Javascript file
    public ActionResult GlobalJS()
    {
        this.SetMIME("text/javascript");
        return PartialView();
    }
}

Then in the GlobalJS view add the javascript code like (adding the //

I noticed that Visual Studio's built-in error detector kind of gets goofy if you try to do this:

var intvar = @(ViewBag.someNumericValue);

Because @(ViewBag.someNumericValue) has the potential to evaluate to nothing, which would lead to the following erroneous JavaScript being generated:

var intvar = ;

If you're certain that someNemericValue will be set to a valid numeric data type, you can avoid having Visual Studio warnings by doing the following:

var intvar = Number(@(ViewBag.someNumericValue));

This might generate the following sample:

var intvar = Number(25.4);

And it works for negative numbers. In the event that the item isn't in your viewbag, Number() evaluates to 0.

No more Visual Studio warnings! But make sure the value is set and is numeric, otherwise you're opening doors to possible JavaScript injection attacks or run time errors.

In controllers action add:

 public ActionResult Index()
        {
            try
            {
                int a, b, c;
                a = 2; b = 2;
                c = a / b;
                ViewBag.ShowMessage = false;
            }
            catch (Exception e)
            {
                ViewBag.ShowMessage = true;
                ViewBag.data = e.Message.ToString();
            }
            return View();    // return View();

        }

in Index.cshtml
Place at the bottom:

<input type="hidden" value="@ViewBag.data" id="hdnFlag" />

@if (ViewBag.ShowMessage)
{
    <script type="text/javascript">
       
        var h1 = document.getElementById('hdnFlag');

        alert(h1.value);
      
    </script>
    <div class="message-box">Some Message here</div>
}

For CoreMVC 3.1, that would be,

@using Newtonsoft.Json
var listInJs =  @Html.Raw(JsonConvert.SerializeObject(ViewBag.SomeGenericList));

onclick="myFunction('@ViewBag.MyValue')"