systemjs/docs/relative-dynamic-loading.md

ES6 modules can check their own name from the global variable __moduleName. __moduleAddress is also available.

In CommonJS and AMD, this is available at module.id.

This allows easy relative dynamic loading, allowing modules to load additional functionality after the initial load:

export function moreFunctionality() {
  return System.import('./extrafunctionality', { name: __moduleName });
}

This can be useful for modules that may only know during runtime which functionality they need to load.