title: Getting all indexes of elements fulfilling a certain condition using Underscore
body: <p>I want to retrieve all the indexes of the elements in an Array which fulfills a certain condition. For ex if I have an array like:</p>
  
  <pre><code>var countries = ["India", "Australia", "United States", "Brazil"];
  </code></pre>
  
  <p>And I would want to get the indexes of those elements whose string length is more than 5, I should get back an array like</p>
  
  <pre><code>[2, 3]
  </code></pre>
  
  <p>Or even an object like :</p>
  
  <pre><code>{
    2: "United States",
    3: "Australia"
  }
  </code></pre>
  
  <p>Is there any inbuilt/native function in JavaScript or underscore.js that I can leverage?</p>