title: Type of lambda expressions in typescript
body: <p>I'm trying to assign a lambda expression which returns a string, to a property, which, according to the API description, accepts the types (String | Object[] | Function).</p>
  
  <pre><code> this._popupTemplate.content = (feature) =&gt; {
              var template: string; 
              //....
              return template;    
        }
  </code></pre>
  
  <p>It seems to be working, however, webstorm says "</p>
  
  <blockquote>
    <p>assigned expression of type (feature:any) => string is not assignable
    to type string</p>
  </blockquote>
  
  <p>"</p>
  
  <p>So I tried using a type assertion: <code>&lt;string&gt;(feature) =&gt; {...}</code> which seems to have no effect. How can I satisfy webstorm (without suppression the information)?</p>