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Pact JS

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Implementation of the consumer driven contract library Pact for Javascript.

From the Pact website:

The Pact family of frameworks provide support for Consumer Driven Contracts testing.

A Contract is a collection of agreements between a client (Consumer) and an API (Provider) that describes the interactions that can take place between them.

Consumer Driven Contracts is a pattern that drives the development of the Provider from its Consumers point of view.

Pact is a testing tool that guarantees those Contracts are satisfied.

Read Getting started with Pact for more information for beginners.

NOTE: This project supersedes Pact Consumer JS DSL.


npm i -S @pact-foundation/[email protected]

Do Not Track

In order to get better statistics as to who is using Pact, we have an anonymous tracking event that triggers when Pact installs for the first time. The only things we track are your type of OS, and the version information for the package being installed. No PII data is sent as part of this request. To respect your privacy, you can disable tracking by simply adding a 'do not track' flag within your package.json file or setting the environment variable PACT_DO_NOT_TRACK=1:

	"name": "some-project",
	"config": {
		"pact_do_not_track": true

See the Changelog for versions and their history.

Using Pact JS

Pact supports synchronous request-response style HTTP interactions and asynchronous interactions with JSON-formatted payloads.

HTTP API Testing

Consumer Side Testing

To use the library on your tests, add the pact dependency:

const { Pact } = require("@pact-foundation/pact")

The Pact class provides the following high-level APIs, they are listed in the order in which they typically get called in the lifecycle of testing a consumer:


Consumer API
API Options Returns Description
new Pact(options) See constructor options below Object Creates a Mock Server test double of your Provider API. If you need multiple Providers for a scenario, you can create as many as these as you need.
setup() n/a Promise Start the Mock Server and wait for it to be available. You would normally call this only once in a beforeAll(...) type clause
addInteraction() Object Promise Register an expectation on the Mock Server, which must be called by your test case(s). You can add multiple interactions per server, and each test would normally contain one or more of these. These will be validated and written to a pact if successful.
verify() n/a Promise Verifies that all interactions specified. This should be called once per test, to ensure your expectations were correct
finalize() n/a Promise Records the interactions registered to the Mock Server into the pact file and shuts it down. You would normally call this only once in an afterAll(...) type clause.
Parameter Required? Type Description
consumer yes string The name of the consumer
provider yes string The name of the provider
port no number The port to run the mock service on, defaults to 1234
host no string The host to run the mock service, defaults to
ssl no boolean SSL flag to identify the protocol to be used (default false, HTTP)
sslcert no string Path to SSL certificate to serve on the mock service
sslkey no string Path to SSL key to serve on the mock service
dir no string Directory to output pact files
log no string File to log to
logLevel no string Log level: one of 'trace', 'debug', 'info', 'error', 'fatal' or 'warn'
spec no number Pact specification version (defaults to 2)
cors no boolean Allow CORS OPTION requests to be accepted, defaults to false
pactfileWriteMode no string Control how the Pact files are written. Choices: 'overwrite' 'update' or 'none'. Defaults to 'overwrite'


The first step is to create a test for your API Consumer. The example below uses Mocha, and demonstrates the basic approach:

  1. Create the Pact object
  2. Start the Mock Provider that will stand in for your actual Provider
  3. Add the interactions you expect your consumer code to make when executing the tests
  4. Write your tests - the important thing here is that you test the outbound collaborating function which calls the Provider, and not just issue raw http requests to the Provider. This ensures you are testing your actual running code, just like you would in any other unit test, and that the tests will always remain up to date with what your consumer is doing.
  5. Validate the expected interactions were made between your consumer and the Mock Service
  6. Generate the pact(s)

Check out the examples folder for examples with Karma Jasmine, Mocha and Jest. The example below is taken from the integration spec.

