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In LoopBack 4, controllers handle the request-response lifecycle for your API. Each function on a controller can be addressed individually to handle an incoming request (like a POST request to /todos), to perform business logic, and to return a response.

Controller is a class that implements operations defined by application’s API. It implements an application’s business logic and acts as a bridge between the HTTP/REST API and domain/database models.

In this respect, controllers are the regions in which most of your business logic will live!

For more information about Controllers, see Controllers.

Create your controller

You can create a REST controller using the CLI as follows:

lb4 controller
? Controller class name: todo
Controller Todo will be created in src/controllers/todo.controller.ts

? What kind of controller would you like to generate? REST Controller with CRUD functions
? What is the name of the model to use with this CRUD repository? Todo
? What is the name of your CRUD repository? TodoRepository
? What is the name of ID property? id
? What is the type of your ID? number
? Is the id omitted when creating a new instance? Yes
? What is the base HTTP path name of the CRUD operations? /todos
   create src/controllers/todo.controller.ts
   update src/controllers/index.ts

Controller Todo was created in src/controllers/

Let’s review the TodoController located in src/controllers/todo.controller.ts. The @repository decorator will retrieve and inject an instance of the TodoRepository whenever an inbound request is being handled. The lifecycle of controller objects is per-request, which means that a new controller instance is created for each request. As a result, we want to inject our TodoRepository since the creation of these instances is more complex and expensive than making new controller instances.

In this example, there are two new decorators to provide LoopBack with metadata about the route, verb and the format of the incoming request body:

  • @post('/todos') creates metadata for @loopback/rest so that it can redirect requests to this function when the path and verb match.
  • @requestBody() associates the OpenAPI schema for a Todo with the body of the request so that LoopBack can validate the format of an incoming request.

Some additional things to note about this example:

  • Routes like @get('/todos/{id}') can be paired with the @param.path decorators to inject those values at request time into the handler function.
  • LoopBack’s @param decorator also contains a namespace full of other “subdecorators” like @param.path, @param.query, and @param.header that allow specification of metadata for those parts of a REST request.
  • LoopBack’s @param.path and @param.query also provide subdecorators for specifying the type of certain value primitives, such as @param.path.number('id').

To view the completed file, see the Todo example.

Now that we’ve wired up the controller, our last step is to tie it all into the Application!

Previous step: Add a repository

Final step: Putting it all together