LoopBack 4 Todo Application Tutorial - Integrate with a geo-coding service
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To call other APIs and web services from LoopBack applications, we recommend to use Service Proxies as a design pattern for encapsulating low-level implementation details of communication with 3rd-party services and providing JavaScript/TypeScript API that’s easy to consume e.g. from Controllers. See Calling other APIs and web services for more details.

In LoopBack, each service proxy is backed by a DataSource, this datasource leverages one of the service connectors to make outgoing requests and parse responses returned by the service.

In our tutorial, we will leverage US Census Geocoder API to convert textual US addresses into GPS coordinates, thus enabling client applications of our Todo API to display location-based reminders,

Configure the backing datasource

Run lb4 datasource to define a new datasource connecting to Geocoder REST service. When prompted for a connector to use, select “REST services”.

$ lb4 datasource
? Datasource name: geocoder
? Select the connector for geocoder: REST services (supported by StrongLoop)
? Base URL for the REST service:
? Default options for the request:
? An array of operation templates:
? Use default CRUD mapping: No
   create src/datasources/geocoder.datasource.json
   create src/datasources/geocoder.datasource.ts
 # npm will install dependencies now
    update src/datasources/index.ts

Datasource geocoder was created in src/datasources/

Edit the newly created datasource configuration to configure Geocoder API endpoints. Configuration options provided by REST Connector are described in our docs here: REST connector.


  "connector": "rest",
  "options": {
    "headers": {
      "accept": "application/json",
      "content-type": "application/json"
  "operations": [
      "template": {
        "method": "GET",
        "query": {
          "format": "{format=json}",
          "benchmark": "Public_AR_Current",
          "address": "{address}"
        "responsePath": "$.result.addressMatches[*].coordinates"
      "functions": {
        "geocode": ["address"]

Implement a service provider

Create a new directory src/services and add the following two new files:

  • src/geocoder.service.ts defining TypeScript interfaces for Geocoder service and implementing a service proxy provider.
  • src/index.ts providing a conventient access to all services via a single import statement.


import {getService, juggler} from '@loopback/service-proxy';
import {inject, Provider} from '@loopback/core';
import {GeocoderDataSource} from '../datasources/geocoder.datasource';

export interface GeoPoint {
   * latitude
  y: number;

   * longitude
  x: number;

export interface GeocoderService {
  geocode(address: string): Promise<GeoPoint[]>;

export class GeocoderServiceProvider implements Provider<GeocoderService> {
    protected datasource: juggler.DataSource = new GeocoderDataSource(),
  ) {}

  value(): GeocoderService {
    return getService(this.datasource);


export * from './geocoder.service';

Register the service for dependency injection

Because @loopback/boot does not support loading of services yet (see issue #1439), we need to add few code snippets to our Application class to take care of this task.


export class TodoListApplication extends BootMixin(
) {
  constructor(options?: ApplicationConfig) {
    // etc., keep the existing code without changes


  setupServices() {

  service<T>(provider: Constructor<Provider<T>>) {
    const key = `services.${provider.name.replace(/Provider$/, '')}`;

Enhance Todo model with location data

Add two new properties to our Todo model: remindAtAddress and remindAtGeo.


export class Todo extends Entity {
  // original code remains unchanged, add the following two properties:

    type: 'string',
  remindAtAddress?: string; // address,city,zipcode

    type: 'string',
  remindAtGeo?: string; // latitude,longitude

Look up address location in the controller

Finally, modify TodoController to look up the address and convert it to GPS coordinates when a new Todo item is created.

Modify the Controller constructor to receive GeocoderService as a new dependency.


export class TodoController {
    @repository(TodoRepository) protected todoRepo: TodoRepository,
    @inject('services.GeocoderService') protected geoService: GeocoderService,
  ) {}

  // etc.

Modify createTodo method to look up the address provided in remindAtAddress property and convert it to GPS coordinates stored in remindAtGeo.


export class TodoController {
  // constructor, etc.

  async createTodo(@requestBody() todo: Todo) {
    if (!todo.title) {
      throw new HttpErrors.BadRequest('title is required');

    if (todo.remindAtAddress) {
      // TODO handle "address not found"
      const geo = await this.geoService.geocode(todo.remindAtAddress);
      // Encode the coordinates as "lat,lng"
      todo.remindAtGeo = `${geo[0].y},${geo[0].x}`;

    return await this.todoRepo.create(todo);

  // other endpoints remain unchanged

Congratulations! Now your Todo API makes it easy to enter an address for a reminder and have the client application show the reminder when the device reaches close proximity of that address based on GPS location.

Previous step: Putting it all together