Find greater of two objects based on key value Javascript Reduce

Question

I've been trying for 3 days straight to figure this out, by this point I don't even know what to search for.

I have an array of objects, I would like to get the greatest "bid_amount" for each "item_id", and then use the item_id, bid_amount and bidder_id in the DOM. I am stuck at the first step.

[
    {
        "item_id": "1",
        "bid_amount": "765432",
        "bidder_id": "298709"
    },
    {
        "item_id": "1",
        "bid_amount": "380",
        "bidder_id": "606396"
    },
    {
        "item_id": "2",
        "bid_amount": "545",
        "bidder_id": "606396"
    },
    {
        "item_id": "2",
        "bid_amount": "525",
        "bidder_id": "317740"
    },
    {
        "item_id": "2",
        "bid_amount": "505",
        "bidder_id": "606396"
    },]

function getBids() {
  var request = new XMLHttpRequest();
  request.open('POST', '/get_bids.php', true);
  request.setRequestHeader('Content-Type', 'application/json; charset=UTF-8');

  request.onload = function() {
    bids = JSON.parse(this.response)

    bids.reduce(function(prev, curr) {
      currItemId = curr.item_id
      currBidAmnt = curr.bid_amount

      const item_id = (prev[curr.item_id] || []);
      const prevBidsForId = prev[currItemId];

      if (prevBidsForId) {
        console.log(' a bid for this item exists, now i need to check if the current bid amount is greater than the previous bid amount for this item ')

        item_id.push(curr)
        prev[currItemId] = item_id
      } else {
        // push current item to prev
        console.log(' a bid for this item does not exist ')

        item_id.push(curr)
        prev[currItemId] = item_id
      }
      return prev
    }, [])
  }

Please show the expected output for this particular input. I don't see any DOM here. Is this a separate question? If so, I suggest removing it to focus on one problem only. Thanks. — ggorlen
– ggorlen, Commented Apr 18, 2022 at 18:59
I'll try work on the DOM part myself. I just needed help on the first step but I thought I'd provide context on what I needed the information for. Sorry, I'm new to posting here! — keithtennant
– keithtennant, Commented Apr 18, 2022 at 23:10

pilchard · Accepted Answer · 2022-04-18 19:40:13Z

You can simply 'group by' item_id and simultaneously store the max bid in the grouped object. Here using a for...of loop but you can shift the logic directly into a reduce if you like. (This stores all the bids in an array keyed by item_id and the max bid is assigned to a seperate property.

const bids = [{ "item_id": "1", "bid_amount": "765432", "bidder_id": "298709" }, { "item_id": "1", "bid_amount": "380", "bidder_id": "606396" }, { "item_id": "2", "bid_amount": "545", "bidder_id": "606396" }, { "item_id": "2", "bid_amount": "525", "bidder_id": "317740" }, { "item_id": "2", "bid_amount": "505", "bidder_id": "606396" }];

const bidsById = {};

for (const bid of bids) {
  (bidsById[bid.item_id] ??= { bids: [], max: bid }).bids.push(bid);
  
  if (bid.bid_amount > bidsById[bid.item_id].max.bid_amount) {
    bidsById[bid.item_id].max = bid;
  }
}

for (const [id, { max }] of Object.entries(bidsById)) {
  console.log(`${id}: ${max.bid_amount}`);
}

If you only want the bid_amount in the result you can simplify

const bids = [{ "item_id": "1", "bid_amount": "765432", "bidder_id": "298709" }, { "item_id": "1", "bid_amount": "380", "bidder_id": "606396" }, { "item_id": "2", "bid_amount": "545", "bidder_id": "606396" }, { "item_id": "2", "bid_amount": "525", "bidder_id": "317740" }, { "item_id": "2", "bid_amount": "505", "bidder_id": "606396" }];

const bidsById = bids.reduce((a, { item_id, bid_amount }) => {
  if (bid_amount > (a[item_id] ??= bid_amount)) {
    a[item_id] = bid_amount;
  }
  return a;
}, {})


for (const [id, max] of Object.entries(bidsById)) {
  console.log(`${id}: ${max}`);
}

Carsten Massmann · Accepted Answer · 2022-04-18 20:19:30Z

1

Here is a short .reduce() based script that will filter out the object with the highest bid:

const bids=[
{
    "item_id": "1",
    "bid_amount": "765432",
    "bidder_id": "298709"
},
{
    "item_id": "1",
    "bid_amount": "380",
    "bidder_id": "606396"
},
{
    "item_id": "2",
    "bid_amount": "545",
    "bidder_id": "606396"
},
{
    "item_id": "2",
    "bid_amount": "525",
    "bidder_id": "317740"
},
{
    "item_id": "2",
    "bid_amount": "505",
    "bidder_id": "606396"
}];

const highest = Object.values(bids.reduce((a,c)=>{
 let aa=a[c.item_id];
 if(!aa||c.bid_amount>aa.bid_amount) a[c.item_id]=c;
 return a }, {}));

console.log(highest);

edited Apr 18, 2022 at 20:19

answered Apr 18, 2022 at 19:03

Carsten Massmann

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3 Comments

Qwerasd Over a year ago

I believe they want the objects with the highest respective bids among other objects of the same item ID

Carsten Massmann Over a year ago

Updated my post, please check again.

keithtennant Over a year ago

Thank you for the help!

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