Merge Duplicate Objects in Array of Objects

Merge duplicate objects in array of objects

I would probably loop through with filter, keeping track of a map of objects I'd seen before, along these lines (edited to reflect your agreeing that yes, it makes sense to make (entry).data always an array):

var seen = {};
data = data.filter(function(entry) {
    var previous;

    // Have we seen this label before?
    if (seen.hasOwnProperty(entry.label)) {
        // Yes, grab it and add this data to it
        previous = seen[entry.label];
        previous.data.push(entry.data);

        // Don't keep this entry, we've merged it into the previous one
        return false;
    }

    // entry.data probably isn't an array; make it one for consistency
    if (!Array.isArray(entry.data)) {
        entry.data = [entry.data];
    }

    // Remember that we've seen it
    seen[entry.label] = entry;

    // Keep this one, we'll merge any others that match into it
    return true;
});

In an ES6 environment, I'd use seen = new Map() rather than seen = {}.

Note: Array.isArray was defined by ES5, so some quite older browsers like IE8 won't have it. It can easily be shimmed/polyfilled, though:

if (!Array.isArray) {
    Array.isArray = (function() {
        var toString = Object.prototype.toString;
        return function(a) {
            return toString.call(a) === "[object Array]";
        };
    })();
}

Side note: I'd probably also always make entry.data an array, even if I didn't see two values for it, because consistent data structures are easier to deal with. I didn't do that above because your end result showed data being just a string when there was only one matching entry. (We've done that above now.)

Live example (ES5 version):

var data = [    {        label: "Book1",        data: "US edition"    },    {        label: "Book1",        data: "UK edition"    },    {        label: "Book2",        data: "CAN edition"    }];snippet.log("Before:");snippet.log(JSON.stringify(data, null, 2), "pre");var seen = {};data = data.filter(function(entry) {    var previous;
    // Have we seen this label before?    if (seen.hasOwnProperty(entry.label)) {        // Yes, grab it and add this data to it        previous = seen[entry.label];        previous.data.push(entry.data);
        // Don't keep this entry, we've merged it into the previous one        return false;    }
    // entry.data probably isn't an array; make it one for consistency    if (!Array.isArray(entry.data)) {        entry.data = [entry.data];    }
    // Remember that we've seen it    seen[entry.label] = entry;
    // Keep this one, we'll merge any others that match into it    return true;});snippet.log("After:");snippet.log(JSON.stringify(data, null, 2), "pre");

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merge duplicate objects in an array and add additional property

You can use reduce to create a unique array that sets admin property to true for duplicate addresses as follows:

const addresses = [{name: 'Paul', id: 2}, {name: 'John', id: 1}, {name: 'John', id: 1}];
const result = addresses.reduce((acc, address) => {  const dup = acc.find(addr => addr.id === address.id);  if (dup) {    dup.admin = true;    return acc;  }  address.admin = false;  return acc.concat(address);}, [])
console.log(result);

merge duplicate objects in an array and combine sub array of each object

I think, reduce() would do the job:

const accounts = [{"Id":103,"accountList":[{}]},{"Id":103,"accountList":[{"tokenId":"5aasdasdsdnjn3434nadd","featureId":2840}]},{"Id":112,"accountList":[{"tokenId":"5d30775bef4a722c38aefaaa","featureId":2877}]},{"Id":112,"accountList":[{"tokenId":"5d30775bef4a722c38aefccc","featureId":2856}]}];

const result = [...accounts
        .reduce((r, o) => {
          const record = r.get(o.Id)||{}
          r.set(o.Id, {
            Id: o.Id,
            accountList: [
              ...(record.accountList||[]),
              ...o.accountList.filter(o => 
                Object.keys(o).length != 0)
            ]            
          })
          return r
        }, new Map())
        .values()]

console.log(result);

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How to merge duplicate object parent items in array?

const data = [
  { "_id": { "month": 5, "year": 2021 }, "total_price": 145111500, "total_quantity": 7 },
  { "_id": { "month": 6, "year": 2021 }, "total_price": 98386000, "total_quantity": 5 },
  { "_id": { "month": 6, "year": 2021 }, "total_price": 32500000, "total_quantity": 3 }
];

const res = [...
  // iterate over the list
  data.reduce((map, item) => {
    // construct key from _id
    const key = `${item._id.month}-${item._id.year}`;
    // get prev map value of key if exists
    const prev = map.get(key);
    // update map, if prev not found, set value as item, or update it with the added values
    map.set(
      key, 
      !prev 
        ? item 
        : { ...item, total_price: prev.total_price + item.total_price, total_quantity: prev.total_quantity + item.total_quantity }
    );
    return map;
  }, new Map)
  // return map values
  .values()
];

console.log(res);

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