How to use the ts-morph.TypeGuards.isUnionTypeNode function in ts-morph

// TODO: discourage native boolean keyword?
  } else if (TypeGuards.isBooleanKeyword(typeNode)) {
    return ok(booleanType());
    // TODO: discourage native string keyword?
  } else if (TypeGuards.isStringKeyword(typeNode)) {
    return ok(stringType());
    // TODO: discourage native number keyword?
  } else if (TypeGuards.isNumberKeyword(typeNode)) {
    return ok(floatType());
  } else if (TypeGuards.isLiteralTypeNode(typeNode)) {
    return parseLiteralType(typeNode);
  } else if (TypeGuards.isArrayTypeNode(typeNode)) {
    return parseArrayType(typeNode, typeTable, lociTable);
  } else if (TypeGuards.isTypeLiteralNode(typeNode)) {
    return parseObjectLiteralType(typeNode, typeTable, lociTable);
  } else if (TypeGuards.isUnionTypeNode(typeNode)) {
    return parseUnionType(typeNode, typeTable, lociTable);
  } else if (TypeGuards.isIndexedAccessTypeNode(typeNode)) {
    return parseIndexedAccessType(typeNode, typeTable, lociTable);
  } else {
    throw new TypeNotAllowedError("unknown type", {
      file: typeNode.getSourceFile().getFilePath(),
      position: typeNode.getPos()
    });
  }
}

return parseTypeReference(typeNode);
  } else if (TypeGuards.isNullLiteral(typeNode)) {
    return NULL;
  } else if (TypeGuards.isBooleanKeyword(typeNode)) {
    return BOOLEAN;
  } else if (TypeGuards.isStringKeyword(typeNode)) {
    return STRING;
  } else if (TypeGuards.isNumberKeyword(typeNode)) {
    return FLOAT;
  } else if (TypeGuards.isLiteralTypeNode(typeNode)) {
    return parseLiteralType(typeNode);
  } else if (TypeGuards.isArrayTypeNode(typeNode)) {
    return parseArrayType(typeNode);
  } else if (TypeGuards.isTypeLiteralNode(typeNode)) {
    return parseObjectLiteralType(typeNode);
  } else if (TypeGuards.isUnionTypeNode(typeNode)) {
    return parseUnionType(typeNode);
  } else if (TypeGuards.isIndexedAccessTypeNode(typeNode)) {
    return parseTypeNode(parseIndexedAccessType(typeNode));
  } else {
    throw new Error("unknown type");
  }
}