在javascript中的promise-aware for循环中更新对象属性时出现问题

Question

I am having a problem accessing updated object properties inside a for loop in Javascript.

let distribution = await ctx.db.query.distribution(
      {
        where: {
          id,
        },
      },
      `
    {
      id
      clients {
        id
        wallet {
          amount
        }
      }
    }`,
  );

let count = 0;

for (const client of distribution.clients) {
   await ctx.db.mutation.updateWallet({
       where: {
         id: client.id,
       },
       data: {
         amount: client.wallet.amount + someAmount,
       },
    });
    distribution.clients[count].wallet.amount = client.wallet.amount + someAmount;
    count++;
  }

In the above code, I execute a graphQL query to fetch all information associated with a distribution. I then want to iterate through the associated clients array and update each client's wallet. The same client may appear multiple times in this array and each time their wallet must be updated.

A problem occurs when a wallet has already been updated in a previous iteration. When I try to update the amount a second time, the client.wallet.amount reflects the initial value of the wallet rather than value after the first update.

Clearly the property clients[count].wallet.amount attribute of the distribution object isn't being updated after each iteration. I thought javascript objects were passed by reference, and therefore the object should be updated after each iteration.

Could someone explain to me why the distribution object property is not being updated and how I can update it correctly?

FYI: I cannot use other loops such as forEach as it is not promise-aware and does not suppose async/await

Answer 1

A problem occurs when a wallet has already been updated in a previous iteration.

Each wallet object is only modified once when the loop runs. Two or more wallet objects in the array may represent the same row in the database, but they are still distinct objects -- changing one will not change the other.

const wallets = [
  { id: 1, amount: 0 },
  { id: 2, amount: 0 },
  { id: 1, amount: 0 },
]
for (const wallet of wallets) {
  wallet.amount = wallet.amount + 5
}
console.log(wallets[0]) // { id: 1, amount: 5 }
console.log(wallets[1]) // { id: 2, amount: 5 }
console.log(wallets[2]) // { id: 1, amount: 5 }

Note that we don't need to use a count variable at all. Because the const wallet is a reference to the original object in the array, if we modify it, we modify the original object.

If you want need to track the amounts by ID, you'd need to implement that logic. For example:

const wallets = [
  { id: 1, amount: 0 },
  { id: 2, amount: 0 },
  { id: 1, amount: 0 },
]
const amountsById = {}
for (const wallet of wallets) {
  amountsById[wallet.id] = (amountsById[wallet.id] || wallet.amount) + 5
}
const updatedWallets = wallets.map(wallet => ({
  ...wallet,
  amount: amountsById[wallet.id],
}))
console.log(updatedWallets[0]) // { id: 1, amount: 10 }
console.log(updatedWallets[1]) // { id: 2, amount: 5 }
console.log(updatedWallets[2]) // { id: 1, amount: 10 }