Javascript ES6 API to model mapper functions

modelMapper.js

import assign from 'lodash/assign';

/**
 * Converts one type of object to another, in this case to/from our Models to
 * plain API response objects.
 * @param {object} sourceObj The source object, either a Model or a plain API response object
 * @param {array} mapping An array of arrays, the inner arrays containing key mappings between objects.
 *                        Inner array format can be either ['modelPropertyKey', ['apiPropertyKey'] or
 *                        ['modelPropertyKey', apiToModelTransformer(), modelToApiTransformer()]
 * @param {bool} isMappingModelToApi True if we are taking a Model object an
 *                                   mapping to an API response object, otherwise false.
 */
function mappingReducer(sourceObj, mapping, isMappingModelToApi) {
  const sourceMapIndex = isMappingModelToApi ? 0 : 1;
  const targetMapIndex = isMappingModelToApi ? 1 : 0;
  const lambdaMapIndex = isMappingModelToApi ? 2 : 1;

  // Iterate through each element of the `mapping` object,
  // and map the source property to the target property.
  return mapping.reduce((targetObj, mapEl) => {
    if (mapEl.length === 3) {
      // We are using mapping functions to generate the result.
      // Process and Object.assign the result.
      if (mapEl[lambdaMapIndex] !== null) {
        const result = mapEl[lambdaMapIndex](sourceObj);
        assign(targetObj, result);
      }
    } else {
      // Just a simple straight mapping conversion.
      targetObj[mapEl[targetMapIndex]] = sourceObj[mapEl[sourceMapIndex]];
    }

    return targetObj;
  }, {});
}

/**
 * Converts a Model to an API response object
 * @param {object} model The model to convert to a POJO
 * @param {array} modelMap An array of arrays, the inner arrays containing key mappings between objects.
 *                         Inner array format can be either ['modelPropertyKey', ['apiPropertyKey'] or
 *                         ['modelPropertyKey', apiToModelTransformer(), modelToApiTransformer()]
 */
export function mapModelToApi(model, modelMap) {
  return mappingReducer(model, modelMap, true);
}

/**
 * Converts a plain API response object to a Model.
 * @param {object} apiObject The API response to convert to a Model
 * @param {array} modelMap An array of arrays, the inner arrays containing key mappings between objects.
 *                         Inner array format can be either ['modelPropertyKey', ['apiPropertyKey'] or
 *                         ['modelPropertyKey', apiToModelTransformer(), modelToApiTransformer()]
 * @param {Prototype} modelPrototype The type of model we are creating (e.g., Referral)
 */
export function mapApiToModel(apiObject, modelMap, modelPrototype) {
  const data = mappingReducer(apiObject, modelMap, false);
  return new modelPrototype(data);
}

modelMapper.test.js

import expect from 'expect';
import { mapModelToApi, mapApiToModel } from './modelMapper';

function TestModel(data) {
  this.testElement = data ? data.testElement : 'WRONG1';
  this.nestedElement = data ? data.nestedElement : 'WRONG2';
}

const exampleTestModel = new TestModel({
  testElement: 'GOOD1',
  nestedElement: {
    nestedElement1: 'GOOD2',
    nestedElement2: 'GOOD3',
  },
});

const apiResponse = {
  test_element: 'GOOD1',
  nested_element_1: 'GOOD2',
  nested_element_2: 'GOOD3',
};

const mapping = [
  ['testElement', 'test_element'],
  [
    'nestedElement',
    apiObj => ({
      nestedElement: {
        nestedElement1: apiObj.nested_element_1,
        nestedElement2: apiObj.nested_element_2,
      },
    }),
    modelObj => ({
      nested_element_1: modelObj.nestedElement.nestedElement1,
      nested_element_2: modelObj.nestedElement.nestedElement2,
    }),
  ],
];

describe('Model mapper functions', () => {
  describe('Model --> API response', () => {
    it('should map basic elements directly', () => {
      const result = mapModelToApi(exampleTestModel, mapping);

      expect(result).toHaveProperty('test_element');
      expect(result.test_element).toBe(exampleTestModel.testElement);
    });

    it('should map nested elements correctly', () => {
      const result = mapModelToApi(exampleTestModel, mapping);

      expect(result).toHaveProperty('nested_element_1');
      expect(result).toHaveProperty('nested_element_2');
      expect(result.nested_element_1).toBe(
        exampleTestModel.nestedElement.nestedElement1
      );
      expect(result.nested_element_2).toBe(
        exampleTestModel.nestedElement.nestedElement2
      );
    });
  });
  describe('API Response --> Model', () => {
    it('should map basic elements directly', () => {
      const result = mapApiToModel(apiResponse, mapping, TestModel);

      expect(result).toHaveProperty('testElement');
      expect(result.testElement).toBe(apiResponse.test_element);
    });

    it('should map flat properties to nested elements', () => {
      const result = mapApiToModel(apiResponse, mapping, TestModel);

      expect(result).toHaveProperty('nestedElement');
      expect(result.nestedElement).toHaveProperty('nestedElement1');
      expect(result.nestedElement).toHaveProperty('nestedElement2');
      expect(result.nestedElement.nestedElement1).toBe(
        apiResponse.nested_element_1
      );
      expect(result.nestedElement.nestedElement2).toBe(
        apiResponse.nested_element_2
      );
    });

    it('should return an object of the correct prototype', () => {
      const result = mapApiToModel(apiResponse, mapping, TestModel);

      expect(result).toBeInstanceOf(TestModel);
    });
  });
});

Thanks @pedrogquirino, much appreciated. There's a little more detail over on the repo page too, if you're interested: https://github.com/jlyman/simple-model-mapper