Middleware Authoring

import type { IMiddleware, MiddlewareNext, SunglassesEvent } from '@drakkar.software/sunglasses-core';

class MyMiddleware implements IMiddleware {
  readonly name = 'MyMiddleware'; // must be unique

  async process(event: SunglassesEvent, next: MiddlewareNext): Promise<SunglassesEvent | null> {
    if (shouldDrop(event)) return null;
    return next({ ...event, properties: { ...event.properties, extra: 'value' } });
  }
}

Rules

1. Never throw — return null to drop
2. Always spread when modifying — { ...event, properties: { ...event.properties } }
3. Never log distinctId or user traits
4. Call next(event) to continue the pipeline
5. Async work is fine — the pipeline awaits each middleware

PiiSanitizer is always prepended before your middleware. See Privacy invariants.

Testing

See Testing.