const path = require("path")
const chai = require("chai")
const { Pact } = require("@pact-foundation/pact")
const chaiAsPromised = require("chai-as-promised")
const expect = chai.expect


describe("Pact", () => {
  // (1) Create the Pact object to represent your provider
  const provider = new Pact({
    consumer: "TodoApp",
    provider: "TodoService",
    port: 1234,
    log: path.resolve(process.cwd(), "logs", "pact.log"),
    dir: path.resolve(process.cwd(), "pacts"),
    logLevel: "INFO",

  // this is the response you expect from your Provider
  const EXPECTED_BODY = [
      id: 1,
      name: "Project 1",
      due: "2016-02-11T09:46:56.023Z",
      tasks: [
        { id: 1, name: "Do the laundry", done: true },
        { id: 2, name: "Do the dishes", done: false },
        { id: 3, name: "Do the backyard", done: false },
        { id: 4, name: "Do nothing", done: false },

  const todoApp = new TodoApp()

  context("when there are a list of projects", () => {
    describe("and there is a valid user session", () => {
      before(() => provider
          // (2) Start the mock server
          // (3) add interactions to the Mock Server, as many as required
          .then(() => provider.addInteraction({
              // The 'state' field specifies a "Provider State"
              state: "i have a list of projects",
              uponReceiving: "a request for projects",
              withRequest: {
                method: "GET",
                path: "/projects",
                headers: { Accept: "application/json" },
              willRespondWith: {
                status: 200,
                headers: { "Content-Type": "application/json" },
                body: EXPECTED_BODY,

      // (4) write your test(s)
      it("generates a list of TODOs for the main screen", async () => {
        const projects = await todoApp.getProjects() // <- this method would make the remote http call
        expect(projects)"projects[0].id", 1)

      // (5) validate the interactions you've registered and expected occurred
      // this will throw an error if it fails telling you what went wrong
      // This should be performed once per interaction test
      afterEach(() => provider.verify())

    // (6) write the pact file for this consumer-provider pair,
    // and shutdown the associated mock server.
    // You should do this only _once_ per Provider you are testing,
    // and after _all_ tests have run for that suite
    after(() =>  provider.finalize())

Provider API Testing

Provider API

Once you have created Pacts for your Consumer, you need to validate those Pacts against your Provider. The Verifier object provides the following API for you to do so:

API Options Returns Description
verifyProvider() See below Promise Start the Mock Server
  1. Start your local Provider service.
  2. Optionally, instrument your API with ability to configure provider states
  3. Then run the Provider side verification step
const { Verifier } = require('@pact-foundation/pact');
let opts = {

new Verifier(opts).verifyProvider().then(function () {
	// do something

Verification Options

Verification Options
Parameter Required Type Description
providerBaseUrl true string Running API provider host endpoint. Required.
provider true string Name of the Provider. Required.
pactUrls true array of strings Array of local Pact file paths or HTTP-based URLs (e.g. from a broker). Required if not using a Broker.
pactBrokerUrl false string URL of the Pact Broker to retrieve pacts from. Required if not using pactUrls.
tags false array of strings Array of tags, used to filter pacts from the Broker.
providerStatesSetupUrl false string DEPRECATED (see stateHandlers). URL to call with a POST request for each providerState defined in a pact (see below for more info).
pactBrokerToken false string Bearer token for Pact Broker authentication. If using Pactflow, you likely need this option.
pactBrokerUsername false string Username for Pact Broker basic authentication. If using Pactflow, you most likely need to use pactBrokerToken
pactBrokerPassword false string Password for Pact Broker basic authentication. If using Pactflow, you most likely need to use pactBrokerToken
publishVerificationResult false boolean Publish verification result to Broker
providerVersion false string Provider version, required to publish verification results to a broker
customProviderHeaders false array of strings Header(s) to add to any requests to the provider service. eg Authorization: Basic cGFjdDpwYWN0. All interactions will receive the header. See requestFilter for when more flexiblility is required in modifying the request to the provider.
timeout false number The duration in ms we should wait to confirm verification process was successful. Defaults to 30000.
requestFilter false object An Express middleware handler (See to modify requests and responses from the provider. See below for more details.
stateHandlers false object Provider state handlers. A map of string -> () => Promise, where each string is the state to setup, and the function is used to configure the state in the Provider. See below for detail.
validateSSL false boolean Allow self-signed certificates. Defaults to true, if not set.
changeOrigin false boolean Changes the origin of the host header to the target URL. Defaults to false, if not set.

