title: Angular2 return true/false with subscription
body: <p>I need to check if user exists in the database and return true if so. I have a function (i am using firebase by the way):</p>
  
  <pre><code>  checkIfUserExists(login: string): boolean {
      this.af.database.list('/users', {
        query: {
          orderByChild: 'login',
          equalTo: login
        }
      }).subscribe(response =&gt; {
        if(response.length === 0 ) return true
      })
      return false
    }
  </code></pre>
  
  <p>The problem is that the function always returns false. It does not wait for checking subscription. Is there some way to fix it?</p>