function groupByVolume(arr) {
var singleObj = arr.reduce(function (allObjects, obj) {
if (obj in allObjects) {
allObjects[obj].volume++
}
else {
allObjects[obj] = {email:obj,volume:1}
}
return allObjects
}, [])
return Object.values(singleObj).sort((a, b) => {
return b.volume - a.volume;
})
}