@stackoverfloweth/mapper

mapper

Basic Setup

Each mapper relies an an underlying set of Profile objects to define what types are supported and how to map between them. Mappers are created with createMapper function, which takes Profile[] as it's only argument.

import { createMapper } from '@stackoverfloweth/mapper'

const mapper = createMapper(profiles)

mapper.map('source-key', source, 'destination-key')

Profiles

Each profile defines a value for sourceKey and destinationKey. These keys must extend string and should be unique, beyond that the choice is irrelevant to the function of the mapper.

Here are a couple simple examples of Profile objects

export const numberToString = {
  sourceKey: 'number',
  destinationKey: 'string',
  map: (source) => {
    return source.toString()
  },
} as const satisfies Profile

export const numberToDate = {
  sourceKey: 'number',
  destinationKey: 'Date',
  map: (source) => {
    return new Date(source)
  },
} as const satisfies Profile

The mapper will use the keys you define to provide type safety when calling map.

mapper.map('number', 123, 'string') // "123"
mapper.map('number', 123, 'Date')   // Wed Dec 31 1969...
mapper.map('number', 123, 'potato') // ERROR TS:2345 Argument of type '"potato"' is not assignable to parameter of type '"string" | "Date"'

Anytime mapper.map is called with source and/or destination keys that are not registered by a profile, it will throw the following error.

'Mapping profile not found'

Loading profiles automatically

This library provides a useful method for automatically loading profiles. If you store all of your profiles in the same folder with a simple barrel file.

└── src
   ├── models
   └── maps
      ├── primitives.ts
      ├── foo.ts
      ├── bar.ts
      └── index.ts

const profiles = await loadProfiles(() => import('src/maps'))

const mapper = createMapper(profiles)

Mapping an array

Because TSource and TDestination are not constrained, you can always define a profile that expects an array.

export const numbersArrayToNumbersSet: Profile<'array', unknown[], 'Set', Set<unknown>> = {
  sourceKey: 'array',
  destinationKey: 'Set',
  map: (source) => {
    return new Set(source)
  },
}

However, if your goal is use the same mapping profile over an array of sources you can use either

const mapped = sources.map(source => mapper.map('source-key', source, 'destination-key'))

or the mapper provides a simpler method mapMany, which takes an array of TSource and returns an array TDestination.

const mapped = mapper.mapMany('source-key', sources, 'destination-key')

Nesting profiles

Sometimes your business logic for mapping from TSource to TDestination might benefit from nesting profiles inside of other profiles. For example, if you have the following models

// src/models/order.ts

export type Order = {
  orderId: string,
  total: number,
  items: Item[],
}

// src/models/item.ts

export type Item = {
  itemId: string,
  title: string,
  description: string,
}

and you need to map from api models

// src/models/api/orderResponse.ts

export type OrderResponse = {
  _id: ObjectId,
  totalInPennies: number,
  items?: ItemResponse[],
}

// src/models/api/itemResponse.ts

export type ItemResponse = {
  _id: ObjectId,
  title: string,
  description?: string,
}

There are a couple opportunities to use the mapper from within the order profile. Both the ObjectId from mongodb and Item mapping logic is likely already encapsulated by another profile. This is where the mapper argument provided to the profile map method comes in handy.

export const orderResponseToOrder: Profile<'OrderResponse', OrderResponse, 'Order', Order> = {
  sourceKey: 'OrderResponse',
  destinationKey: 'Order',
  map: (source, mapper) => {
    return {
      orderId: mapper.map('ObjectId', source._id, 'string'),
      total: number,
      items: mapper.map('ItemResponse', source.items ?? [], 'Item'),
    }
  },
}

However, there's a catch. Because the type safety is controlled by your application supplying the profiles to createMapper, the library itself cannot have an accurate type for mapper here. The type provided by default is AnyMapper, which unlike the mapper you get back from createMapper will have no constraints on keys or types.

The solution to this is to take advantage of ts duck typing by defining your own type for MapFunction that has narrowed types but still satisfies the requirement of Profile.map.

export const mapper = createMapper(profiles)
export type Mapper = typeof mapper

export type MapFunction<TSource, TDestination> = (source: TSource, mapper: Mapper) => TDestination

Now you can rewrite your order profile with type safety

import { MapFunction } from 'src/services'

const map: MapFunction<OrderResponse, Order> = (source, mapper) => {
  return {
    orderId: mapper.map('ObjectId', source._id, 'string'),
    total: number,
    items: mapper.map('ItemResponse', source.items ?? [], 'Item'),
  }
}

export const orderResponseToOrder: Profile<'OrderResponse', OrderResponse, 'Order', Order> = {
  sourceKey: 'OrderResponse',
  destinationKey: 'Order',
  map,
}

Notes

• What you chose to name the profile doesn't matter to the mapper. In these examples I used the pattern ${sourceKey}To${destinationKey} but this key is not currently used by loadProfiles() in any way.