I need to be able to access strings held in my C# code in JavaScript. To test, I have tried displaying a message box with the C# string in JavaScript (I am using this string literal and the message box as an example scenario):
alert(<%: "TEST" %>);
When this code runs, no message box is displayed. On the other hand, a message box is displayed with this code:
alert(<%: 6 %>);
Why is it that I can use integers but not strings? Is there any way around this?
Thanks.
You need to add quotes around the string; otherwise, the browser sees alert(TEST);, which is incorrect. To prevent cross-site scripting attacks, you also need to properly escape special characters. Calling HttpUtility.JavaScriptStringEncode lets you do both:
alert(<%= HttpUtility.JavaScriptStringEncode("TEST", true) %>);
Note: If this JavaScript snippet appears inside an HTML attribute like onclick, you may need to change <%= to <%: so that the double quotes are also HTML encoded.
Why is it that I can use integers but not strings?
Because you need to put strings in quotes:
alert("<%: "TEST" %>");
The key here, as always, is to look at what the browser actually receives. With your original code, what the browser sees is:
alert(TEST);
...which is trying to use the variable TEST, not a literal string.
Now in the above, I've assumed the string won't have any " in it or other things that aren't valid within a JavaScript string literal. That's not usually a good assumption to make.
If you're using a recent version of .Net or using JSON.Net (see this question for details), you can output the string using a JSON serializer, which will ensure that anything within it that may be problematic is properly encoded/escaped. For instance, with JSON.Net, you might use:
// With JSON.Net
alert(<%: JsonConvert.ToString("TEST") %>);
// With a recent version of .Net
alert(<%: HttpUtility.JavaScriptStringEncode("TEST", true) %>);
The problem is in how this translates into JavaScript:
alert(<%: "TEST" %>);
becomes
alert(TEST);
This is a problem because it assumes there is a variable named TEST that you'd like to display the value of, but most likely, TEST is undefined. What you probably want to do is this:
alert('<%: "TEST" %>');
But since this is MVC 4, you can use the Json.Encode method to be a little cleaner, like this:
alert(<%: Json.Encode("TEST") %>);
Both of thse will translate to
alert('TEST');
This should display a message box with the string 'TEST'.
