<span class="hljs-keyword">class</span> Car <span class="hljs-keyword">extends</span> Vehicle {
  <span class="hljs-keyword">constructor</span>(<span class="hljs-params">speed, cost</span>) {
    <span class="hljs-keyword">super</span>(speed);

    <span class="hljs-keyword">var</span> c = Symbol(<span class="hljs-string">'cost'</span>);
    <span class="hljs-keyword">this</span>[c] = cost;

    <span class="hljs-keyword">this</span>.intro = <span class="hljs-string">`This is a car runs at
      <span class="hljs-subst">${speed}</span>.`</span>;
  }
}