Read more about Verifying Pacts.

API with Provider States

If you have defined any states in your consumer tests, the Verifier can put the provider into the right state prior to sending the request. For example, the provider can use the state to mock away certain database queries. To support this, set up a handler for each state using hooks on the stateHandlers property. Here is an example from our e2e suite:

let opts = {
  stateHandlers: {
    "Has no animals": () => {
      return Promise.resolve(`Animals removed from the db`)
    "Has some animals": () => {
      return Promise.resolve(`Animals added to the db`)
    "Has an animal with ID 1": () => {
      return Promise.resolve(`Animals added to the db`)

return new Verifier(opts).verifyProvider().then(...)

As you can see, for each state ("Has no animals", ...), we configure the local datastore differently. If this option is not configured, the Verifier will ignore the provider states defined in the pact and log a warning.

Read more about Provider States.

Modify Requests Prior to Verification (Request Filters)

Sometimes you may need to add things to the requests that can't be persisted in a pact file. Examples of these are authentication tokens with a small life span. e.g. an OAuth bearer token: Authorization: Bearer 0b79bab50daca910b000d4f1a2b675d604257e42.

For these cases, we have two facilities that should be carefully used during verification:

  1. the ability to specify custom headers to be sent during provider verification. The flag to achieve this is customProviderHeaders.
  2. the ability to modify a request/response and modify the payload. The flag to achieve this is requestFilter.

Example API with Authorization

For example, to have an Authorization bearer token header sent as part of the verification request, set the verifyProvider options as per below:

let token
let opts = {
  provider: 'Animal Profile Service',
  stateHandlers: {
    "is authenticated": () => {
      token = "1234"
      Promise.resolve(`Valid bearer token generated`)
    "is not authenticated": () => {
      token = ""
      Promise.resolve(`Expired bearer token generated`)

  // this middleware is executed for each request, allowing `token` to change between invocations
  // it is common to pair this with `stateHandlers` as per above, that can set/expire the token
  // for different test cases
  requestFilter: (req, res, next) => {
    req.headers["Authorization"] = `Bearer: ${token}`

  // This header will always be sent for each and every request, and can't be dynamic
  // (i.e. passing a variable instead of the bearer token)
  customProviderHeaders: ["Authorization: Bearer 1234"]

return new Verifier(opts).verifyProvider().then(...)

As you can see, this is your opportunity to modify\add to headers being sent to the Provider API, for example to create a valid time-bound token.

Important Note: You should only use this feature for things that can not be persisted in the pact file. By modifying the request, you are potentially modifying the contract from the consumer tests!

Publishing Pacts to a Broker

Sharing is caring - to simplify sharing Pacts between Consumers and Providers, we have created the Pact Broker.

The Broker:

  • versions your contracts
  • tells you which versions of your applications can be deployed safely together
  • allows you to deploy your services independently
  • provides API documentation of your applications that is guaranteed to be up-to date
  • visualises the relationships between your services
  • integrates with other systems, such as Slack or your CI server, via webhooks
  • ...and much much more.

Host your own, or signup for a free hosted Pact Broker.

let pact = require('@pact-foundation/pact-node');
let opts = {

pact.publishPacts(opts)).then(function () {
	// do something

Publishing options

Publishing Options
Parameter Required Type Description
providerBaseUrl false string Running API provider host endpoint.
pactFilesOrDirs true array of strings Array of local Pact files or directories containing pact files. Path must be absolute. Required.
pactBroker true string The base URL of the Pact Broker. eg. Required.
pactBrokerToken false string Bearer token for Pact Broker authentication. Optional. If using Pactflow, you likely need this option
pactBrokerUsername false string Username for Pact Broker basic authentication. Optional. If using Pactflow, you most likely need to use pactBrokerToken
pactBrokerPassword false string Password for Pact Broker basic authentication. Optional. If using Pactflow, you most likely need to use pactBrokerToken
consumerVersion true string The consumer application version; e.g. '1.0.0-cac389f'. (See more info on versioning)
tags false array of strings Tag your pacts, often used with your branching, release or environment strategy e.g. ['prod', 'test']

Publishing Verification Results to a Pact Broker

If you're using a Pact Broker (e.g. a hosted one at, you can publish your verification results so that consumers can query if they are safe to release.

