start.js

(() => {
  requestMock()
    .then(res => res.data.arr)
    .then(arr => {
      if(arr.data.length) {
        return requestMock()
          .then(nextArr => {
            return nextArr.map(nextEl => nextEl+5)
          })
        } else {
          return arr.map(el => el+2)
        }
      }
    })
    .then(arr => console.log(arr));
})();