Application context

There are several ways of mounting the Nest application. You can create either a web app, microservice or just a Nest application context. Nest context is a wrapper around the Nest container, which holds all instantiated classes. We can grab an existing instance from within any imported module directly using application object. Hence, you can take advantage of the Nest framework everywhere, including CRON jobs and even build a CLI on top of it.

Getting started

In order to create a Nest application context, we use the following syntax:

@@filename()
async function bootstrap() {
  const app = await NestFactory.createApplicationContext(ApplicationModule);
  // application logic...
}
bootstrap();

Afterward, Nest allows you to pick any instance registered within Nest application. Let’s imagine that we have a TasksService in the TasksModule. This class provides a set of usable methods, which we want to call from within CRON job.

@@filename()
const app = await NestFactory.create(ApplicationModule);
const tasksService = app.get(TasksService);

And that’s it. To grab TasksService instance we had to use get() method. We didn’t have to go through entire modules tree, the get() method act like a query that search for an instance in each registered module automatically. However, if you prefer a strict context checking, you can always switch to it using strict: true options object that has to be passed as the second argument of get() method. Then, you have to go through all modules to pick up a particular instance from the selected context.

@@filename()
const app = await NestFactory.create(ApplicationModule);
const tasksService = app.select(TasksModule).get(TasksService, { strict: true });

info Hint In non-strict mode, the root module is selected by default. In order to select any other module, you need to go through entire modules tree (step by step).

If you want the node application to close after the script finishes (useful for CRON jobs), add await app.close() to the end of your bootstrap function:

@@filename()
async function bootstrap() {
  const app = await NestFactory.createApplicationContext(ApplicationModule);
  // application logic...
  await app.close();
}
bootstrap();