Using Step Examples

Overview

Step examples are structured { in, out } pairs (with optional mapping for destinations) that define the expected input/output behavior of each step in a flow. They serve as:

• Test fixtures for automated it.each testing
• Simulation data for walkeros push --simulate
• MCP context for AI-assisted development
• Documentation showing real-world usage

Every source, transformer, and destination can ship step examples alongside its code.

The Three Type Zones

Each step in a flow sits at a boundary between arbitrary external formats and the walkerOS event model:

           Source              Transformer           Destination
  ┌─────────────────┐   ┌─────────────────┐   ┌─────────────────┐
  │  in: arbitrary   │   │  in: walkerOS   │   │  in: walkerOS   │
  │  (HTTP req, DOM) │   │      Event      │   │      Event      │
  │        │         │   │        │         │   │        │         │
  │        ▼         │   │        ▼         │   │        ▼         │
  │  out: walkerOS   │   │  out: walkerOS  │   │  out: arbitrary  │
  │      Event       │   │   Event | false │   │  (gtag, API)     │
  └─────────────────┘   └─────────────────┘   └─────────────────┘

  arbitrary ──────────── walkerOS.Event ──────────── arbitrary

• Source converts platform-specific input into a walkerOS event
• Transformer receives and returns walkerOS events (or false to filter)
• Destination converts a walkerOS event into vendor-specific output

The in/out/trigger Format

Every step example is an object with in (input), out (array of observable effects), and optional trigger (how to invoke):

type StepEffect = readonly [callable: string, ...args: unknown[]];
type StepOut = readonly StepEffect[];

interface StepExample {
  in: unknown; // Platform-specific input
  out: StepOut; // Array of effect tuples - see shape rules below
  trigger?: { type?: string; options?: unknown }; // How to invoke
  mapping?: unknown; // Destination mapping rule
  command?: 'config' | 'consent' | 'user' | 'run'; // Route to walker command
}

out is always an array of effect tuples, even for a single effect. Each tuple is [callable, ...args]. The first element is the callable's public name (the SDK function users would write). Remaining elements are the arguments.

Callable conventions

Component type	Callable	Notes
Destination (SDK function)	'gtag', 'fbq', 'ttq.track'	Public SDK name. Dotted paths render literally.
Destination (method on global)	'analytics.track', 'dataLayer.push'	Method notation.
Destination (HTTP)	'fetch', 'sendServer'	The actual call users would make. NOT env.* prop names - those are internal plumbing.
Source	'elb'	The walker public push API.
Transformer	'return'	Reserved keyword. Renders as return <value> (no parens).

'return' is reserved. Don't use it for anything else.

Empty out: [] means the step produced no observable effect (filtered input, transformer passthrough, validator rejection). Reserved only for cases where the destination/source deliberately emits nothing.

Source Step Example - Server (Express)

in is an HTTP request shape, out is a tuple of the elb() call the source makes:

export const checkoutPost: Flow.StepExample = {
  trigger: { type: 'POST' },
  in: {
    method: 'POST',
    path: '/collect',
    body: { name: 'order complete', data: { id: 'ORD-123', total: 149.97 } },
  },
  out: [
    ['elb', { name: 'order complete', data: { id: 'ORD-123', total: 149.97 } }],
  ],
};

Source Step Example - Browser

in is an HTML string, out is a tuple of the elb() call:

export const clickEvent: Flow.StepExample = {
  trigger: { type: 'click', options: 'button' },
  in: '<button data-elb="cta" data-elb-cta="label:Sign Up" data-elbaction="click:click">Sign Up</button>',
  out: [
    [
      'elb',
      {
        name: 'cta click',
        data: { label: 'Sign Up' },
        trigger: 'click',
        entity: 'cta',
        action: 'click',
      },
    ],
  ],
};

