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Pact.js Utils Consumer Helpers

Principle

Use createProviderState, toJsonMap, setJsonContent, and setJsonBody from @seontechnologies/pactjs-utils to build type-safe provider state tuples and reusable PactV4 JSON callbacks for consumer contract tests. These helpers eliminate manual JsonMap casting and repetitive inline builder lambdas.

Rationale

Problems with raw consumer helper handling

• JsonMap requirement: Pact's .given(stateName, params) requires params to be JsonMap — a flat object where every value must be string | number | boolean | null
• Type gymnastics: Complex params (Date objects, nested objects, null values) require manual casting that TypeScript can't verify
• Inconsistent serialization: Different developers serialize the same data differently (e.g., dates as ISO strings vs timestamps)
• Verbose .given() calls: Repeating state name and params inline makes consumer tests harder to read
• Repeated interaction callbacks: PactV4 interactions duplicate inline (builder) => { ... } blocks for body/query/header setup

Solutions

• createProviderState: Returns a [string, JsonMap] tuple that spreads directly into .given() — one function handles name and params
• toJsonMap: Explicit coercion rules documented and tested — Date→ISO string, null→"null" string, nested objects→JSON string
• setJsonContent: Curried callback helper for request/response builders — set query, headers, and/or body from one reusable function
• setJsonBody: Body-only shorthand for setJsonContent({ body }) — ideal for concise .willRespondWith(...) bodies

Pattern Examples

Example 1: Basic Provider State Creation

import { PactV3, MatchersV3 } from '@pact-foundation/pact';
import { createProviderState } from '@seontechnologies/pactjs-utils';

const provider = new PactV3({
  consumer: 'movie-web',
  provider: 'SampleMoviesAPI',
  dir: './pacts',
});

describe('Movie API Contract', () => {
  it('should return movie by id', async () => {
    // createProviderState returns [stateName, JsonMap] tuple
    const providerState = createProviderState({
      name: 'movie with id 1 exists',
      params: { id: 1, name: 'Inception', year: 2010 },
    });

    await provider
      .given(...providerState) // Spread tuple into .given(name, params)
      .uponReceiving('a request for movie 1')
      .withRequest({ method: 'GET', path: '/movies/1' })
      .willRespondWith({
        status: 200,
        body: MatchersV3.like({ id: 1, name: 'Inception', year: 2010 }),
      })
      .executeTest(async (mockServer) => {
        const res = await fetch(`${mockServer.url}/movies/1`);
        const movie = await res.json();
        expect(movie.name).toBe('Inception');
      });
  });
});

Key Points:

• createProviderState accepts { name: string, params: Record<string, unknown> }
• Both name and params are required (pass params: {} for states without parameters)
• Returns [string, JsonMap] — spread with ... into .given()
• params values are automatically converted to JsonMap-compatible types
• Works identically with HTTP (PactV3) and message (MessageConsumerPact) pacts

Example 2: Complex Parameters with toJsonMap

import { toJsonMap } from '@seontechnologies/pactjs-utils';

// toJsonMap conversion rules:
// - string, number, boolean → passed through
// - null → "null" (string)
// - undefined → "null" (string, same as null)
// - Date → ISO string (e.g., "2025-01-15T10:00:00.000Z")
// - nested object → JSON string
// - array → comma-separated string via String() (e.g., [1,2,3] → "1,2,3")

const params = toJsonMap({
  id: 42,
  name: 'John Doe',
  active: true,
  score: null,
  createdAt: new Date('2025-01-15T10:00:00Z'),
  metadata: { role: 'admin', permissions: ['read', 'write'] },
});

// Result:
// {
//   id: 42,
//   name: "John Doe",
//   active: true,
//   score: "null",
//   createdAt: "2025-01-15T10:00:00.000Z",
//   metadata: '{"role":"admin","permissions":["read","write"]}'
// }

Key Points:

• toJsonMap is called internally by createProviderState — you rarely need it directly
• Use it when you need explicit control over parameter conversion outside of provider states
• Conversion rules are deterministic: same input always produces same output

Example 3: Provider State Without Parameters

import { createProviderState } from '@seontechnologies/pactjs-utils';

// State without params — second tuple element is empty object
const emptyState = createProviderState({ name: 'no movies exist', params: {} });
// Returns: ['no movies exist', {}]

await provider
  .given(...emptyState)
  .uponReceiving('a request when no movies exist')
  .withRequest({ method: 'GET', path: '/movies' })
  .willRespondWith({ status: 200, body: [] })
  .executeTest(async (mockServer) => {
    const res = await fetch(`${mockServer.url}/movies`);
    const movies = await res.json();
    expect(movies).toEqual([]);
  });

Example 4: Multiple Provider States

import { createProviderState } from '@seontechnologies/pactjs-utils';

