pan class="hl opt">=</span> <span class="hl kwa">function</span><span class="hl opt">(</span>arg<span class="hl opt">,</span> cb<span class="hl opt">) {</span>
      <span class="hl kwa">if</span> <span class="hl opt">(!</span>arg<span class="hl opt">) {</span>
        <span class="hl kwa">return</span> <span class="hl kwd">cb</span><span class="hl opt">(</span><span class="hl str">'null file'</span><span class="hl opt">);</span>
      <span class="hl opt">}</span> <span class="hl kwa">else</span> <span cla