ckfn</var>) is false, throw a <emu-val>TypeError</emu-val> exception.</li><li>Let <var>entries</var> be the List that is <var>map</var>.<var class="field">[[MapData]]</var>.</li><li>Let <var>accumulator</var> be <emu-val>undefined</emu-val></li><li>Let <var>first</var> be <emu-val>true</emu-val>.</li><li>If <var>initialValue</var> is present, then<ol><li>Set <var>accumulator</var> to <var>initialValue</var></li></ol></li><li>For each Record { <var class="field">[[Key]]</var>, <var class="field">[[Value]]</v