title: What does it mean when TsLint says "expected callSignature to have a typedef."
body: <p>I have a function in my code:</p>
  
  <pre><code>networkStop = (action: string = null) =&gt; {
      this.action[action] = false;
      this.net = false;
      this.netd = false;
  }
  </code></pre>
  
  <p>I'm getting a TsLint error saying:</p>
  
  <pre><code>Message 4   TsLint: expected callSignature to have a typedef.
  </code></pre>
  
  <p>Can someone explain what this means?</p>