title: Code coverage on multiple files
body: <p>I have a application in node.js using mocha framework .
  I have two javascript source files ,for which I want to take the code coverage (viz a.js and b.js).
  I am using istanbul for this purpose .</p>
  
  <p>Here the problem is I am not getting how to take code coverage for multiple files .
  I am using the following format :</p>
  
  <pre><code>istanbul cover node_modules/mocha/bin/_mocha a.js 
  
  
  istanbul cover node_modules/mocha/bin/_mocha a.js b.js 
  </code></pre>
  
  <p>but unfortunately both the commands are giving the same code coverage, I think its taking only a.js code . Is there any solution for finding the code coverage for multiple files?</p>