@kitbag/mapper

A simple and versatile mapping utility for Typescript.

Getting Started

Get started with the documentation

Installation

# bun
bun add @kitbag/mapper
# yarn
yarn add @kitbag/mapper
# npm
npm install @kitbag/mapper

Basic Setup

import mapper from '@kitbag/mapper'

const profiles = [
  createProfile('number', 'string', (source: number): boolean => source.toString())
  createProfile('number', 'Date', (source: number): Date =>new Date(source))
]

mapper.register(profiles)

const dateValue = mapper.map('number', 123, 'Date') // Wed Dec 31 1969...

Kitbag Mapper relies an an underlying set of Profile objects to define what types are supported and how to map between them. To add profiles to Kitbag Mapper, use register.

Type Safety

In order to have type safety when using Kitbag Mapper, you must register your profiles within the Register interface under the namespace @kitbag/mapper.

declare module '@kitbag/mapper' {
  interface Register {
    profiles: typeof profiles
  }
}

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 Kitbag Mapper.

Profiles can be created with the createProfile function, which takes 3 arguments

Argument	Type
SourceKey	`string`
DestinationKey	`string`
MapFunction	`(source: TSource) => TDestination`

import { createProfile } from '@kitbag/mapper'

createProfile('number', 'string', (source: number): boolean => source.toString())

Alternatively, you can define profiles directly by using the Profile type.

import { Profile } from '@kitbag/mapper'

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

or together in an array

import { Profiles } from '@kitbag/mapper'

export const profiles = [
  {
    sourceKey: 'number',
    destinationKey: 'string',
    map: (source: number): string => {
      return source.toString()
    },
  },
  {
    sourceKey: 'number',
    destinationKey: 'Date',
    map: (source: number): Date => {
      return new Date(source)
    },
  }
] as const satisfies Profiles

Note the satisfies operator requires Typescript v4.9+.

Assuming you declared your own Register interface from Type Safety. Kitbag 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"'

ProfileNotFoundError

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

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
   └── maps
      ├── foo.ts
      ├── bar.ts
      ├── index.ts
      └── primitives
        ├── string.ts
        ├── number.ts
        ├── boolean.ts
        └── index.ts

import mapper, { loadProfiles } from '@kitbag/mapper'
import * as profilesToLoad from '@/maps'

const profiles = loadProfiles(profilesToLoad)

mapper.register(profiles)

ProfileTypeError

With most use cases involving an import that is not type safe, it's not unreasonable to assume something that doesn't satisfy Profile will get passed in. If loadProfiles is provided with anything that doesn't satisfy Profile, it will throw ProfileTypeError.

Mapping an array

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

export const arrayToSet = {
  sourceKey: 'array',
  destinationKey: 'Set',
  map: function <T>(source: T[]): Set<T> {
    return new Set<T>(source)
  },
} as const satisfies Profile

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 Kitbag 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 Kitbag Mapper from within the order profile. Both the ObjectId from mongodb and Item mapping logic is likely already encapsulated by another profile. The same mapper can be imported and used within a Profile.

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

Notes

Profile name

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

Missing types or source type `never`

If you're seeing map as (sourceKey: string, source: unknown, destinationKey: string) => unknown, this likely means you missed setting the Register interface. See more about type safety.

This could also be the result of your profiles not using as const satisfies Profile.

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

kitbagjs/mapper