[Vuejs]-Why do I receive Promise {<pending>} with Vuejs

You need to call .json() on the fetch response, and await the result.

And you need to make your function async and await the promise from the first function.

TotalIncomeData(day, branch_office_id) {
  return fetch(...).then(rawResponse => rawResponse.json())
                                                    ^^^^^^ 
 },
 async getTotalIncomes(day, branch_office_id) {
 ^^^^^
     console.log(await this.TotalIncomeData(day, branch_office_id))
                 ^^^^^
 },