mergesort.js

function mergesort(list){
    if(list.length > 1){
        let middle = list.length/2
        return merge(mergesort(list.slice(0, middle)), mergesort(list.slice(middle, list.length)))
    }else{
        return list
    }
}

function merge(listA, listB){
    if(listA.length === 0){return listB}
    if(listB.length === 0){return listA}

    if(listA[0] < listB[0]){
        return [listA[0]].concat(merge(listA.slice(1, listA.length), listB))
    }else{
        return [listB[0]].concat(merge(listA, listB.slice(1, listB.length)))
    }
}