It looks like this:

screenshot of verification result

You need to specify the following when constructing the pact object:

let opts = {
  provider: 'Animal Profile Service',
  publishVerificationResult: true,
  providerVersion: "1.0.0",
  provider: "Foo",


NOTE: You need to be retrieving pacts from the broker for this feature to work.

Asynchronous API Testing

Since version v6.0.0 or later

Modern distributed architectures are increasingly integrated in a decoupled, asynchronous fashion. Message queues such as ActiveMQ, RabbitMQ, SQS, Kafka and Kinesis are common, often integrated via small and frequent numbers of microservices (e.g. lambda.).

Furthermore, the web has things like WebSockets which involve bidirectional messaging.

Pact supports these use cases, by abstracting away the protocol and focussing on the messages passing between them.

For further reading and introduction into this topic, see this article and our asynchronous examples for a more detailed overview of these concepts.


A Consumer is the system that will be reading a message from a queue or some other intermediary - like a DynamoDB table or S3 bucket - and be able to handle it.

From a Pact testing point of view, Pact takes the place of the intermediary (MQ/broker etc.) and confirms whether or not the consumer is able to handle a request.

The following test creates a contract for a Dog API handler:

const {
} = require("@pact-foundation/pact")

// 1 Dog API Handler
const dogApiHandler = function(dog) {
  if (! && ! && !dog.type) {
    throw new Error("missing fields")

  // do some other things to dog...
  // e.g.

// 2 Pact Message Consumer
const messagePact = new MessageConsumerPact({
  consumer: "MyJSMessageConsumer",
  dir: path.resolve(process.cwd(), "pacts"),
  pactfileWriteMode: "update",
  provider: "MyJSMessageProvider",

describe("receive dog event", () => {
  it("accepts a valid dog", () => {
    // 3 Consumer expectations
    return (
        .given("some state")
        .expectsToReceive("a request for a dog")
          id: like(1),
          name: like("rover"),
          type: term({ generate: "bulldog", matcher: "^(bulldog|sheepdog)$" }),
          "content-type": "application/json",

        // 4 Verify consumers' ability to handle messages


  1. The Dog API - a contrived API handler example. Expects a dog object and throws an Error if it can't handle it.
    • In most applications, some form of transactionality exists and communication with a MQ/broker happens.
    • It's important we separate out the protocol bits from the message handling bits, so that we can test that in isolation.
  2. Creates the MessageConsumer class
  3. Setup the expectations for the consumer - here we expect a dog object with three fields
  4. Pact will send the message to your message handler. If the handler returns a successful promise, the message is saved, otherwise the test fails. There are a few key things to consider:
    • The actual request body that Pact will send, will be contained within a Message object along with other context, so the body must be retrieved via content attribute.
    • All handlers to be tested must be of the shape (m: Message) => Promise<any> - that is, they must accept a Message and return a Promise. This is how we get around all of the various protocols, and will often require a lightweight adapter function to convert it.
    • In this case, we wrap the actual dogApiHandler with a convenience function synchronousBodyHandler provided by Pact, which Promisifies the handler and extracts the contents.

Provider (Producer)

A Provider (Producer in messaging parlance) is the system that will be putting a message onto the queue.

