---

name: understanding-destinations description: Use when working with destinations, understanding the destination interface, or learning about env pattern and configuration. Covers interface, lifecycle, env mocking, and paths.

Understanding walkerOS Destinations

Overview

Destinations receive processed events from the collector and deliver them to third-party tools (analytics, marketing, data warehouses).

Core principle: Destinations transform and deliver. They don't capture or process—that's sources and collector.

Destination Interface

See packages/core/src/types/destination.ts for canonical interface.

The env Pattern

Destinations use dependency injection via env for external APIs. This enables testing without mocking.

Copy// Destination defines its env type
export interface Env extends DestinationWeb.Env {
  window: {
    gtag: Gtag.Gtag;
    dataLayer: unknown[];
  };
}

// Destination uses env, not globals
async function push(event, context) {
  const { env } = context;
  env.window.gtag('event', mappedName, mappedData);
}

Testing with env

REQUIRED SKILL: See testing-strategy for full testing patterns.

Copyimport { mockEnv } from '@walkeros/core';
import { examples } from '../dev';

const calls: Array<{ path: string[]; args: unknown[] }> = [];
const testEnv = mockEnv(examples.env.push, (path, args) => {
  calls.push({ path, args });
});

await destination.push(event, { ...context, env: testEnv });

expect(calls).toContainEqual({
  path: ['window', 'gtag'],
  args: ['event', 'purchase', expect.any(Object)],
});

Destination Config

Copyconfig: {
  settings: { /* destination-specific */ },
  mapping: { /* event transformation rules */ },
  data: { /* global data mapping */ },
  consent: { /* required consent states */ },
  policy: { /* processing rules */ },
  queue: boolean,  // queue events
  dryRun: boolean, // test mode
}

Destination Paths

Template Destination

Use as starting point: packages/web/destinations/plausible/

Transformer Wiring

Destinations can wire to post-collector transformer chains via the before property:

Copydestinations: {
  gtag: {
    code: destinationGtag,
    before: 'redact'  // First transformer to run before this destination
  }
}

The transformer chain runs after collector enrichment, before the destination receives events. Each destination can have its own chain. See understanding-transformers for chain details.

Related

Skills:

• understanding-mapping skill - Configure transformations
• understanding-transformers skill - Transformer chaining to destinations
• testing-strategy skill - Test with env pattern
• create-destination skill - Create new destination

Source Files:

• packages/core/src/types/destination.ts - Interface

Package READMEs:

• packages/web/destinations/gtag/README.md - gtag example
• packages/web/destinations/plausible/README.md - Plausible (template)

Documentation:

• Website: Destinations - Overview
• Website: Create Your Own - Guide