Javascript remove duplicate in a list of objects merging some properties

Question

I have this list:

const debts = [
  {
    amount: 10,
    debtor: "Mark",
    creditor: "John"
  },
  {
    amount: 20,
    debtor: "Mark",
    creditor: "John"
  },
  {
    amount: 10,
    debtor: "Mark",
    creditor: "Tom"
   }
];

...and I want to merge the elements with the same debtor and creditor calculating the total amount, like this

const debts = [
  {
    amount: 30, // 10 + 20
    debtor: "Mark",
    creditor: "John"
  },
  {
    amount: 10,
    debtor: "Mark",
    creditor: "Tom"
   }
];

Could anyone help me? Thanks

Answer 1

If you're interested in a solution using Ramda, here's a suggestion:

const {
  pipe,
  mergeWithKey,
  map,
  reduce,
  values,
  groupBy,
  props,
  join
} = R;

const debts = [{
    amount: 10,
    debtor: "Mark",
    creditor: "John"
  },
  {
    amount: 20,
    debtor: "Mark",
    creditor: "John"
  },
  {
    amount: 10,
    debtor: "Mark",
    creditor: "Tom"
  }
];

const mergeAmount = mergeWithKey((key, left, right) => key === 'amount' ? left + right : left);
const groupKey = pipe(props(['debtor', 'creditor']), join(' ~> '));

const process =
  pipe(
    groupBy(groupKey),
    map(reduce(mergeAmount, {})),
    values);

console.log(process(debts));

<script src="https://cdnjs.cloudflare.com/ajax/libs/ramda/0.25.0/ramda.min.js"></script>

---

The idea is to split the process in three steps:

1. Group debtors and creditors together
2. For each debtor/creditor group, merge the amount
3. Extract each debtor/creditor group into an array

Step 1: groupBy(groupKey)

Step 1 gets the original debts array

{
    "Mark ~> John": [
        {
            amount: 10,
            creditor: "John",
            debtor: "Mark"
        },
        {
            amount: 20,
            creditor: "John",
            debtor: "Mark"
        }
    ],
    "Mark ~> Tom": [
        {
            amount: 10,
            creditor: "Tom",
            debtor: "Mark"
        }
    ]
}

Step 2: map(reduce(mergeAmount, {}))

Step 2 gets the output from step 1

{
    "Mark ~> John": {
        amount: 30,
        creditor: "John",
        debtor: "Mark"
    },
    "Mark ~> Tom": {
        amount: 10,
        creditor: "Tom",
        debtor: "Mark"
    }
}

Step 3: values

Step 3 gets the output from step 2

[
    {
        amount: 30,
        creditor: "John",
        debtor: "Mark"
    },
    {
        amount: 10,
        creditor: "Tom",
        debtor: "Mark"
    }
]