Javascript: check if variable is false or if it doesn't exist at all
<p>I want to write a function that uses argument value. If argument does not exist it will use value true by default. </p>

<pre><code>var check = function(truthValue){
    var val = truthValue || true;
    console.log(val);
};
</code></pre>

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