title: What does this for in loop mean and why it just prints out 0 and 1?
body: <p>It is so funny that the following for in loop prints out 0 and 1. 
  Anybody helps to explain: 1 what does it mean for variable in a lambda function? 2. why prints out 0 and 1? (I changed the ['a', 'b'] to [1,2], it still prints 0 and 1.</p>
  
  <pre><code>for (var f in d = function(){}, ['a', 'b']){console.log(f)}
  </code></pre>
  
  <p>prints out</p>
  
  <pre><code>0  
  1 
  </code></pre>