Providers

Providers are generally constructors that can be instantiated. It can also be any factory method and declared values. They will all be registered in the container for dependency lookup and injection.

Class Provider

JavaScript constructors that do not require additional parameters are all available providers.

class UserService {
  async getUser() {
    // fetch and process user
  }
}

In most cases, since the function name will be lost after the production build, you must specify the name of the provider through token option.

const provider = {
  token: 'UserService',
  useClass: UserService
};

The token can also be a Symbol:

// Specify a symbol as the name of the provider.
const token = Symbol('svc:user');
const provider = {
  token,
  useClass: UserService
};

Factory Provider

Use factory methods to customize instance creation, you can return any instance including promise.

Create Customized Instance

const roles = ['ADMIN'];

const provider = {
  token: 'AuthService',
  useFactory() {
    // customize creation
    return new RoleAuthService(roles);
  }
};

Create Async Instance

Sometimes, the instance that needs to be created depends on some remote data, and we can return a promise through the factory method.

// async factory
async function createAuthService() {
  const roles = await fetchRoles();

  return new RoleAuthService(roles);
}

Definition of provider:

const provider = {
  token: 'AuthService',
  useFactory: createAuthService
};

Value Provider

Declared values such as configuration, constants, etc.

export const CONSTANTS = {
  BASE_URL: '/path/to/base/url'
};

Definition of provider:

const provider = {
  token: 'AppConstant',
  useValue: CONSTANTS
};