reduceMerge.js

// quick idea to reduce an array by automatically merging result of iteratee(current, i, src) onto acc
const reduceMerge = (arr, iteratee) =>
  arr.reduce((acc, cur, i, src) => {
    const val = iteratee(cur, i, src)
    // allow falsy returns to result in noop for this iteration
    if (!val) {
      return acc
    } else if (val.constructor === Object) {
      return Object.assign(acc, val)
    }
    throw new TypeError(`Cannot reduce and merge non-object value: ${val}`)
  }, {})

const sampleData = [
  { id: 654, firstName: 'John', lastName: 'Doe' },
  { id: 41321, firstName: 'Alias', lastName: 'Pseudonym' },
]

reduceMerge(sampleData, ({ id, firstName, lastName }) => ({
  [id]: `${firstName} ${lastName}`,
}))
// --> {654: "John Doe", 41321: "Alias Pseudonym"}