Transformer Step Example

in is a walkerOS event, out is a ['return', value] tuple. An empty array means the transformer passed the event through unchanged:

export const step = {
  orderPasses: {
    in: { name: 'order complete', data: { id: 'ORD-123' } },
    out: [['return', { name: 'order complete', data: { id: 'ORD-123' } }]],
  },
  debugFiltered: {
    in: { name: 'debug test', data: { message: 'noise' } },
    out: [['return', false]], // Transformer rejects
  },
  passthrough: {
    in: { name: 'page view' },
    out: [], // No modification
  },
};

Destination Step Example

in is a walkerOS event, mapping is the mapping rule that transforms it, out is an array of call tuples - one per observable effect the destination produces. Multi-call events (e.g., GA4 + Ads + GTM for a single walker event) flatten into a single array in execution order:

import type { Flow } from '@walkeros/core';
import { getEvent } from '@walkeros/core';

export const purchase: Flow.StepExample = {
  in: getEvent('order complete', { timestamp: 1700000000 }),
  mapping: {
    name: 'Purchase',
    data: { map: { value: 'data.total', currency: { value: 'EUR' } } },
  },
  out: [
    [
      'fbq',
      'track',
      'Purchase',
      { value: 555, currency: 'EUR' },
      { eventID: '1700000000-gr0up-1' },
    ],
  ],
};

Multi-tool example (one walker event produces GA4, Ads, GTM in order):

out: [
  ['gtag', 'event', 'purchase', { transaction_id: 'o1', value: 555 }],
  ['gtag', 'event', 'conversion', { send_to: 'AW-123', value: 555 }],
  ['dataLayer.push', { event: 'purchase', ecommerce: { ... } }],
],

Each export is a self-contained Flow.StepExample - no intermediate variables, no all aggregation. The mapping field ties the mapping rule to the example so tests can register it dynamically: { [event.entity]: { [event.action]: example.mapping } }.

Consumers iterate all examples via Object.entries(examples.step) - export * as step exposes every named export directly.

Command Step Example

Some destinations need to respond to walker commands (walker consent, walker user, walker run), not events. Set the command field to route in through elb('walker <command>', in) instead of pushing it as an event:

export const consentGranted: Flow.StepExample = {
  command: 'consent',
  in: { marketing: true, functional: true },
  out: [
    'consent',
    'update',
    { ad_storage: 'granted', analytics_storage: 'granted' },
  ],
};

Supported commands: config, consent, user, run.

• When command is absent (default), in is pushed as a regular event via elb(in).
• When command is set, mapping is not applied - commands don't flow through event mapping.
• The out format is destination-specific. For gtag it's [action, subAction, params] matching gtag(...) calls.

Writing Examples

Best Practices

1. Use camelCase names -- descriptive of the scenario (checkoutPost, debugFiltered, pageViewBasic)
2. Cover happy path and edge cases -- include at least one positive and one negative case for transformers
3. Use realistic data -- real-looking event names, IDs, and values
4. Keep examples minimal -- only include fields relevant to the step's logic; omit empty/default-value fields (context: {}, nested: [], user: {}, etc.)
5. Export from examples/step.ts -- follow the existing dev.ts structure
6. Title and describe public examples -- add title (2-5 words) and description (one short sentence, 10-25 words) for every example that should appear in docs. Mark test-only fixtures with public: false.

File Structure

// src/examples/step.ts - only Flow.StepExample exports, nothing else
import type { Flow } from '@walkeros/core';
import { getEvent } from '@walkeros/core';

export const purchase: Flow.StepExample = {
  in: getEvent('order complete', { timestamp: 1700000000 }),
  mapping: { name: 'Purchase', data: { map: { value: 'data.total' } } },
  out: ['track', 'Purchase', { value: 555 }, { eventID: '1700000000-gr0up-1' }],
};

// src/examples/index.ts
export * as env from './env';
export * as step from './step';

The file exports only Flow.StepExample objects. No intermediate variables, no all, no config. Consumers iterate via Object.entries(examples.step).