As per the Consumer case, Pact takes the position of the intermediary (MQ/broker) and checks to see whether or not the Provider sends a message that matches the Consumer's expectations.

const { MessageProvider, Message } = require("@pact-foundation/pact")

// 1 Messaging integration client
const dogApiClient = {
  createDog: () => {
    return new Promise((resolve, reject) => {
        id: 1,
        name: "fido",
        type: "bulldog",

describe("Message provider tests", () => {
  // 2 Pact setup
  const p = new MessageProviderPact({
    messageProviders: {
      "a request for a dog": () => dogApiClient.createDog(),
    provider: "MyJSMessageProvider",
    providerVersion: "1.0.0",
    pactUrls: [

  // 3 Verify the interactions
  describe("Dog API Client", () => {
    it("sends some dogs", () => {
      return p.verify()


  1. Our API client contains a single function createDog which is responsible for generating the message that will be sent to the consumer via some message queue
  2. We configure Pact to stand-in for the queue. The most important bit here is the messageProviders block
    • Similar to the Consumer tests, we map the various interactions that are going to be verified as denoted by their description field. In this case, a request for a dog, maps to the createDog handler. Notice how this matches the original Consumer test.
  3. We can now run the verification process. Pact will read all of the interactions specified by its consumer, and invoke each function that is responsible for generating that message.

Pact Broker Integration

As per HTTP APIs, you can publish contracts and verification results to a Broker.


Matching makes your tests more expressive making your tests less brittle.

Rather than use hard-coded values which must then be present on the Provider side, you can use regular expressions and type matches on objects and arrays to validate the structure of your APIs.

NOTE: Make sure to start the mock service via the Pact declaration with the option specification: 2 to get access to these features.

Match common formats

Often times, you find yourself having to re-write regular expressions for common formats. We've created a number of them for you to save you the time:

Matchers API
method description
boolean Match a boolean value (using equality)
string Match a string value
integer Will match all numbers that are integers (both ints and longs)
decimal Will match all real numbers (floating point and decimal)
hexadecimal Will match all hexadecimal encoded strings
iso8601Date Will match string containing basic ISO8601 dates (e.g. 2016-01-01)
iso8601DateTime Will match string containing ISO 8601 formatted dates (e.g. 2015-08-06T16:53:10+01:00)
iso8601DateTimeWithMillis Will match string containing ISO 8601 formatted dates, enforcing millisecond precision (e.g. 2015-08-06T16:53:10.123+01:00)
rfc3339Timestamp Will match a string containing an RFC3339 formatted timestapm (e.g. Mon, 31 Oct 2016 15:21:41 -0400)
iso8601Time Will match string containing times (e.g. T22:44:30.652Z)
ipv4Address Will match string containing IP4 formatted address
ipv6Address Will match string containing IP6 formatted address
uuid Will match strings containing UUIDs
email Will match strings containing Email address

Match based on type

const { like, string } = Matchers

  state: "Has some animals",
  uponReceiving: "a request for an animal",
  withRequest: {
    method: "GET",
    path: "/animals/1",
  willRespondWith: {
    status: 200,
    headers: {
      "Content-Type": "application/json; charset=utf-8",
    body: {
      id: 1,
      name: string("Billy"),
      address: like({
        street: "123 Smith St",
        suburb: "Smithsville",
        postcode: 7777,

Note that you can wrap a like around a single value or an object. When wrapped around an object, all values and child object values will be matched according to types, unless overridden by something more specific like a term.

Match based on arrays

Matching provides the ability to specify flexible length arrays. For example:

pact.eachLike(obj, { min: 3 })

Where obj can be any javascript object, value or Pact.Match. It takes optional argument ({ min: 3 }) where min is greater than 0 and defaults to 1 if not provided.

