Group objects by a string field

group.js

function group (rows, discriminant) {
  const asMap = rows.reduce(
    (acc, item) => {
      const key = item[discriminant]
      return {
        ...acc,
        [key]: acc[key] ? [...acc[key], item] : [item]
      }
    },
    {}
  )
  const asArray = Object.keys(asMap).reduce(
    (acc, key) => [
      ...acc,
      [...asMap[key]]
    ],
    []
  )
  return {
    asMap,
    asArray
  }
}