Type alias MediatorMiddleware<Context, EventName>

Represents a middleware function for Mediator events. Middlewares execute before event listeners and can observe, transform, or cancel events.

Returns

Either undefined (pass through), modified input data, or a cancel event.

Example

// Logger middleware (observes only)
const logger: MediatorMiddleware<MyContext, string> = (context, input, event) => {
  console.log(`Event ${event} triggered`)
  // No return - passes through unchanged
}

// Validation middleware (can cancel)
const validator: MediatorMiddleware<MyContext, string> = (context, input, event) => {
  if (input.pendingChanges && 'count' in input.pendingChanges && input.pendingChanges.count < 0) {
    return { cancel: true } // Stop processing
  }
  return input // Pass through
}

// Transform middleware (modifies data)
const transformer: MediatorMiddleware<MyContext, string> = (context, input, event) => {
  return {
    pendingChanges: {
      ...(input.pendingChanges ?? {}),
      timestamp: Date.now()
    }
  }
}
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