Below is an example that uses all of the Pact Matchers.

const { somethingLike: like, term, eachLike } = pact

const animalBodyExpectation = {
  id: 1,
  first_name: "Billy",
  last_name: "Goat",
  animal: "goat",
  age: 21,
  gender: term({
    matcher: "F|M",
    generate: "M",
  location: {
    description: "Melbourne Zoo",
    country: "Australia",
    post_code: 3000,
  eligibility: {
    available: true,
    previously_married: false,
  children: eachLike({ name: "Sally", age: 2 }),

// Define animal list payload, reusing existing object matcher
// Note that using eachLike ensure that all values are matched by type
const animalListExpectation = eachLike(animalBodyExpectation, {

  state: "Has some animals",
  uponReceiving: "a request for all animals",
  withRequest: {
    method: "GET",
    path: "/animals/available",
  willRespondWith: {
    status: 200,
    headers: {
      "Content-Type": "application/json; charset=utf-8",
    body: animalListExpectation,

Match by regular expression

If none of the above matchers or formats work, you can write your own regex matcher.

The underlying mock service is written in Ruby, so the regular expression must be in a Ruby format, not a Javascript format.

const { term } = pact

  state: "Has some animals",
  uponReceiving: "a request for an animal",
  withRequest: {
    method: "GET",
    path: "/animals/1",
  willRespondWith: {
    status: 200,
    headers: {
      "Content-Type": "application/json; charset=utf-8",
    body: {
      id: 100,
      name: "billy",
      gender: term({
        matcher: "F|M",
        generate: "F",


GraphQL is simply an abstraction over HTTP and may be tested via Pact. There are two wrapper APIs available for GraphQL specific testing: GraphQLInteraction and ApolloGraphQLInteraction.

These are both lightweight wrappers over the standard DSL in order to make GraphQL testing a bit nicer.

See the history, and below for an example.

Tutorial (60 minutes)

Learn everything in Pact JS in 60 minutes:



Asynchronous APIs

Using Pact in non-Node environments

Pact requires a Node runtime to be able to start and stop Mock servers, write logs and other things.

However, when used within browser or non-Node based environments - such as with Karma or ng-test - this is not possible.

To address this challenge, we have released a separate 'web' based module for this purpose - pact-web. Whilst it still provides a testing DSL, it cannot start and stop mock servers as per the pact package, so you will need to coordinate this yourself prior to and after executing any tests.

To get started, install pact-web and Pact Node:

npm install --save-dev @pact-foundation/pact-web @pact-foundation/pact-node

If you're not using Karma, you can start and stop the mock server using Pact Node or something like Grunt Pact.

Using Pact with Karma

We have create a plugin for Karma, which will automatically start and stop any Mock Server for your Pact tests.

Modify your karma.conf.js file as per below to get started:

    // Load pact framework - this will start/stop mock server automatically
    frameworks: ['pact'],

    // Load the pact and default karma plugins
    plugins: [

    // load pact web module
    files: [

    // Configure the mock service
    pact: [{
      port: 1234,
      consumer: 'KarmaMochaConsumer',
      provider: 'KarmaMochaProvider',
      logLevel: 'DEBUG',
      log: path.resolve(process.cwd(), 'logs', 'pact.log'),
      dir: path.resolve(process.cwd(), 'pacts')

Check out the Examples for how to use the Karma interface.

Using Pact with RequireJS

The module name should be "Pact" - not "pact-js". An example config with a karma test might look like the following:

In client-spec.js change the define to:

define(['client', 'Pact'], function (example, Pact) {

In test-main.js:

  baseUrl: "/base",
  paths: {
    Pact: "node_modules/pact-web/pact-web",
    client: "js/client",
  deps: allTestFiles,
  callback: window.__karma__.start,

See this Stack Overflow question for background, and this gist with a working example.


If you are having issues, a good place to start is setting logLevel: 'DEBUG' when configuring the new Pact({...}) object.

Alpine + Docker


Parallel tests

Pact tests are inherently stateful, as we need to keep track of the interactions on a per-test basis, to ensure each contract is validated in isolation from others. However, in larger test suites, this can result in slower test execution.

Modern testing frameworks like Ava and Jest support parallel execution out-of-the-box, which

The good news is, parallel test execution is possible, you need to ensure that:

  1. Before any test run invocation, you remove any existing pact files, to prevent invalid / stale interactions being left over from previous test runs
  2. Each test is fully self-contained, with its own mock server on its own port
  3. You set the option pactfileWriteMode to "merge", instructing Pact to merge any pact documents with the same consumer and provider pairing at the end of all test runs.

