Explicitly declare modifications to an existing object type via a fluent interface which yields a mapping function to the derived type. Demonstration of TypeScript meta-programming through mapped and conditional types.

fluent_mapper_builder_gist.ts

/** 
 * _Explicitly declare modifications_ to an existing object type via a fluent interface which yields a mapping function 
 * from the original data type to the type derived from the modification commands. 
 * 
 * The value over authoring a simple mapping function directly
 * * harder to make a mistake due to the explicit nature
 * * easier to read.
 * 
 * This is _not_ production quality code but rather a _proof of concept_ gist for applying conditional/mapped
 * types to mapper generation. A real-world usage might involve also generating the type-guard function
 * and integrating the entire thing with some kind of Document Database migration system.
 * 
 * **NOTE**: This code file can be copy/pasted into the [TypeScript Playground](http://www.typescriptlang.org/play/)
 * 
 * **NOTE**: Type arguments are named descriptively using camelcase; just like any other argument/variable. 
 **/
module FluentMapperBuilder {
    /** Produce a new type by removing a set of properties from a given an object type. */
    export type RemoveProperties<originalObject extends {}, propertiesToRemove extends (keyof originalObject)[]> =
        propertiesToRemove extends (infer propertiesToRemoveUnion)[]
        ? Pick<originalObject, Exclude<keyof originalObject, propertiesToRemoveUnion>>
        : never;

    /** Produce a new object type changing the type of one of the properties. */
    export type ChangePropertyType<originalObject extends {}, nameOfPropertyToChange extends keyof originalObject, newPropertyType> =
        {
            [currentProperty in keyof originalObject]: currentProperty extends nameOfPropertyToChange
                ? newPropertyType
                : originalObject[currentProperty]
        };

    /** Produce a new object type by renaming a property. */
    export type RenameProperty<originalObject, originalPropertyName extends keyof originalObject, newPropertyName extends string | symbol> =
        // Drop the original name
        Pick<originalObject, Exclude<keyof originalObject, originalPropertyName>>
        & // Add the new property name with the original type
        Record<newPropertyName, originalObject[originalPropertyName]>;
   
    /** A fluent builder for strongly typed object to object transforms. */
    export interface IObjectTypeMapperBuilder<originalObjectInPipeline, transformThisObject> {
        removeProperties<removeProperties extends (keyof transformThisObject)[]>(
            removeSpecification: removeProperties
        ): IObjectTypeMapperBuilder<originalObjectInPipeline, RemoveProperties<transformThisObject, removeProperties>>;

        addProperty<newPropertyName extends string>(newPropertyName: Exclude<newPropertyName, keyof transformThisObject>): {
            withDefaultValue<newPropertyType>(defaultValue: newPropertyType): IObjectTypeMapperBuilder<originalObjectInPipeline, transformThisObject & Record<newPropertyName, newPropertyType>>;
        };

        renameProperty<existingPropertyName extends keyof transformThisObject, newPropertyName extends string>(
            existingPropertyName: existingPropertyName,
            newPropertyName: newPropertyName
        ): IObjectTypeMapperBuilder<originalObjectInPipeline, RenameProperty<transformThisObject, existingPropertyName, newPropertyName>>;

        mapProperty<newType, propertyName extends keyof transformThisObject>(
            propertyName: propertyName,
            map: (val: transformThisObject[propertyName]) => newType
        ): IObjectTypeMapperBuilder<originalObjectInPipeline, ChangePropertyType<transformThisObject, propertyName, newType>>;

        getMapper(): (original: originalObjectInPipeline) => transformThisObject;
    }

    // Interface is exported but the class is not for encapsulation. 
    class ObjectTypeMapperBuilder<
        originalObjectInPipeline extends {}, 
        transformThisObject extends {}
    > implements IObjectTypeMapperBuilder<originalObjectInPipeline, transformThisObject> {

        constructor(private readonly _transform: (original: originalObjectInPipeline) => transformThisObject) { }

        public addProperty<newPropertyName extends string>(newPropertyName: Exclude<newPropertyName, keyof transformThisObject>) {
            const { _transform } = this;
            return { withDefaultValue }

            function withDefaultValue<newPropertyType>(defaultValue: newPropertyType) {
                return new ObjectTypeMapperBuilder((originalObject: originalObjectInPipeline): transformThisObject & Record<newPropertyName, newPropertyType> => {
                    const thisObject = _transform(originalObject);              
                    return {
                        ...thisObject
                        , [newPropertyName]: defaultValue
                    } as any;
                });    
            }
        }
        
