I dunno what i'm doing but i know it's just help people

array.js

// ISSUE https://www.facebook.com/permalink.php?story_fbid=2923264041231670&id=100006443087773
// we have some data with item is
// { id: number, name: string, jumlahDonasi: number }
const data = [
    {
        id: 1,
        name: "c",
        jumlahDonasi: 2000,
    },
     {
        id: 1,
        name: "c",
        jumlahDonasi: 5000,
    },
     {
        id: 2,
        name: "d",
        jumlahDonasi: 2000,
    },
]

// we want to transform item of data, to be like this
// { id: number, name: string, jumlahDonasi: array(number) }
// #1
let reduceDonation1 = (acc, cur, idx, src) => {
  let maybeExist = acc.findIndex(item => item.id === cur.id); // will return 0,1,N by length of array and -1 when nothing
 
  if(maybeExist >= 0){ 
    // we assume cur.id exist in `acc` & so we can push to acc[index].julahDonasi
    // possibly error push when `jumlahDonasi` is not an array so you can make it more safely
    acc[maybeExist].jumlahDonasi.push(cur.jumlahDonasi)
    return acc 
  }
 
  acc.push({...cur, jumlahDonasi: [cur.jumlahDonasi]})
  return acc 
}

data.reduce(reduceDonation, []) // correctly when initiator is empty array