title: TypeScript error: Type '() => boolean' is not assignable to type 'boolean'
body: <p>If array of <code>user_list.length == 0</code>, the &quot;clear user&quot; button should be disabled. When I am calling <code>this.disabledButton</code> function, it throws an error saying</p>
  <blockquote>
  <p>Type '() =&gt; boolean' is not assignable to type 'boolean'</p>
  </blockquote>
  <p>Below is my logic.</p>
  <pre><code>&lt;section&gt;
       &lt;Button
          id='clear-user'
          disabled={this.disabledButton}
          onClick={this.onClearAllUserButton}&gt;
          {'Clear User List'}
       &lt;/Button&gt;
  &lt;/section&gt;
  </code></pre>
  <p>Function defined:</p>
  <pre><code> private disabledButton= (): boolean =&gt; {
      if (this.props.user_list.length == 0) {
        return true
      }
      return false
    }
  </code></pre>
  <p>Is my calling wrong?</p>