<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>; } }