Pushing element into array inside for loop

Question

I want to dynamically generate a 50 element array from a single element, by just modifying one value on each loop.

const eventRecords = { Records: [] }
      for (let i = 0; i < 50; i++) {
        const aRecord = Object.assign({}, eventS3Chunk.Records[0])
        aRecord.s3.object.key = `F7kdfh2Dj3j2s8/uploads/10000_users_without_password-20190102T030405Z/${i}.csv`
        eventRecords.Records.push(Object.assign({}, aRecord))
      }

eventRecords.Records end up with 50 copies of same element with s3.object.key = F7kdfh2Dj3j2s8/uploads/10000_users_without_password-20190102T030405Z/49.csv.

Object.assign is not a deep copy, for copying nested object one method is JSON.parse(JSON.stringify(obj)) — Chris Li
– Chris Li, Commented Aug 28, 2019 at 13:24
Take a look into deep cloning. In your example, the s3 and/or object is being passed by reference and therefore assigning the key to one, affects all records. — Jeff Hechler
– Jeff Hechler, Commented Aug 28, 2019 at 13:25

Taki · Accepted Answer · 2019-08-28 13:26:28Z

4

it's because you're creating a shallow copy of Records[0], use JSON.parse(JSON.stringify(eventS3Chunk.Records[0])); :

const eventS3Chunk = {
  Records: [{
    s3: {
      object: {
        key: "a"
      }
    }
  }]
};

const eventRecords = Array.from({
  length: 50
}, (_, i) => {
  const aRecord = JSON.parse(JSON.stringify(eventS3Chunk.Records[0]));
  aRecord.s3.object.key = `F7kdfh2Dj3j2s8/uploads/10000_users_without_password-20190102T030405Z/${i}.csv`;
  return aRecord;
});

console.log(eventRecords)

answered Aug 28, 2019 at 13:26

Taki

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