尝试如下：

const items = [
   { id: 1, account: 'acct_101' },
   { id: 2, account: 'acct_52' },
   { id: 3, account: 'acct_33' },
   { id: 4, account: 'acct_101' },
   { id: 5, account: 'acct_101' },
]

const output = items.reduce((prev, curr) => {
  if(prev[curr.account]) {
    prev[curr.account].push(curr)
  } else {
    prev[curr.account] = [curr]
  }
  return prev
}, {})

console.log([output])

更简短的形式：

const items = [
   { id: 1, account: 'acct_101' },
   { id: 2, account: 'acct_52' },
   { id: 3, account: 'acct_33' },
   { id: 4, account: 'acct_101' },
   { id: 5, account: 'acct_101' },
]

const output = items.reduce((prev, curr) => {
  prev[curr.account] = (prev[curr.account] ?? []).concat(curr)
  return prev
}, {})

console.log([output])

使用Array.prototype.reduce()

对于你想要的输出，我认为你需要使用减少高阶函数。
因此，您的代码将是：

const itemGroups = items.reduce((acc, item) => {
  const account = item.account;
  if (acc[account]) {
    acc[account].push(item);
  } else {
    acc[account] = [item];
  }
  return acc;
}, {});

console.log(itemGroups);

输出：

{
  acct_101: [
    { id: 1, account: 'acct_101' },
    { id: 4, account: 'acct_101' },
    { id: 5, account: 'acct_101' }
  ],
  acct_52: [ { id: 2, account: 'acct_52' } ],
  acct_33: [ { id: 3, account: 'acct_33' } ]
}

const arr = [{
    id: 1,
    account: 'acct_101'
  },
  {
    id: 2,
    account: 'acct_52'
  },
  {
    id: 3,
    account: 'acct_33'
  },
  {
    id: 4,
    account: 'acct_101'
  },
  {
    id: 5,
    account: 'acct_101'
  },
];
const retObj = {};
for (let i = 0; i < arr.length; i++) {

  retObj[arr[i]["account"]] = arr.filter(item => item.account == arr[i]["account"]);
}
console.log(retObj)