merge object & sum a single property javascript

Question

i have an array like this

[
  {
    item_guid: "57e7a1cd6a3f3669dc03db58"
    quantity:3
  },
  {
    item_guid: "57e77b06e0566d496b51fed5"
    quantity:3
  },
  {
    item_guid: "57e7a1cd6a3f3669dc03db58"
    quantity:3
   },
  {
    item_guid: "57e77b06e0566d496b51fed5"
    quantity:3
  }
]

What i want from this to merge similar item_guid into one object and quantity get summed up like this

[
  {
    item_guid: "57e7a1cd6a3f3669dc03db58"
    quantity:6
   },
  {
    item_guid: "57e77b06e0566d496b51fed5"
    quantity:6
  }
]

I know this can be achieved some loop, instead I'm looking for solutions that uses lodash or EcmaScript2015 code to make our life easier

Answer 1

A simple approach:

Loop over data and create an object where key will be unique id and its value will be quantity. Then loop back over object's keys and create objects.

var data = [{ item_guid: "57e7a1cd6a3f3669dc03db58", quantity: 3 }, { item_guid: "57e77b06e0566d496b51fed5", quantity: 3 }, { item_guid: "57e7a1cd6a3f3669dc03db58", quantity: 3 }, { item_guid: "57e77b06e0566d496b51fed5", quantity: 3 }]

var r = {};
data.forEach(function(o){
  r[o.item_guid] = (r[o.item_guid] || 0) + o.quantity;
})

var result = Object.keys(r).map(function(k){
  return { item_guid: k, quantity: r[k] }
});
console.log(result)