javascript:switch-caseで不等号

switch-caseで不等号による条件分岐。

var answer=prompt("How old are you?")
switch(true){
    case answer<20:
        console.log("you are under " + answer)
        break;
    case answer<30:
        console.log("you are under "+ answer)
        break;
    case answer<40:
        console.log("you are under "+ answer)
        break;
    default:
        console.log("have fun")
}