Question

I’m not sure what is wrong with this binding statement.

css: { 'error': responseStatus < 0, 'success': responseStatus > 0 }

Here is a working example: http://jsfiddle.net/awoehler/zEn3t/

My server will respond JSON response containing either response.MESSAGE or response.ERROR and I want to apply a class based on which response I receive.

viewUserModel = {           
    serverResponse: ko.observable(),
    responseStatus: ko.observable( 0 ),  // -1, 0, or 1 
    displayServerResponse: ko.observable( false )
//....
}

Answer 1

When referring to observables in expression, they must be called as function to get their value. So changing this:

css: { 'error': responseStatus < 0, 'success': responseStatus > 0 }

to this:

css: { 'error': responseStatus() < 0, 'success': responseStatus() > 0 }

should make your code work.