Nighttime Peanut Migrations

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    1.0.2 • Public • Published


    Simple data mapping utility for easier hexagonal architecture implementation.

    Main purpose of the library is to reduce the amount of code that you need to write to map your objects.


    A method that allows you to map objects synchronously or asynchronously using specification. To map an object you need to pass two parameters to the Map function:

    1. specification object
    2. object that you want to map

    As a result the function will return the object that is the result of mapping.

    Specification object

    Is the object where you specify how to acquire properties of the destination object from the source object.

    Each property has a name that corresponds to the specific property P of the destination object. Every property is a function that takes a deep copy of the source object and returns a value that should be put in the destination object at P key.

    Map specification looks like this

    export type ObjectMapSpec<O1 extends DataObject, O2 extends DataObject> = {
        [P in keyof O2]: (o: O1) => O2[P]
    } & {[MapperSpecOptionsSym]?: {async?: false}};
    // or async version
    export type AsyncObjectMapSpec<O1 extends DataObject, O2 extends DataObject> = {
        [P in keyof O2]: (o: O1) => O2[P] | Promise<O2[P]>
    } & {[MapperSpecOptionsSym]: {async: true}};      // you need to set async option to true


    Simple object

    type UserInfo = {
        userId: string,
        colorOfEyes: string,
        role: 'user' | 'admin',
        city: string,
        location: [number, number],
    type UserInfoView = {
        colorOfEyes: [number, number, number],
        role: 'user' | 'admin',
        city: string,
        location: string,
    const UserInfoUserInfoViewMapSpec: ObjectMapSpec<UserInfo, UserInfoView> = {
        role: o => o.role,
        city: o =>,
        location: o => o.location[0] + '.' + o.location[1],
        colorOfEyes: o => MapColor(o.colorOfEyes)
    const userInfo = {
        userId: 'dsf79f8s98f7',
        role: 'user',
        colorOfEyes: '2FA8FF',
        city: 'London',
        location: [2141241, 509520523],
    const userInfoView = Map(UserInfoUserInfoViewMapSpec, userInfo);

    Result will be:

      "role": "user",
      "city": "LONDON",
      "location": "2141241.509520523",
      "colorOfEyes": [47, 168, 255]

    Mapping nested objects

    1. Using specification to map nested objects For bigger nested objects with a lot of mapping logic it's recommended to put mapping logic into another specification and use Map function inside the specification like so:
    const userMapSpec = {
        userInfo: o => Map(UserInfo_UserInfoViewMapSpec, o.userInfo)
        // ...
    1. Mapping by hand Smaller objects can be mapped directly without creating another specification.
    userInfo: o => ({
        role: o.userInfo.role,
        location: o.userInfo.location,
        colorOfEyes: MapColor(o.userInfo.colorOfEyes)


    npm i easymapper

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    • francoisvong