title: jasmine - how to check if an argument exists?
body: <p>good day.</p>
  
  <p>im testing to see function got all her args.</p>
  
  <p>i know what value two of her args must have,</p>
  
  <p>but for the third arg, i just want to test if it exists.</p>
  
  <pre><code>expect(myFunction).toHaveBeenCalledWithMatcher({
      a: 1,
      b: 2,
      c: dont know its val but want it to exist
  });
  </code></pre>
  
  <p>thanks in advance</p>