You cannot store a value in ViewBag from javascript. ViewBag is a server side concept and exists only on the server. Javascript runs on the client. As far as storing some data from ViewBag into a javascript variable is concerned you could use the following:

<script type=text/javascript>

    var foo = @Html.Raw(Json.Encode(ViewBag.FooBar))

</script>

Now this being said I always advice people against using ViewBag/ViewData in ASP.NET MVC. I recommend using strongly typed view and view models. So your code will look like this:

@model MyViewModel

<script type=text/javascript>

    var foo = @Html.Raw(Json.Encode(Model))

</script>