If statement in a switch

I am working on a question for a course for learning javascript. I am running into trouble trying to add an if statement inside of a switch. I currently have:

var user = prompt("Are you ready for battle?!").toUpperCase();
switch(user) {
    case'YES':
        if(YES && NO) {
           console.log("Great, let's do it.");
        } else {
            console.log("Bye");
        }
        console.log("Great! It will be a long exciting battle.");
        break;  
    case'NO':
        console.log("Come back when you are ready.");
        break;
    case'MAYBE':
        console.log("Go away. This is only for the brave");
        break;
        default:
        console.log("You obviously do not belong here. It was a simple yes/no question.")
}

The question is this:

Add some if/else statements to your cases that check to see whether one
condition and another condition are true, as well as whether one condition 
or another condition are true. Use && and || at least one time each.

The error I am getting is this: ReferenceError: YES is not defined

What can I put in the if's condition to make this work or how do I define YES?