title: Javascript: check if variable is false or if it doesn't exist at all
body: <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>