title: What is fesm2015 and how it relates to esm2015?
body: <p>I know that esm2015 refers to ecmascript modules described by the <a href="https://www.ecma-international.org/ecma-262/6.0/#sec-modules" rel="noreferrer">ecmascript 2015 specification (modules section)</a></p>
  <p>In some libraries under <code>node_modules/</code> I see, next to the directory <code>esm2015/</code>, another one  called <code>fesm2015/</code>.</p>
  <ul>
  <li><strong>What is fesm2015</strong>?</li>
  <li>Does it <strong>extend esm2015</strong>?</li>
  <li>Is there any <strong>specification describing</strong> it?</li>
  </ul>