masking a string

maskValue.ts

/**
   * Replaces value inside {value} with a spaced representation of the masked array
   * and then replaces the Nth caret with the provided key, returning the new masked
   * value.
   * @param {(RegExp | string)[]} mask
   * @param {string} key
   * @param {number} caretPos
   * @param {string} value
   * @returns <{str: string, pos: number}>
   */
  maskValue(mask: (RegExp|string)[], key: string, caretPos: number, value?: string) {
    if (caretPos < 0 || caretPos > mask.length || value.length > mask.length) return {str: value, pos: 0};
    if (!value) value = mask.map(k => k instanceof RegExp ? '0' : k).join('');

    const returnValue = (str: string, pos: number) => { return {str, pos}; };
    if (!key) return returnValue(value, 0);

    if (!(mask[caretPos] instanceof RegExp) && (mask[caretPos + 1] && (<RegExp>mask[caretPos + 1]).test(key)))
      return returnValue(value.substr(0, caretPos) + mask[caretPos] + key + value.substr(caretPos + 2), caretPos + 2);
    if ((mask[caretPos] instanceof RegExp && !(<RegExp>mask[caretPos]).test(key)) ||
      (!(mask[caretPos] instanceof RegExp) && mask[caretPos] !== key))
      return returnValue(value.substr(0, caretPos), caretPos);

    return returnValue(value.substr(0, caretPos) + key + value.substr(caretPos + 1), caretPos + 1);
  }