How do you access a Dictionary<int, string> in an external Javascript file?

If all you are doing is populating a drop down menu, like most people, then you don't have to worry about that. You can simply build the options right from C# in your Razor view.

Populating a razor dropdownlist from a List<object> in MVC

@Html.LabelFor(m => m.SelectedUserRoleId)
@Html.DropDownListFor(m => m.SelectedUserRoleId, Model.UserRoles)

I that's not your use case, then you can populate your JS array in a slightly different manner.

Razor MVC Populating Javascript array with Model Array

var myArray = [];

@foreach (var d in Model.data)
{
    @:myArray.push("@d");  // for basic data
    //  or
    @:myArray.push(ClassMember1: "@d.ClassMember1", ClassMember2: "@d.ClassMember2"); // for objects
}