// Some interactions require multiple provider states
// Call .given() multiple times with different states
await provider
  .given(...createProviderState({ name: 'user is authenticated', params: { userId: 1 } }))
  .given(...createProviderState({ name: 'movie with id 5 exists', params: { id: 5 } }))
  .uponReceiving('an authenticated request for movie 5')
  .withRequest({
    method: 'GET',
    path: '/movies/5',
    headers: { Authorization: MatchersV3.like('Bearer token') },
  })
  .willRespondWith({ status: 200, body: MatchersV3.like({ id: 5 }) })
  .executeTest(async (mockServer) => {
    // test implementation
  });

Example 5: When to Use setJsonBody vs setJsonContent

import { MatchersV3 } from '@pact-foundation/pact';
import { setJsonBody, setJsonContent } from '@seontechnologies/pactjs-utils';

const { integer, string } = MatchersV3;

await pact
  .addInteraction()
  .given('movie exists')
  .uponReceiving('a request to get movie by name')
  .withRequest(
    'GET',
    '/movies',
    setJsonContent({
      query: { name: 'Inception' },
      headers: { Accept: 'application/json' },
    }),
  )
  .willRespondWith(
    200,
    setJsonBody({
      status: 200,
      data: { id: integer(1), name: string('Inception') },
    }),
  );

Key Points:

• Use setJsonContent when the interaction needs query, headers, and/or body in one callback (most request builders)
• Use setJsonBody when you only need jsonBody and want the shorter .willRespondWith(status, setJsonBody(...)) form
• setJsonBody is equivalent to setJsonContent({ body: ... })

Key Points

• Spread pattern: Always use ...createProviderState() — the tuple spreads into .given(stateName, params)
• Type safety: TypeScript enforces { name: string, params: Record<string, unknown> } input (both fields required)
• Null handling: null becomes "null" string in JsonMap (Pact requirement)
• Date handling: Date objects become ISO 8601 strings
• No nested objects in JsonMap: Nested objects are JSON-stringified — provider state handlers must parse them
• Array serialization is lossy: Arrays are converted via String() (e.g., [1,2,3] → "1,2,3") — prefer passing arrays as JSON-stringified objects for round-trip safety
• Message pacts: Works identically with MessageConsumerPact — same .given() API
• Builder reuse: setJsonContent works for both .withRequest(...) and .willRespondWith(...) callbacks (query is ignored on response builders)
• Body shorthand: setJsonBody keeps body-only responses concise and readable
• Matchers check type, not value: string('My movie') means "any string", integer(1) means "any integer". The example values are arbitrary — the provider can return different values and verification still passes as long as the type matches. Use matchers only in .willRespondWith() (responses), never in .withRequest() (requests) — Postel's Law applies.
• Reuse test values across files: Interactions are uniquely identified by uponReceiving + .given(), not by placeholder values. Two test files can both use testId: 100 without conflicting. On the provider side, shared values simplify state handlers — idempotent handlers (check if exists, create if not) only need to ensure one record exists. Use different values only when testing different states of the same entity type (e.g., movieExists(100) for happy paths vs. movieNotFound(999) for error paths).

Related Fragments

• pactjs-utils-overview.md — installation, decision tree, design philosophy
• pactjs-utils-provider-verifier.md — provider-side state handler implementation
• contract-testing.md — foundational patterns with raw Pact.js

Anti-Patterns

Wrong: Manual JsonMap assembly

// ❌ Manual casting — verbose, error-prone, no type safety
provider.given('user exists', {
  id: 1 as unknown as string,
  createdAt: new Date().toISOString(),
  metadata: JSON.stringify({ role: 'admin' }),
} as JsonMap);

Right: Use createProviderState

// ✅ Automatic conversion with type safety
provider.given(
  ...createProviderState({
    name: 'user exists',
    params: { id: 1, createdAt: new Date(), metadata: { role: 'admin' } },
  }),
);

Wrong: Inline state names without helper

// ❌ Duplicated state names between consumer and provider — easy to mismatch
provider.given('a user with id 1 exists', { id: '1' });
// Later in provider: 'user with id 1 exists' — different string!

Right: Share state constants

// ✅ Define state names as constants shared between consumer and provider
const STATES = {
  USER_EXISTS: 'user with id exists',
  NO_USERS: 'no users exist',
} as const;

provider.given(...createProviderState({ name: STATES.USER_EXISTS, params: { id: 1 } }));

Wrong: Repeating inline builder lambdas everywhere

// ❌ Repetitive callback boilerplate in every interaction
.willRespondWith(200, (builder) => {
  builder.jsonBody({ status: 200 });
});

Right: Use setJsonBody / setJsonContent

// ✅ Reusable callbacks with less boilerplate
.withRequest('GET', '/movies', setJsonContent({ query: { name: 'Inception' } }))
.willRespondWith(200, setJsonBody({ status: 200 }));

Source: @seontechnologies/pactjs-utils consumer-helpers module, pactjs-utils sample-app consumer tests