groupBy.js

function groupBy (collection, func, callback) {	
  if (typeof func !== 'function') {
    callback('please provide a function to group by')
    return
  }
  
  if (typeof callback !== 'function') {
    callback('please provide a function as a callback')
    return
  }
  
  var callReturnCount = 0
  var result = {}
  var collectionLength = collection.length
 
  for(var start=0;start <collection.length;start++) {
    var collectionItem = collection[start]
    processItem(collectionItem)
  }
  
  function processItem (item) {
    function serverCallback (val) { 
      if (result[val]){
        result[val].push(item)
      } else {
        result[val] = [item]
      }
      callReturnCount += 1
      if (callReturnCount === collectionLength) {
        callback(result)
      }
    }
    func(item, serverCallback)
  }
}


let collection = [
  {name: 'Abhilash', age: 30, company: 'inmobi'},
  {name: 'Abhinav', age: 20, company: 'jiffle'},
  {name: 'Rajesh', age: 30, company: 'google'},
  {name: 'rahul', age: 20, company: 'jiffle'},
  {name: 'akashdeep', age: 30, company: 'google'},
]

groupBy(collection, function(item, cb) {
  setTimeout(() => cb(item.name), Math.random() * 500);
}, function(group) {
  console.log(group)
})