Express.JS, mongoose, Joi

const Users = mongoose.model('users', new mongoose.Schema({
  username: {
    type: String,
    required: true,
    unique: true
  },
  color:{
    type: String,
    required: true,
    unique: true
  },
}, { timestamps: true }));

const main = async (req, res, callback, {
  internalCode = -1,
  joiSchema = null
}) => {
  const params = { ...req.body, ...req.query };
  let data;
  try {
    let value = {};
    if (joiSchema) {
      const { error: joiError, value: joiValue = {} } = await joiSchema.validate(params);
      if (joiError) {
        console.error(joiError);
        throw `Joi error: ${JSON.stringify(joiError)}.`;
      } else {
        value = { ...joiValue };
      }
    }
    await connectDB();
    data = await callback(value);
  } catch (err) {
    const { code, index, keyPattern, keyValue, message } = err;
    data = { code, index, keyPattern, keyValue, message };
    console.error('ERROR', data);
  } finally {
    res.json(data);
  }
}

const joiPaginationSchema = Joi.object({
  skip: Joi.number().default(0),
  limit: Joi.number().default(PAGE_LIMIT)
});

const joiSchemaFromModel = model => {
  const obj = {};
  const schemaObj = model.schema.obj;
  Object.keys(schemaObj).forEach(field => {
    const fieldType = schemaObj[field].type;
    if (fieldType === String) {
      obj[field] = Joi.string();
    }
  });
  return Joi.object(obj);
}

const joiModelSchema = model => joiPaginationSchema.concat(joiSchemaFromModel(model));

const paginationAggregation = ({
  skip: $skip,
  limit: $limit
}) => [
  { $skip },
  { $limit }
];

const GEThandlerFromModel = model => async (req, res) => {
  const joiSchema = joiModelSchema(model);
  main(req, res, async (values = {}) => {
    const { skip, limit } = values;
    const $match = {};
    const aggregation = [
      { $match },
      ...paginationAggregation({ skip, limit })
    ];
    return await model.aggregate(aggregation);
  }, { joiSchema, internalCode: 52 });
}

const POSThandlerFromModel = model => async (req, res) => {
  const joiSchema = joiModelSchema(model);
  main(req, res, async (values = {}) => {
    return await model.create(values);
  }, { joiSchema, internalCode: 52 });
}

const mapMethods = {
  GET: GEThandlerFromModel,
  POST: POSThandlerFromModel
}

const RESTAPIHandler = model => async (req, res) => {
  mapMethods[req.method](model)(req, res);
}

app.all('/users', RESTAPIHandler(Users));