props: {
    alertType: {
       validator: function (value) {
           return [ "success", "info", "warning", "danger" ].indexOf(value) >= 0;
       },
       default: "danger"
    },
// Some more things
 computed: {
     classes: { //Compute the correct classes for the alert type
         var classesObj ={
             'alert-dismissible': this.dismissable
         };
         classesObj["alert-"+this.alertType]=true; //Problem if invalid
         return classesObj;
     }
 }