u-clause id="Map.keyBy">
            <h1><span class="secnum">4.1</span> Map.keyBy( <var>iterable</var>, <var>callbackfn</var>)</h1>
            <emu-import href="./map/key-by.html"><p>When the <code>keyBy</code> method is called, the following steps are taken:</p><emu-alg><ol><li>Let <var>C</var> be the <emu-val>this</emu-val> value.</li><li>If IsConstructor(<var>C</var>) is <emu-val>true</emu-val><ol><li>Let <var>map</var> be ?&nbsp;Construct(<var>C</var>)</li></ol></li><li>Else,<ol><li>Throw a <emu-val>T