title: Why for..of does not log the foo value?
body: <p>I am looking at a sample from mozilla documents about JS and here is this snippet that shows that For..of prints 3, 5, 7, but why not "hello"?</p>
  
  <pre><code>let arr = [3, 5, 7];
  arr.foo = "hello";
  
  for (let i in arr) {
     console.log(i); // logs "0", "1", "2", "foo"
  }
  
  for (let i of arr) {
     console.log(i); // logs "3", "5", "7" ?????
  }
  </code></pre>