*keys() {

        /*
         * Because this iterator just returns a subset of the information
         * returned by the `entries()` iterator, we just use `entries()`
         * and take the information we care about.
         */
        for (const [key] of this.entries()) {
            yield key;
        }
    }
Create an iterator that returns each key in the hash map.
@returns {Iterator} An iterator on the hash map keys.