just a simple reduce with some domain, pre-refactoring

flag_combine.js

import {List, Map} from 'immutable'

test('test name goes here', () => {
  const currentFlags = List([
    Map({id: 7, tag: 'flag/bbb:JAR'}),
    Map({id: 6, tag: 'flag/bbb'}),
    Map({id: 9, tag: 'flag/ccc:ADV'}),
    Map({id: 8, tag: 'flag/bbb:ELI'}),
    Map({id: 10, tag: 'flag/ccc:JAR'})
  ])
  const expectedResults = Map([
    [List([7, 6, 8]), 'bbb'],
    [List([9, 10]), 'ccc'],
  ])
  expect(availableFlagsOptions(currentFlags)).toStrictEqual(Map(expectedResults))
})

export function availableFlagsOptions (currentFlags) {
  let availableFlags = Map()
  if (currentFlags) {
    const reversedMultiIdMapping = currentFlags.reduce((accumulator, flag) => {
      const flagBaseName = getFlagBaseName(flag)
      let ids = accumulator.get(flagBaseName) || List()
      ids = ids.push(flag.get('id'))
      let result = accumulator.set(flagBaseName, ids)
      return result
    }, Map())

    availableFlags = reversedMultiIdMapping.mapEntries(([k, v]) => [v, k])
  }
  return availableFlags
}

const FLAG_PREFIX = 'flag/'
const PENDING_FLAG_PREFIX = `${FLAG_PREFIX}pending/`

export function getFullFlagName (flag) {
  return flag.get('tag')
}

export function getFlagName (flag) {
  const flagName = getFullFlagName(flag)
  if (flagName.startsWith(PENDING_FLAG_PREFIX)) {
    return flagName.slice(PENDING_FLAG_PREFIX.length)
  } else if (flagName.startsWith(FLAG_PREFIX)) {
    return flagName.slice(FLAG_PREFIX.length)
  }
  return flagName
}

export function getFlagBaseName (flag) {
  const flagName = getFlagName(flag)
  return flagName.split(':')[0]
}

export function flagIsPending (flag) {
  return getFullFlagName(flag).startsWith(PENDING_FLAG_PREFIX)
}