Sort an array of Objects and take N elements

Question

in a NodeJS service I have an array with objects that have these properties:

• batchType: string
• batchId: string (is a hash)
• transactionId: string (it's a hash)

This array stores all transactions of different types of batches.

Basically what I need is to be able to get N items from the array, but respecting certain rules:

1. At least get 1 item from each type of batch
2. At least get 1 item for each batchId
3. Sometimes the array could have only one type of batch

This is an example of the array:

let batchTransactionsArray = [
  { batchType: 'type1', batchId: '123', transactionId: 'ffasf23' },
  { batchType: 'type1', batchId: '312', transactionId: '423' },
  { batchType: 'type1', batchId: '123', transactionId: '534' },
  { batchType: 'type1', batchId: '312', transactionId: '86' },
  { batchType: 'type2', batchId: '111', transactionId: '97' },
  { batchType: 'type1', batchId: '312', transactionId: '1945' },
  { batchType: 'type1', batchId: '123', transactionId: '79' },
  { batchType: 'type1', batchId: '312', transactionId: '79' },
  { batchType: 'type3', batchId: '425', transactionId: '1555645' },
  { batchType: 'type1', batchId: '123', transactionId: 'fg5' },
  { batchType: 'type1', batchId: '123', transactionId: 'jkh5' },
  { batchType: 'type1', batchId: '312', transactionId: '53j' },
  { batchType: 'type1', batchId: '111', transactionId: '4545' },
  { batchType: 'type2', batchId: '111', transactionId: '534l' },
  { batchType: 'type1', batchId: '111', transactionId: 'jkg435' },
  { batchType: 'type1', batchId: '111', transactionId: 'gfxg23' },
  { batchType: 'type1', batchId: '111', transactionId: '7asdt' },
  { batchType: 'type1', batchId: '222', transactionId: 'jdsa7' },
  { batchType: 'type3', batchId: '663', transactionId: '12423445' },
  { batchType: 'type1', batchId: '111', transactionId: '89saf6' },
  { batchType: 'type1', batchId: '111', transactionId: '12h3g' },
  { batchType: 'type1', batchId: '111', transactionId: '4h3k2hj' },
  { batchType: 'type3', batchId: '663', transactionId: '145' }
];

And an example of the output I need is (if I want 5 transactions from the array):

[{ batchType: 'type1', batchId: '123', transactionId: '534' },
 { batchType: 'type1', batchId: '312', transactionId: '86' },
 { batchType: 'type2', batchId: '111', transactionId: '97' },
 { batchType: 'type2', batchId: '111', transactionId: '534l' },
 { batchType: 'type3', batchId: '663', transactionId: '145' }
]

The criteria for sorting transactionIds would be random, there is no specific order to meet.

I was trying some lodash functions like groupBy and sortBy but no luck yet.

Here is a jsfiddle were I was playing with this: https://jsfiddle.net/20jh3ze7/

I really appreciate suggestions.

Answer 1

You can do this in steps:

• first add unique batchType elements (this meets the first criterion)
• then add unique batchId elements (this meets the second criterion)
• fill the array with remaining elements if there's still some space left (to go up to N elements)

To check for uniqueness, you can use a Set. Since you said you're using lodash, you can use _.groupBy to group your array by batchItem and batchId to get unique elements from each group.

let batchTransactionsArray = [
  { batchType: 'type1', batchId: '123', transactionId: 'ffasf23' },
  { batchType: 'type1', batchId: '312', transactionId: '423' },
  { batchType: 'type1', batchId: '123', transactionId: '534' },
  { batchType: 'type1', batchId: '312', transactionId: '86' },
  { batchType: 'type2', batchId: '111', transactionId: '97' },
  { batchType: 'type1', batchId: '312', transactionId: '1945' },
  { batchType: 'type1', batchId: '123', transactionId: '79' },
  { batchType: 'type1', batchId: '312', transactionId: '79' },
  { batchType: 'type3', batchId: '425', transactionId: '1555645' },
  { batchType: 'type1', batchId: '123', transactionId: 'fg5' },
  { batchType: 'type1', batchId: '123', transactionId: 'jkh5' },
  { batchType: 'type1', batchId: '312', transactionId: '53j' },
  { batchType: 'type1', batchId: '111', transactionId: '4545' },
  { batchType: 'type2', batchId: '111', transactionId: '534l' },
  { batchType: 'type1', batchId: '111', transactionId: 'jkg435' },
  { batchType: 'type1', batchId: '111', transactionId: 'gfxg23' },
  { batchType: 'type1', batchId: '111', transactionId: '7asdt' },
  { batchType: 'type1', batchId: '222', transactionId: 'jdsa7' },
  { batchType: 'type3', batchId: '663', transactionId: '12423445' },
  { batchType: 'type1', batchId: '111', transactionId: '89saf6' },
  { batchType: 'type1', batchId: '111', transactionId: '12h3g' },
  { batchType: 'type1', batchId: '111', transactionId: '4h3k2hj' },
  { batchType: 'type3', batchId: '663', transactionId: '145' }
];

function getNItems(array, N = 5) {
  if (N >= array.length) return array;
  let batchTypeGroups = _.groupBy(array, e => e.batchType),
      res  = new Set(), batchIds = new Set();
  
  // add unique batchTypes
  for (let [batchType, elements] of Object.entries(batchTypeGroups)) {
    if (res.size === N) break;
    let d = elements.find(e => !batchIds.has(e.batchId));
    if (d) {
      res.add(d);
      batchIds.add(d.batchId);
    } else {
      res.add(elements[0]);
    }
  }
  
  // add remaining unique batchIds
  let batchIdGroups = _.groupBy(array.filter(e => !batchIds.has(e.batchId)), e => e.batchId);
  for (let [batchId, elements] of Object.entries(batchIdGroups)) {
    if (res.size === N) break;
    res.add(elements[0]);
  }
  
  // add any remaining elements until we have N elements
  for (let e of array) {
    if (res.size === N) break;
    res.add(e);
  }
  
  return Array.from(res);
}

console.log(getNItems(batchTransactionsArray, 5));

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