Example for refactoring

generator.js

const alphabetArray = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'.split('')

const generateLetterSequence = (start, end, alphabet) => {
    const positionStart = alphabet.indexOf(start)
    const positionEnd = alphabet.indexOf(end) + 1
    return alphabet.slice(positionStart, positionEnd)
}

const generateNumberSequence = (start, end) => {
    const numberArray = []
    for (let i = start; i < end + 1; ++i) {
        numberArray.push(i)
    }
    return numberArray
}


const generateSequenceOfLettersAndNumbers = (positions, numbers) => {
    return positions.map(letter => {
        const newArray = []
        numbers.forEach(number => {
            newArray.push(`${letter}-${number}`)
        })
        return newArray
    })
}


const flat = (arr) => {
    const newArray = []
    arr.forEach(element => {
        element.forEach(el => {
            newArray.push(el)
        })
    })
    return newArray
}

const joinWarehouseAndShelfs = (warehouse, shelfs) => {
    return warehouse.map(element => {
        const newArray = []
        shelfs.forEach(el => {
            newArray.push(`${element}-${el}`)
        })
        return newArray
    })
}

const addSortNumberToArray = (positions, sortNumber, sortNumberIncrement) => {
    const arrayWithSortNumber = positions.map((element, index, array) => {
        return { [element]: sortNumber + (index * sortNumberIncrement) }
    })
    return arrayWithSortNumber
}


module.exports = async (id, params, putItem) => {
    const { locationId, sortNumber, sortNumberIncrement, warehouseStart, warehouseEnd, shelfNumberStart, shelfNumberEnd, shelfRowStart, shelfRowEnd, shelfPartStart, shelfPartEnd } = params
    const warehousePosition = generateLetterSequence(warehouseStart, warehouseEnd, alphabetArray)
    const shelfRows = generateLetterSequence(shelfRowStart, shelfRowEnd, alphabetArray)
    const shelfNumbers = generateNumberSequence(shelfNumberStart, shelfNumberEnd)
    const shelfParts = generateNumberSequence(shelfPartStart, shelfPartEnd)
    const warehouseAndShelfs = generateSequenceOfLettersAndNumbers(warehousePosition, shelfNumbers)
    const shelfRowsAndParts = generateSequenceOfLettersAndNumbers(shelfRows, shelfParts)
    const warehouseAndShelfsFlat = flat(warehouseAndShelfs)
    const shelfRowsAndPartsFlat = flat(shelfRowsAndParts)
    const warehouseAndShelfPositions = joinWarehouseAndShelfs(warehouseAndShelfsFlat, shelfRowsAndPartsFlat)
    const warehouseAndShelfPositionsFlat = flat(warehouseAndShelfPositions)
    const warehousePlan = addSortNumberToArray(warehouseAndShelfPositionsFlat, sortNumber, sortNumberIncrement)
    const response = await putItem(id, locationId, warehousePlan)
    return {
        id,
        locationId,
        storageBins: warehousePlan
    }
}