Rule: examples live only on the ./dev subpath

Examples (and schemas) must be exported only from src/dev.ts (the ./dev package subpath), never from a production entry (src/index.ts / the package main). Production bundles tree-shake on the import path: if examples is reachable from the main entry, the bundler inlines every example (and its getEvent fixtures) into the shipped JS, bloating consumer bundles with test-only data. Importing them from ./dev keeps them available for simulation and testing while leaving production output clean.

A no-restricted-syntax ESLint rule enforces this, so a export ... examples from a production entry fails lint.

Metadata: title, description, public

Every Flow.StepExample accepts three optional metadata fields that control how it surfaces in docs and MCP output:

• title?: string - overrides the default camelCase-to-spaced heading in website docs. Keep it short (2-5 words), human-readable.
• description?: string - one short sentence (10-25 words) rendered above each example in docs and surfaced in MCP flow_examples output. Explains what the example demonstrates, not how.
• public?: boolean - defaults to true. When false, the example is excluded from the website docs render and from default MCP flow_examples output. It still runs in tests and remains callable via CLI/MCP flow_simulate.

export const purchase: Flow.StepExample = {
  title: 'Purchase',
  description:
    'A completed order mapped to the Meta Pixel Purchase standard event.',
  in: getEvent('order complete', { timestamp: 1700000000 }),
  mapping: { name: 'Purchase', data: { map: { value: 'data.total' } } },
  out: [['fbq', 'track', 'Purchase', { value: 555, currency: 'EUR' }]],
};

// Internal fixture - runs in tests, hidden from docs and default MCP output.
export const debugFiltered: Flow.StepExample = {
  public: false,
  in: { name: 'debug test', data: { message: 'noise' } },
  out: [['return', false]],
};

When to mark public: false

Mark public: false when:

• The example is a transformer rejection (out: [['return', false]]) that only proves filtering works.
• It's an error-path or malformed-input case.
• It's a redundant variant kept only for regression coverage.
• It's an internal fixture used by cross-package tests.

Keep public (omit the field) when:

• It's the happy path.
• It teaches a distinct mapping or usage pattern.
• It's the only example for some entity/action the package handles.

Source Trigger Metadata

Source step examples can include a trigger field for simulation:

{
  in: '<button data-elb="cta">Sign Up</button>',
  trigger: { type: 'click' },
  out: { name: 'cta click', data: { label: 'Sign Up' } }
}

When simulating via CLI or MCP, the step example maps to SourceInput:

• in -> content (the actual source input)
• trigger -> trigger (which mechanism to fire)

Destination and transformer examples don't use trigger.

Simulating with Step Examples

Use the --step flag to target a specific step, then provide the event as SourceInput ({ content, trigger? }):

# Simulate a source step with trigger metadata
walkeros push flow.json --simulate source.browser --event '{"content":"<html>...","trigger":{"type":"click"}}'

The MCP flow_examples tool returns trigger metadata alongside in/out, and mapping for destination examples, giving full visibility into how input events are transformed to vendor-specific output.

Validating Examples

Cross-step example validation is included automatically when validating a flow:

# Validate flow config including step example compatibility
walkeros validate flow.json

Flow validation checks that:

• Source out types match transformer in types
• Transformer out types match destination in types
• Connected steps have compatible examples

Testing with Examples

The primary use of step examples is automated testing with it.each.

Source Tests (createTrigger)

Sources export createTrigger from their examples. It follows the unified Trigger.CreateFn interface: receives Collector.InitConfig, lazily starts the flow, and returns a trigger function that simulates real-world invocations.

Each package implements createTrigger differently:

• Browser: Injects HTML into DOM, starts flow, dispatches native events
• Express: Boots real HTTP server, sends fetch() requests

