Work has been going non-stop on the upcoming LoopBack 4.0, and we’d like to share some of what we’ve just added in the current release!

We’ve added the new controller generation command to the lb4 CLI toolkit, which you can install with npm i -g @loopback/cli.

How controllers work in LoopBack 4

Controllers are what sit in the middle of the request-response lifecycle of LoopBack 4. Incoming requests are routed by the RestServer to an instance of a controller that has been configured to handle the appropriate route. Controllers often work with a repository and a model definition in order to facilitate CRUD operations between the client making requests and whatever datasources might be wired up to your LoopBack 4 application.

Previously, you would have to build most of the boilerplate for a controller yourself. Now, all you need to do is run a command and answer a few simple questions!

Using the new controller template tool

Install the new loopback CLI toolkit:

$ npm i -g @loopback/cli

Run the command to download the template application:

$ lb4 example
? What example would you like to clone?
  codehub: A GitHub-like application we used to use to model LB4 API.
❯ getting-started: An application and tutorial on how to build with LoopBack 4.

Jump into the loopback4-example-getting-started directory and install the dependencies required to run it.

$ cd loopback4-example-getting-started
$ npm i

At the time of this writing, the CLI template is a closer representation of the end goal than the original TodoController in loopback4-example-getting-started, so we’re going to replace the existing TodoController (todo.controller.ts) with a new one. Delete the controller from the controllers folder. Additionally, since this will break the existing test suite, we’ll remove that as well.

$ rm ./src/controllers/todo.controller.ts
$ rm ./test/unit/controllers/todo.controller.test.ts

Run the lb4 controller command and generate a replacement.

$ lb4 controller

At the prompt, enter the name of the controller. You don’t need to add the word “Controller” at the end.

? Controller class name: Todo
? What kind of controller would you like to generate?
  Empty Controller
❯ REST Controller with CRUD functions

Next, select from the list of available models. In this example, we only have one model.

? What is the name of the model to use with this CRUD repository?
❯ Todo

Now select from the available repositories. This example is also limited to one repository.

? What is the name of your CRUD repository?
❯ TodoRepository

Finally, define what type is being used for the Model ID. This is currently limited to numbers, strings, and objects.

? What is the type of your ID?
❯ number

The tool will generate the new controller file in the original location.

   create src/controllers/todo.controller.ts

Controller Todo is now created in src/controllers/

If we look inside the controller file, it should look something like this.

import {Filter, Where} from '@loopback/repository';
import {post, param, get, put, patch, del} from '@loopback/openapi-v2';
import {inject} from '@loopback/context';
import {Todo} from '../models';
import {TodoRepository} from '../repositories';

export class TodoController {

    public todoRepository : TodoRepository,
  ) {}

  async create(@param.body('obj') obj: Todo)
    : Promise<Todo> {
    return await this.todoRepository.create(obj);

  async count(@param.query.string('where') where: Where) : Promise<number> {
    return await this.todoRepository.count(where);

  async find(@param.query.string('filter') filter: Filter)
    : Promise<Todo[]> {
      return await this.todoRepository.find(filter);

  async updateAll(@param.query.string('where') where: Where,
    @param.body('obj') obj: Todo) : Promise<number> {
      return await this.todoRepository.updateAll(where, obj);

  async deleteAll(@param.query.string('where') where: Where) : Promise<number> {
    return await this.todoRepository.deleteAll(where);

  async findById(@param.path.number('id') id: number) : Promise<Todo> {
    return await this.todoRepository.findById(id);

  async updateById(@param.path.number('id') id: number, @param.body('obj')
   obj: Todo) : Promise<boolean> {
    return await this.todoRepository.updateById(id, obj);

  async deleteById(@param.path.number('id') id: number) : Promise<boolean> {
    return await this.todoRepository.deleteById(id);

You can then modify the template to add business logic to these handler functions, or add totally new ones. Feel free to start the app and play around with it! In this example, I grab the existing todos from the sample database and pipe them into jq for some pretty formatting.

npm start
Server is running on port 3000
$ curl | jq
    "id": 1,
    "title": "Take over the galaxy",

Call for action

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