Why typeof not working inside if statement

function myDom(){
    var message = document.getElementById('show');
    var xx = document.getElementById('demo').value;
    try{
        if(xx === "") throw "can not be empty";
        if(typeof xx === "number") throw "can not be number";
        //if(xx < 100) throw "less than hundreds!"
   }catch(err){
       message.innerHTML = "input " + err;
   }
}