This should cover all major types:
public class ViewBagUtils
{
public static string ToJavascriptValue(dynamic val)
{
if (val == null) return "null";
if (val is string) return val;
if (val is bool) return val.ToString().ToLower();
if (val is DateTime) return val.ToString();
if (double.TryParse(val.ToString(), out double dval)) return dval.ToString();
throw new ArgumentException("Could not convert value.");
}
}
And in your .cshtml file inside the <script> tag:
@using Namespace_Of_ViewBagUtils
const someValue = @ViewBagUtils.ToJavascriptValue(ViewBag.SomeValue);
Note that for string values, you'll have to use the @ViewBagUtils expression inside single (or double) quotes, like so:
const someValue = "@ViewBagUtils.ToJavascriptValue(ViewBag.SomeValue)";