How can I create an Array of objects from arr array in javascript using spread operator? - Trying to fit all of the code on to one line

Need to create one object per objectid and create an array of contacts for that object.
I have done it without spread operator but wondering how this can be done using spread operator.

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const arr = [
  {
    "objectid": 1,
    "userid": "1",
    "associationid": "123"
  },
  {
    "objectid": 1,
    "userid": "2",
    "associationid": "456"
  },  
  {
    "objectid": 2,
    "userid": "2",
    "associationid": "456"
  }
];

//My working code without using spread operator
let res = Object.values(
arr.reduce((acc, {objectid, userid, associationid}) => {
(acc[objectid] ??= {objectid, contacts : []}).contacts
.push({isdelete: '1', contactid: userid, associationid});
return acc;
}, {})
);
console.log(res);

//Tried the following code using spread operator but not working
const res = arr.reduce((acc, {objectid,userid,associationid}) =>
  ({ ...acc, (acc[objectid] ??= {objectid, contacts : []}).contacts.push({isdelete: '1', contactid: userid, associationid}) }),
  {})
  
console.log(res);

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You are trying to mutate an array within an object while spreading the object at the same time, which is syntactically incorrect. I am using the spread operator to keep the previous contacts and add the new one.

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const arr = [{
    "objectid": 1,
    "userid": "1",
    "associationid": "123"
  },
  {
    "objectid": 1,
    "userid": "2",
    "associationid": "456"
  },
  {
    "objectid": 2,
    "userid": "2",
    "associationid": "456"
  }
];

const res = arr.reduce((acc, {
  objectid,
  userid,
  associationid
}) => {
  if (!acc[objectid]) {
    acc[objectid] = {
      objectid,
      contacts: []
    };
  }

  return {
    ...acc,
    [objectid]: {
      ...acc[objectid],
      contacts: [
        ...acc[objectid].contacts,
        {
          isdelete: '1',
          contactid: userid,
          associationid
        }
      ]
    }
  };
}, {});

console.log(Object.values(res));

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You can try this code I hope it answers your question

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const arr = [{
    "objectid": 1,
    "userid": "1",
    "associationid": "123"
  },
  {
    "objectid": 1,
    "userid": "2",
    "associationid": "456"
  },
  {
    "objectid": 2,
    "userid": "2",
    "associationid": "456"
  }
];

const res = Object.values(
  arr.reduce((acc, {
    objectid,
    userid,
    associationid
  }) => {
    acc[objectid] = {
      ...acc[objectid],
      objectid,
      contacts: [
        ...(acc[objectid]?.contacts ?? []),
        {
          isdelete: '1',
          contactid: userid,
          associationid
        },
      ],
    };
    return acc;
  }, {})
);

console.log(res);

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You can use the comma operator to always return the accumulator after modifying it.

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const arr=[{objectid:1,userid:"1",associationid:"123"},{objectid:1,userid:"2",associationid:"456"},{objectid:2,userid:"2",associationid:"456"}];
let res = Object.values(
  arr.reduce((acc, {objectid, userid, associationid}) => 
    ((acc[objectid] ??= {objectid, contacts : []}).contacts
      .push({isdelete: '1', contactid: userid, associationid}), acc), {})
);
console.log(res);

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