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

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?

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Answer

Use

if (typeof val ===
            'undefined') val = true;