Javascript merge multiple arrays, de-duped by property name, O(N1) using reduce

script.js

const array1 = [
  { name: 'a', creationDate: new Date() },
  { name: 'b', creationDate: new Date() },
]
const array2 = [
  { name: 'a', creationDate: new Date() },
  { name: 'c', creationDate: new Date() },
]

const mergeBy = (...arrays) => {
  const prop_index = arrays.length - 1, key = arrays[prop_index]
  if (typeof arrays[prop_index] !== 'string') throw Error('Last argument must be a string')
  arrays.splice(-1, 1)

  return Object.values(
    arrays.reduce((acc, cur) => {
      cur.forEach(prop => acc[prop[key]] = acc[prop[key]] ? Object.assign(acc[prop[key]], prop) : prop)
      return acc
    }, {})
  )
}

console.log('--------')
console.log(mergeBy(array1, array2, 'name'))
console.log('--------')
console.log(mergeBy(array1, array2, 'creationDate'))

// .etc:
// console.log(mergeBy(array1, array2, array3, 'name'))
// console.log(mergeBy(array1, array2, array3, array4, 'name'))