Define Object to Object mapping using Typescript decorators

import { Mapper, Converter } from "./typevert";

class A {
    aField: string = "common_field";
    aCollection: Number[] = [1, 2, 3, 4];
}

class B {
    bField: string;
    bCollection: Number[];
}

@Mapper({ sourceType: A, targetType: B }, [
    { source: "aField", target: "bField" },
    { source: "aCollection", target: "bCollection", isCollection: true },
])
class AToBMapper extends Converter<A, B> {}

const a = new A();
const aToBConverter = new AToBMapper();
const resultB = aToBConverter.convert(a);

A common problem when converting classes one to another is that you have to write a lot of boilerplate mapping functions or converters. For example mapping Mongo Entities to your internal objects could produce a significant number of nested mappings, null checks, etc.

Typevert aims to solve this problem.

Instead of:

class DocumentEntity {
    name: string;
    payload: string;
}

class UserEntity {
    id: string;
    roles: string[];
    documents: DocumentEntity[];
}

class Document {
    name: string;
    format: string;
    payload: string;
}

class User {
    id: string;
    roles: string[];
    documents: DocumentEntity[];
}

function mapUserEntityToUser(userEntity: UserEntity) {
    if (userEntity == null) {
        return null;
    }
    const user = new User();
    user.id = userEntity.id;
    user.roles = userEntity.roles == null ? null : userEntity.roles.map(role => role.toUpperCase());
    user.documents = user.documents == null ? null : userEntity.documents.map(doc => mapDocuments(doc));
    return user;
}

function mapDocuments(documentEntity: DocumentEntity) {
    if (documentEntity == null) {
        return null;
    }
    const document = new Document();
    document.name = documentEntity.name;
    document.format = documentEntity.name == null ? null : documentEntity.name.split(".")[1];
    document.payload = documentEntity.payload;
    return document;
}

You can just write:

@Mapper({ sourceType: DocumentEntity, targetType: Document }, [
    { source: "name", target: "name" },
    { source: "format", target: "name", expr: name => name.split(".")[1] },
    { source: "payload", target: "payload" },
])
class DocumentMapper extends Converter<DocumentEntity, Document> {}

@Mapper({ sourceType: DocumentEntity, targetType: Document }, [
    { source: "id", target: "id" },
    { source: "roles", target: "roles", isCollection: true, expr: role => role.toUpperCase() },
    { source: "documents", target: "documents", isCollection: true, converter: DocumentMapper },
])
class UserMapper extends Converter<DocumentEntity, Document> {}

• TypeScript 3.2+
• Node 8+
• emitDecoratorMetadata and experimentalDecorators must be enabled in tsconfig.json

npm install typevert -S

The @Mapper decorator adds the target class the mappingFunction method. It admits converting your source object to target according to the mappings.

Mapper decorator checks that your class is a child of the Converter<IN, OUT> abstract class.

Decorator accepts two arguments:

• MapperDeclaration - which contains mapper description: name, source type and target type

  export interface MapperDeclaration<IN, OUT> {
      name?: string; // Name of the mapper
      sourceType: Constructor<IN>; // Constructor of the source type
      targetType: Constructor<OUT>; // Constructor of the target type
  }

• Mappings - array of rules how to convert one field to another

  export class MappingRules<SourceField, TargetField, SourceFieldType, TargetFieldType> {
      source!: SourceField; // String Field name from the source object which would be mapped
      target!: TargetField; // String Field name from the target object where to map
      default?: TargetFieldType; // Default value if source field is null
      isCollection?: Boolean = false; // Flag that enables Array.map converting for this field
      expr?: (x: SourceFieldType) => TargetFieldType; // Expression for manual converting or preparing field
      converter?: Constructor<Converter<SourceFieldType, TargetFieldType>>; // Converter constructor for nested objects
  }

  Options in mapping rules have order when expr and converter are present :

  1. Exec converting by expr
  2. Exec converting by converter.convert

  Additionally:

  • If source field is null then default value will be set and none expr or converter called
  • If isCollection then for each object in collection mapping will be performed
  • If none expr or converter is set and field value is not null then common assigment is performing