sql-style joins using key decoration in javascript

group.js

function mergeData(...arrs) {
    const getKey = el => el.affId + ',' + el.offId
    const full = concat(arrs)
    const groupedObj = groupBy(full, getKey)
    const grouped = Object.values(groupedObj)
    const merged = grouped.map(mergeObjects)
    return merged
}

function concat(arrs) {
    const cat = (a, b) => [...a, ...b]
    return arrs.reduce(cat, [])
}

function groupBy(arr, keyfunc) {
    let res = {}
    for (el of arr) {
        let key = keyfunc(el)
        if (res[key])
            res[key].push(el)
        else
            res[key] = [el]
    }
    return res
}

function mergeObjects(arr) {
    const assign = (acc, el) => Object.assign(acc, el)
    return arr.reduce(assign, {})
}


function example() {
    const netArr = [
        {affId: '1', offId: '200', revenue: '12'},
        {affId: '2',  offId: '201', revenue: '3'},
        {affId: '3',  offId: '202'}
    ]
    const tuneArr = [
        {affId: '1', offId: '200', revenue: '12'},
        {affId: '2',  offId: '201', revenue: '3'},
        {affId: '3',  offId: '202'}
    ]
    const tabArr = [
        {affId: '1', cost: '4', clicks: '5',
         actions: '10', cost: '.5', offId: '200'},
        {affId: '2',  cost: '2', clicks: '5',
         actions: '30', cost: '50.0', offId: '201'},
        {affId: '3',  cost: '5', clicks: '8',
         actions: '310', cost: '1.5', offId: '202'}
    ]

    return mergeData(netArr, tuneArr, tabArr)
}

module.exports = {
    mergeData,
    example
}