Use a spy destination to capture events:

import type { Destination, WalkerOS } from '@walkeros/core';
import { sourceExpress } from '@walkeros/server-source-express';
import { examples } from '@walkeros/server-source-express/dev';

describe('Step Examples', () => {
  it.each(Object.entries(examples.step))('%s', async (name, example) => {
    const events: WalkerOS.Event[] = [];
    const spy: Destination.Instance = {
      type: 'spy',
      config: { init: true },
      push: jest.fn((event) => {
        events.push(JSON.parse(JSON.stringify(event)));
      }),
    };

    const instance = await examples.createTrigger({
      consent: { functional: true },
      sources: {
        express: { code: sourceExpress, config: { settings: { port: 0 } } },
      },
      destinations: { spy: { code: spy } },
    });

    await instance.trigger(example.trigger?.type)(example.in);

    const found = events.find((e) => e.name === example.out.name);
    expect(found).toBeDefined();
  });
});

Browser source note: The browser source's elbLayer processes events via a detached promise chain (fire-and-forget). For interactive triggers (click, submit), poll until events arrive:

while (!events.length) await Promise.resolve();

CMP Source Tests (Consent Assertion)

CMP sources push walker consent commands, not regular events. Assert on collector consent state instead of using a spy destination:

const instance = await examples.createTrigger({
  consent: {},
  sources: {
    usercentrics: {
      code: sourceUsercentrics,
      config: { settings: {} },
    },
  },
});

await instance.trigger(
  example.trigger?.type,
  example.trigger?.options,
)(example.in);

// Yield for detached elb('walker consent') chain
while (!Object.keys(instance.flow!.collector.config.consent || {}).length)
  await Promise.resolve();

expect(instance.flow!.collector.config.consent).toEqual(
  expect.objectContaining(expected),
);

Server Function Handler Tests (collector.sources)

Server function handlers (fetch, AWS Lambda, GCP CloudFunction) don't own servers. Their createTrigger accesses the source instance from collector.sources after startFlow and calls source.push() with platform-native types:

function findSource(collector) {
  for (const source of Object.values(collector.sources || {})) {
    if (source.type === 'fetch') return source;
  }
}

// In trigger:
const source = findSource(flow.collector);
const response = await source.push(request);

Destination Functional Tests

For destinations, use startFlow + elb() to run events through the real collector pipeline. This verifies the full flow including mapping:

import { startFlow } from '@walkeros/collector';
import { clone } from '@walkeros/core';
import { examples } from './dev';

describe('Step Examples', () => {
  it.each(Object.entries(examples.step))('%s', async (name, example) => {
    const event = example.in as WalkerOS.Event;
    const mapping = example.mapping as Rule | undefined;

    const mockFn = jest.fn();
    const env = clone(examples.env.push);
    env.window.fbq = mockFn;

    const dest = jest.requireActual('.').default;
    const { elb } = await startFlow({});

    // Build mapping config from event entity/action
    const mappingConfig = mapping
      ? { [event.entity]: { [event.action]: mapping } }
      : undefined;

    elb('walker destination', {
      code: { ...dest, env },
      config: {
        settings: { pixelId: '1234567890' },
        mapping: mappingConfig,
      },
    });

    await elb(event);
    expect(mockFn).toHaveBeenLastCalledWith(...(example.out as unknown[]));
  });
});

Transformer Tests

For transformers that can return false:

import { examples } from '../dev';

describe('transformer', () => {
  it.each(Object.entries(examples.step))(
    '%s',
    async (name, { in: input, out: expected }) => {
      const result = await transformer.push(input, context);
      if (expected === false) {
        expect(result).toBe(false);
      } else {
        expect(result).toEqual(expected);
      }
    },
  );
});

Checklist

When adding step examples to a package or flow:

• Create src/examples/step.ts with Flow.StepExample exports
• Export from src/examples/index.ts
• Use camelCase, descriptive example names
• Include at least one happy-path example
• For transformers, include a false (filtered) case
• Add it.each test using step examples
• Verify examples compile: npm run build
• Run tests: npm run verify:touched -- <pkg> (L1)
• Add title and description to every public example.
• Mark internal/test-only examples with public: false.

Related Skills

• testing-strategy - Testing patterns and env mocking
• understanding-flow - Architecture and data flow
• using-cli - CLI simulate and validate commands
• create-destination - Destination creation workflow
• create-source - Source creation workflow
• create-transformer - Transformer creation workflow