Merge two arrays of objects but with uniqueness on a certain key/value

Question

I have two Javascript arrays of objects with various keys/values.

I'm trying to achieve a new array of objects with selected keys/values from each of the original arrays, but have uniqueness on a specific key/value.

Example:

const startDate = [
   {
    name: 'John', //Don't need this
    Id: 'ae570d88-809b-45b1-bc20-69b569e361ce', //This should be the 'unique' key
    datePosted: '2020-04-04T00:01:20.000Z' //This will be the start date
   }
]

const endDate = [
   {
    name: 'James', //Don't need this
    Id: 'ae570d88-809b-45b1-bc20-69b569e361ce', //This should be the 'unique' key
    datePosted: '2021-04-04T00:01:20.000Z' //This will be the end date
   }
]

const desiredOutput = [
    {
    'ae570d88-809b-45b1-bc20-69b569e361ce': { 
        startDate: '2020-04-04T00:01:20.000Z',
      endDate: '2021-04-04T00:01:20.000Z'
    }
  }
]

const desiredOutput2 = [
    {
     Id: 'ae570d88-809b-45b1-bc20-69b569e361ce',
     startDate: '2020-04-04T00:01:20.000Z',
     endDate: '2021-04-04T00:01:20.000Z'
  }
]

I've tried using the JS spread operator but can't figure out the renaming of the key to startDate/endDate and adding both to the same object within the array based on uniqueness of the 'Id' key.

Either of the two desiredOutputs would work great

The left top is for html when you have JS code use the bottom left box in the grid view. — Aalexander, Feb 04 '21 at 09:19
Relevant, but I am not sure it's a full duplicate: [How to group an array of objects by key](https://stackoverflow.com/q/40774697) — VLAZ, Feb 04 '21 at 09:26

score 7 · Accepted Answer · answered Feb 04 '21 at 09:25

You can map over the startDate array and find the endDate based on the Id.

const startDate = [
   {
    name: 'John', //Don't need this
    Id: 'ae570d88-809b-45b1-bc20-69b569e361ce', //This should be the 'unique' key
    datePosted: '2020-04-04T00:01:20.000Z' //This will be the start date
   }
]

const endDate = [
   {
    name: 'James', //Don't need this
    Id: 'ae570d88-809b-45b1-bc20-69b569e361ce', //This should be the 'unique' key
    datePosted: '2021-04-04T00:01:20.000Z' //This will be the end date
   }
]

const desiredOutput = startDate.map((startObj) => {  
  const foundEndDate = endDate.find((endObj) => endObj.Id === startObj.Id);
  
  return {
    Id: startObj.Id,
    startDate: startObj.datePosted,
    endDate: foundEndDate.datePosted,
  };
});

console.log(desiredOutput);

score 2 · Answer 2 · answered Feb 04 '21 at 09:29

You could take an object with same Id as key and merge the two arrays.

const
    startDate = [{ name: 'John', Id: 'ae570d88-809b-45b1-bc20-69b569e361ce', datePosted: '2020-04-04T00:01:20.000Z' }],
    endDate = [{ name: 'James', Id: 'ae570d88-809b-45b1-bc20-69b569e361ce', datePosted: '2021-04-04T00:01:20.000Z' }],
    result = Object
        .entries(Object.entries({ startDate, endDate }).reduce((r, [k, v]) => {
            v.forEach(o => r[o.Id] = { ...r[o.Id], [k]: o.datePosted })
            return r;
        }, {}))
       .map(a => Object.fromEntries([a]));

console.log(result);

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Merge two arrays of objects but with uniqueness on a certain key/value

2 Answers2