title: How to create a function with return types in TypeScript?
body: <p>I'm using Aurelia and TypeScript for my web application. 
  I created following 'user_checking' function to check users and status as follows.</p>
  
  <pre><code>user_checking(registerdata, tested_users) {
    return registerdata.userid &amp;&amp; typeof tested_users[registerdata.userid] == 'undefined';
  }
  </code></pre>
  
  <p>This function has two parameters and also a return type. I don't see any kind of error there. But when I run the application, I'm getting following error.</p>
  
  <blockquote>
    <p>error TS2363: The right-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type. [07:46:58] gulp-notify: [Error running Gulp] Error: </p>
  </blockquote>
  
  <p>Can anyone please tell me what's the error in this function. </p>