How do i combine data values in array to create a new array

Question

var players = [{id: 17, amount: 126},
{id: 17, amount: 45},
{id: 12, amount: 44},
{id: 12, amount: 23}];

How would i turn the above array into a new array adding the amount for each id.

var newArray=[{id:17.amount:171},{id:12,amount:67}];

Answer 1

You could group the same id with a hash table and add the amount, where necessary.

var players = [{ id: 17, amount: 126 }, { id: 17, amount: 45 }, { id: 12, amount: 44 }, { id: 12, amount: 23 }],
    grouped = players.reduce(function (hash) {
        return function (r, a) {
            (hash[a.id] = hash[a.id] || r[r.push({ id: a.id, amount: 0 }) - 1]).amount += a.amount;
            return r;
        };
    }(Object.create(null)), []);

console.log(grouped);

Basically this line

(hash[a.id] = hash[a.id] || r[r.push({ id: a.id, amount: 0 }) - 1]).amount += a.amount;

checks if hash[a.id] exists and if not, then this part

r[r.push({ id: a.id, amount: 0 }) - 1]

is perfomed, which contains of two parts, a part to access the result set and inside it pushes a new objetc set to the result set. While push returns the new length of the array, it need to decrease the index to get the last inserted element.

r[                                - 1]
  r.push({ id: a.id, amount: 0 })

After that, take the property amount and increase by the amount of the actual element.

Answer 2

You could also change the output format a little, to make it easier to search:

var players = [{ id: 17, amount: 126 }, { id: 17, amount: 45 }, { id: 12, amount: 44 }, { id: 12, amount: 23 }],
    result = {};

for(var i = 0; i < players.length; i++){
  result[players[i].id] = 
      (result[players[i].id] || 0) + // The existing amount, or 0 for a new `id`
       players[i].amount;           // Add the current amount
}

console.log(result);
console.log('Amount for 17: ' + result[17]);

This way, you don't have to filter the result on it's id value, to get the amount.

Answer 3

You could push element in a a new array if id is new or just add amount to the previous if it already exists :

var players = [{ id: 17, amount: 126 }, { id: 12, amount: 103 }, { id: 17, amount: 45 }, { id: 12, amount: 44 }, { id: 12, amount: 23 }];
var results = [];
results.push(players.sort((a,b) => a.id-b.id)[0]);
var shift = 0;
players.forEach((item, index, arr) => {
   if (index < arr.length - 1){
     if (item.id === arr[index+1].id) {
       results[index + shift].amount += arr[index+1].amount;
       shift--;
     }
     else results.push(players[index+1]);
   }
});
console.log(results); // [ { id: 12, amount: 170 }, { id: 17, amount: 171 } ]