Spreading an array inside an object gives unexpected result – Javascript

I have two arrays of objects with the same keys and I want to combine them and create a new array of objects. I am able to get almost everything right, but the issue occurs when I want to spread them inside the new array. The spread automatically includes a key value pair with the index and the value which is not what i want. What am I missing? Please advice.

This is my implementation:

const currentData = [{
  FIRST_NAME: 'ABC',
  MIDDLE_NAME: 'DEF',
  LAST_NAME: 'GHI'
}]

const historicalData = [{
  FIRST_NAME: 'ABC1',
  MIDDLE_NAME: 'DEF1',
  LAST_NAME: 'GHI1'
}, {
  FIRST_NAME: 'ABC2',
  MIDDLE_NAME: 'DEF2',
  LAST_NAME: 'GHI2'
}, {
  FIRST_NAME: 'ABC3',
  MIDDLE_NAME: 'DEF3',
  LAST_NAME: 'GHI3'
}]

const rowDataKeys = Object.keys(currentData[0])

const rowData = rowDataKeys.map((i) => {
  const resp = historicalData.map((j, index) => {
    return {
      [`history${index+1}`]: historicalData[index][i]
    }
  })
  return {
    field: i,
    current: currentData[0][i],
    ...resp
  }
})

console.log(rowData)

Expected Output:

[{
    field: 'FIRST_NAME',
    current: 'ABC',
    history1: 'ABC1',
    history2: 'ABC2',
    history3: 'ABC3'
}, {
    field: 'MIDDLE_NAME',
    current: 'DEF',
    history1: 'DEF1',
    history2: 'DEF2',
    history3: 'DEF3'
}, {
    field: 'LAST_NAME',
    current: 'GHI',
    history1: 'GHI1',
    history2: 'GHI2',
    history3: 'GHI3'
}]

Answer

The type of resp in the code snippet before is an array of objects.

const resp = historicalData.map((j, index) => {
  return {
    [`history${index+1}`]: historicalData[index][i]
  }
});

You likely want the type to be an object directly.

You can use Array.reduce for this:

const currentData = [{
  FIRST_NAME: 'ABC',
  MIDDLE_NAME: 'DEF',
  LAST_NAME: 'GHI'
}]

const historicalData = [{
  FIRST_NAME: 'ABC1',
  MIDDLE_NAME: 'DEF1',
  LAST_NAME: 'GHI1'
}, {
  FIRST_NAME: 'ABC2',
  MIDDLE_NAME: 'DEF2',
  LAST_NAME: 'GHI2'
}, {
  FIRST_NAME: 'ABC3',
  MIDDLE_NAME: 'DEF3',
  LAST_NAME: 'GHI3'
}]

const rowDataKeys = Object.keys(currentData[0])

const rowData = rowDataKeys.map((i) => {
  const resp = historicalData.reduce((acc, j, index) => ({
    ...acc,
    [`history${index+1}`]: historicalData[index][i],
  }), {})
  return {
    field: i,
    current: currentData[0][i],
    ...resp
  }
})

console.log(rowData)