Module Singleton

Definition

The Module Singleton pattern exploits the ES module cache to maintain a unique global state. A private module variable is shared by all importers — no static class or global registry required.

Diagram

graph TD
    A["@granit/react-authentication"] -->|"setTokenGetter()"| Singleton["_tokenGetter\n(module variable)"]
    B["@granit/api-client\n(interceptor)"] -->|"reads"| Singleton

Implementation in Granit

Singleton variable	Package	Purpose
_tokenGetter	@granit/api-client	Bearer token getter shared between auth and HTTP client
Global log level	@granit/logger	Shared log level across all logger instances

Rationale

Token management requires coordination between @granit/react-authentication (which obtains tokens from Keycloak) and @granit/api-client (which injects them into HTTP requests). A module singleton avoids direct package coupling — the auth package calls setTokenGetter() once during startup, and the interceptor reads it on every request.

Usage example

// @granit/api-client — private module variable
let _tokenGetter: (() => Promise<string | undefined>) | null = null;

export function setTokenGetter(
  getter: () => Promise<string | undefined>
): void {
  _tokenGetter = getter;
}

// Axios interceptor reads _tokenGetter on every request
instance.interceptors.request.use(async (req) => {
  if (_tokenGetter) {
    const token = await _tokenGetter();
    if (token) req.headers.Authorization = `Bearer ${token}`;
  }
  return req;
});

useKeycloakInit calls setTokenGetter() internally — the application never wires this manually.

Further reading

• Singleton — refactoring.guru