Create Nested Immutable Record with full type support

createCarRecord.js

// @flow strict

/**
 * Purpose: convert object
 * {
 *   name: 'Andrew',
 *   email: '[email protected]',
 *   car: {
 *     type: 'Ford',
 *     cost: 10,
 *     usageDetails: {
 *       distanceTravelled: 0,
 *       averageMileage: 0
 *     }
 *   }
 * }
 *
 * into immutable object
 *
 * result:
 *
 * [Flow]
 * car: ?RecordOf<{
 *   cost: ?number,
 *   type: ?string,
 *   usageDetails: ?RecordOf<{
 *     averageMileage: ?number,
 *     distanceTravelled: ?number
 *   }>
 * }>
 */

const _UsageDetails: RecordFactory<{
  distanceTravelled: ?number,
  averageMileage: ?number
}> = Record({ distanceTravelled: null, averageMileage: null })

const _Car: RecordFactory<{
  type: ?string,
  cost: ?number,
  usageDetails: ?$Call<typeof _UsageDetails, {}>
}> = Record({ type: null, cost: null, usageDetails: null })

const createCarRecord = (
  obj: Object
): $Call<RecordFactory<{
    name: ?string,
    email: ?number,
    car: ?$Call<typeof _Car, {}>
  }>,
  {}> => {
  // start with converting the nested object fields
  const usageDetails = _UsageDetails(obj.car.usageDetails)

  const car = _Car({
    ...obj.car,
    usageDetails
  })

  return Record({
    name: null,
    email: null,
    car: null
  })({
    ...obj,
    car
  })
}

const carRecord = createCarRecord({
  name: 'Andrew',
  email: '[email protected]',
  car: {
    type: 'Ford',
    cost: 10,
    usageDetails: {
      distanceTravelled: 0,
      averageMileage: 0
    }
  }
})

const car = carRecord.car

export default createCarRecord