title: How do I add TypeScript types to a JavaScript module without switching to TypeScript?
body: <p>I guess the title is somewhat confusing, but I wasn't able to put it in a more concise way in a short title. However, this is what I want to do:</p>
  
  <ul>
  <li>I have an npm module that is written in JavaScript, not TypeScript.</li>
  <li>Some of the users of this module are using TypeScript, so it would be nice if I could provide type annotations for my JavaScript code.</li>
  <li>But I do not want to switch to TypeScript, but I want to stick with pure JavaScript.</li>
  </ul>
  
  <p>So, what I am looking for is some kind of an external type declaration manifest, or something like that.</p>
  
  <p>Is this possible? If so, how?</p>
  
  <p>Please bear with me if this is a naive question to TypeScript users, but I have absolutely no experience with TypeScript, and hence I do not know what to search the web for. Maybe if someone can push me into the right direction by giving me a name for what I am looking for, I am able to find a solution on my own.</p>