javascript - Javascript: check if variable is false or if it doesn't exist at all

Question

I want to write a function that uses argument value. If argument does not exist it will use value true by default.

var check = function(truthValue){
    var val = truthValue || true;
    console.log(val);
};

The problem is that if I pass value false to it,as it will still use default value. So how do I check if varible exists and use that value in Javascript?