Node js - функция для возврата массива объектов, прочитанных из базы данных sequelize

Вот что нужно сделать, чтобы получить ожидаемый результат:

function getInvoicesCount() {
    ...
    return  db.userInvoices.findAll({ //<-------- 1. First add return here
        ...
    }).then(result => {
        ...
        return promiseInvoices; //<----------- 2. Put this line inside the then
    });
    // return promiseInvoices; //<----------- Remove this line from here    
}

getInvoicesCount().then(data => {
    console.info(data); // <------- Check the output here
})

Explanation for this :

To get .then when you can function , function should return promise , but in you case you are just returning a blank array ,

As per the sequlize doc , db.userInvoices.findAll returns the promise so all you need to do is add return before the this function , First step is done

You were return promiseInvoices; at wrong place , why ? , coz at that you will never get the result , as the code above it run in async manner as it is promise , so you will always get the balnk array , to get the expected result you should return it from db.userInvoices.findAll's then function as shown above in code

Вы должны вернуть результаты, чтобы получить к нему доступ в цепочке.

function getInvoicesCount() {
  const promiseInvoices = []
  let userInvCount = 0
  let deletedUserInvCount = 0
  let userInvAmount = 0
  let deletedUserInvAmount = 0
  const monthWiseInvCount = []

  return db.userInvoices
    .findAll({
      attributes: [
        'deleted_at',
        [sequelize.fn('COUNT', sequelize.col('id')), 'count'],
        [sequelize.fn('SUM', sequelize.col('invoice_amount')), 'amount'],
        [sequelize.fn('MONTH', sequelize.col('invoice_date')), 'month'],
      ],
      group: ['invoice_date', 'deleted_at'],
      paranoid: false,
    })
    .then((result) => {
      result.forEach((element) => {
        userInvCount += element.dataValues.count
        userInvAmount += element.dataValues.amount
        if (element.dataValues.deleted_at != null) {
          deletedUserInvAmount += element.dataValues.amount
          deletedUserInvCount += element.dataValues.count
        }
        monthWiseInvCount.push(element.dataValues)
      })
      if (monthWiseInvCount.map(a => a === 'deleted_at')) {
        monthWiseInvCount.map(a => delete a.deleted_at)
      }
      return promiseInvoices.push(
        userInvCount,
        userInvAmount,
        deletedUserInvCount,
        deletedUserInvAmount,
        monthWiseInvCount,
      )
    })
}

Теперь вы можете использовать

getInvoicesCount().then(() => {
  //do something here
})

getData(htmlData,tags)
.then(function(data) { 
     console.info(data); //use data after async call finished
})
.catch(function(e) {
    console.info(e);
});

function getData() {
  return new Promise(function(resolve, reject) {
    //call async task and pass the response
    resolve(resp); 
  });
}

Обычно вы можете вернуть обещание следующим образом:

function returnPromise() {
    return new Promise((resolve, reject) => {
        resolve({foo: 'bar'});
    });
}

Итак, хотя другой ответ полностью верен, вы можете использовать приведенную выше структуру для создания функции, которая возвращает обещание, подобное этому:

function getInvoicesCount() {
  return new Promise((resolve, reject) => {
    let promiseInvoices = [];
    let userInvCount = 0;
    let deletedUserInvCount = 0;
    let userInvAmount = 0;
    let deletedUserInvAmount = 0;
    let monthWiseInvCount = [];

  db.userInvoices
    .findAll({
      attributes: [
        'deleted_at',
        [sequelize.fn('COUNT', sequelize.col('id')), 'count'],
        [sequelize.fn('SUM', sequelize.col('invoice_amount')), 'amount'],
        [sequelize.fn('MONTH', sequelize.col('invoice_date')), 'month']
      ],
      group: ['invoice_date', 'deleted_at'],
      paranoid: false
    })
    .then(result => {
      result.forEach(function(element) {
        userInvCount += element.dataValues.count;
        userInvAmount += element.dataValues.amount;
        if (element.dataValues.deleted_at != null) {
          deletedUserInvAmount += element.dataValues.amount;
          deletedUserInvCount += element.dataValues.count;
        }
        monthWiseInvCount.push(element.dataValues);
      });
      if (monthWiseInvCount.map(a => a === 'deleted_at')) {
        monthWiseInvCount.map(a => delete a.deleted_at);
      }
      promiseInvoices.push(
        userInvCount,
        userInvAmount,
        deletedUserInvCount,
        deletedUserInvAmount,
        monthWiseInvCount
      );
      resolve(promiseInvoices); // When you are done, you resolve
    })
    .catch(err => reject(err)); // If you hit an error - reject
});
}

Однако это довольно большая функция, рекомендуется разделить ее на более мелкие части.