n class="nx">callback</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">if</span> <span class="p">(</span><span class="k">this</span><span class="p">.</span><span class="nx">index</span> <span class="o">==</span> <span class="kc">undefined</span><span class="p">)</span> <span class="k">this</span><span class="p">.</span><span class="nx">index</span> <span class="o">=</span> <span class="mi">0</span>
  <span class="kd">var</span> <span class="nx">buf</span> <span class="o">=</span> <s