t</var>) is not Object, throw a <emu-val>TypeError</emu-val> exception.</li><li>If <var>set</var> does not have a <var class="field">[[SetData]]</var> internal slot, throw a <emu-val>TypeError</emu-val> exception.</li></ol></li><li>If <var>separator</var> is <emu-val>undefined</emu-val>, let <var>sep</var> be the single-element String <code>","</code>.</li><li>Else, let <var>sep</var> be ?&nbsp;ToString(<var>separator</var>).</li><li>Let <var>R</var> be the empty String.</li><li>Let <var>k</var> be <emu-val