lass="secnum">3.13</span> Map.prototype.update(<var>key</var>, <var>callback</var> [, <var>thunk</var>])</h1>
            <emu-import href="./map/prototype-update.html"><emu-note type="editor"><span class="note">Editor's Note</span><div class="note-contents">
  This code serves as placeholder until formal spec is written for this method.
</div></emu-note><pre><code class="javascript hljs"><span class="hljs-keyword">function</span> <span class="hljs-title function_">update</span>(<span class="hljs-params">ke