        public removeProperties<removeProperties extends (keyof transformThisObject)[]>(
            removeProperties: removeProperties
        ): IObjectTypeMapperBuilder<originalObjectInPipeline, RemoveProperties<transformThisObject, removeProperties>> {
            return new ObjectTypeMapperBuilder((originalObject: originalObjectInPipeline): RemoveProperties<transformThisObject, removeProperties> => {
                const thisObject = this._transform(originalObject);              
                const newObj: { [index: string]: any } = {};
                Object.keys(thisObject).forEach((property) => {
                    if (removeProperties.indexOf(property as any) === -1)
                        newObj[property] = (thisObject as any)[property];
                });
                return newObj as any;
            });
        }

        public renameProperty<existingPropertyName extends keyof transformThisObject, newPropertyName extends string>(
            existingPropertyName: existingPropertyName,
            newPropertyName: newPropertyName
        ): IObjectTypeMapperBuilder<originalObjectInPipeline, RenameProperty<transformThisObject, existingPropertyName, newPropertyName>> {
            return new ObjectTypeMapperBuilder((originalObject: originalObjectInPipeline): RenameProperty<transformThisObject, existingPropertyName, newPropertyName> => {
                const thisObject = this._transform(originalObject);              
                const newObj: { [index: string]: any } = {};
                Object.keys(thisObject).forEach((property) => {
                    newObj[property === existingPropertyName ? newPropertyName : property] = (thisObject as any)[property];
                });
                return newObj as any;
            });
        }

        public mapProperty<newType, propertyName extends keyof transformThisObject>(
            propertyName: propertyName,
            transform: (val: transformThisObject[propertyName]) => newType
        ): IObjectTypeMapperBuilder<originalObjectInPipeline, ChangePropertyType<transformThisObject, propertyName, newType>> {
            return new ObjectTypeMapperBuilder((originalObject: originalObjectInPipeline): ChangePropertyType<transformThisObject, propertyName, newType> => {
                const thisObject = this._transform(originalObject);
                return {
                    ...thisObject
                    , [propertyName]: transform(thisObject[propertyName])
                } as any;
            });
        }

        public getMapper(): (original: originalObjectInPipeline) => transformThisObject {
            return this._transform;
        }

    }

    /** Start building a mapper that maps from this type. */ 
    export const from = <T>(): IObjectTypeMapperBuilder<T, T> => 
        new ObjectTypeMapperBuilder<T, T>(it => it);
}

//----------------------------------------------------------------------------
// Example Usage
//----------------------------------------------------------------------------
module ExampleTypesAndMappers {
    // Starting data type
    export type Person = {
        fName: string;
        lName: string;
        dob: Date;
        selfEsteem: "Low" | "Medium" | "High";
        flurbNibble: number;
    }

    // An enumerated string data type
    export type Gender = "Unknown" | "Female" | "Male" | "Other";

    // mapper
    const dateToDateJson = (date: Date) => ({ 
        year: date.getFullYear()
        , month: date.getMonth()
        , date: date.getDate() 
    });

    // declare a mapper function using fluent interface
    export const mapToPersonV2 = FluentMapperBuilder.from<Person>()
        .addProperty("weightInLbs").withDefaultValue<number | null>(null)
        .addProperty("gender").withDefaultValue<Gender>("Unknown")
        // Uncomment the line below to watch the compiler complain because gender cannot be added again
        //.addProperty("gender").withDefaultValue("Male")
        .mapProperty("dob", dateToDateJson)
        .renameProperty("lName", "lastName")
        .renameProperty("fName", "firstName")
        .removeProperties(["selfEsteem", "flurbNibble"])
        .getMapper();

    // get the result type from the mapper
    export type PersonV2 = ReturnType<typeof mapToPersonV2>;
}


const originalJim: ExampleTypesAndMappers.Person = {
    fName: "Jim"
    , lName: "Smith"
    , dob: new Date(1980, 1, 1)
    , selfEsteem: "Low"
    , flurbNibble: 42
};
    
const jimV2 = ExampleTypesAndMappers.mapToPersonV2(originalJim);

// Hover over a field to inspect type/structure.
[
    jimV2.firstName
    , jimV2.lastName
    , jimV2.gender
    , jimV2.dob
    , jimV2.weightInLbs
    //, jimV2.flurbNibble
    //, jimV2.selfEsteem
];
console.log("Original", originalJim);
console.log("Mapped", jimV2);
alert("Check the Dev console for output.");