title: Filter array by letter
body: <p>Pardon me if this has been asked but I searched and didn't find the specific implementation of my problem.</p>
  
  <p>Anyway, I'm currently learning high-order functions in JavaScript and I'm at the array.prototype.filter function. I understand its purpose (as its name so conveniently implies) but I'm having trouble implementing this:</p>
  
  <p>So, say I have an array of names, like this:</p>
  
  <pre><code>var names = ["Anna", "Bob", "Charles", "Daniel",
           "Allison", "Beatrice", "Cindy", "Fiona"];
  </code></pre>
  
  <p>And then I want to, say, filter that array by all entries that start with the letter "A". I'm aware of the fact that I could do this:</p>
  
  <pre><code>var filteredNames = names.filter(function(word) {
      return word[0] === "A";
  });
  </code></pre>
  
  <p>And that would work just fine. But say I want to be less explicit and make it more adaptable to more situations. Say I want to program the filtering so that I can say "return only the entries that have the letter "x" at index [y]", for example "return only the entries that have the letter "F" at index[3].</p>
  
  <p>How can I achieve that?</p>