When all of your tests have completed, the result is the union of the all of the interactions from each test case in the generated pact file.

See the following examples for working parallel tests:

Splitting tests across multiple files

Pact tests tend to be quite long, due to the need to be specific about request/response payloads. Often times it is nicer to be able to split your tests across multiple files for manageability.

You have a number of options to achieve this feat:

  1. Consider implementing the Parallel tests guidelines.

  2. Create a Pact test helper to orchestrate the setup and teardown of the mock service for multiple tests.

    In larger test bases, this can significantly reduce test suite time and the amount of code you have to manage.

    See this example and this issue for more.

  3. Set pactfileWriteMode to merge in the Pact() constructor

    This will allow you to have multiple independent tests for a given Consumer-Provider pair, without it clobbering previous interactions, thereby allowing you to incrementally build up or modify your pact files.

    This feature addresses the use case of "my pact suite takes bloody ages to run, so I just want to replace the interactions that have been run in this test execution" and requires careful management

    NOTE: If using this approach, you must be careful to clear out existing pact files (e.g. rm ./pacts/*.json) before you run tests to ensure you don't have left over requests that are no longer relevant.

    See this PR for background.

Re-run specific verification failures

If you prefix your test command (e.g. npm t) with the following two environment variables, you can selectively run a specific interaction during provider verification.

For the e2e example, let's assume we have the following failure:

3 interactions, 2 failures

Failed interactions:

* A request for all animals given Has some animals

* A request for an animal with id 1 given Has an animal with ID 1

If we wanted to target the second failure, we can extract the description and state as the bits before and after the word "given":

PACT_DESCRIPTION="a request for an animal with ID 1" PACT_PROVIDER_STATE="Has an animal with ID 1" npm t

Also note that PACT_DESCRIPTION is the failing description and PACT_PROVIDER_STATE is the corresponding providerState from the pact file itself.


Under the hood, Pact JS spins up a Ruby Mock Service. On some systems, this may take more than a few seconds to start. It is recommended to review your unit testing timeout to ensure it has sufficient time to start the server.

See here for more details.

Note on Jest

Jest uses JSDOM under the hood which may cause issues with libraries making HTTP request.

You'll need to add the following snippet to your package.json to ensure it uses the proper Node environment:

"jest": {
  "testEnvironment": "node"

Also, from Jest 20, you can add the environment to the top of the test file as a comment. This will allow your pact test to run along side the rest of your JSDOM env tests.

 * @jest-environment node

Jest also runs tests in parallel by default, which can be problematic with Pact which is stateful. See parallel tests to see how to make it run in parallel, or run Jest with the --runInBand option to run them sequentially.

See this issue for background, and the Jest example for a working example.

Note on Angular

Angular's HttpClient filters out many headers from the response object, this may cause issues when validating a response in tests.

You'll need to add the additional header Access-Control-Expose-Headers, this will allow specified headers to be passed to the response object. This can be done by declaring the header in the willRespondWith section of your interaction:

"willRespondWith": {
  "headers": {
    "Access-Control-Expose-Headers": like("My-Header"),
    "My-Header": "..."

See this issue for background.


If your standard tricks don't get you anywhere, setting the logLevel to DEBUG and increasing the timeout doesn't help and you don't know where else to look, it could be that the binaries we use to do much of the Pact magic aren't starting as expected.

Try starting the mock service manually and seeing if it comes up. When submitting a bug report, it would be worth running these commands before hand as it will greatly help us:


...and also the verifier (it will whinge about missing params, but that means it works):



  1. Fork it
  2. Create your feature branch from the relevant tree (e.g. [v5] or [v6]) (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request

If you would like to implement Pact in another language, please check out the Pact Spec and have a chat to one of us on the pact-dev Google group.

The vision is to have a compatible Pact implementation in all the commonly used languages, your help would be greatly appreciated!


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