toHiragana(str) {
    return [...str]
      .map((ch) =>
        ch > '\u30a0' && ch < '\u30f7'
          ? String.fromCharCode(ch.charCodeAt(0) + this.shiftToHiragana)
          : ch
      )
      .join('');
  }
Converts katakana characters to hiragana
@param {String} str Input string