Komondor core library

Usage no npm install needed!

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komondor is your friendly guard dog for writing tests across boundaries.

Check out my pair programming schedule if you are interested in working together on this project.

The problem

Boundary is where two systems meet and communicate with each other using data structures and primitive types.

For example, making calls to a remote server or a component written by another team in another language.

When we write tests that needs to communicate across the boundary, we often need to create a test double to similate the behavior.

This allow us to write tests that are fast to run and also decouple from the remote system, so that we don't have to configure the remote system to produce the expected behavior.

However, if we only have these tests, we will not be able to catch breakage when the remote system changes its behavior.

Traditionally, we will have a suite of system integration test to make sure the system is working as a whole. However, these tests are hard to configure and slow. Most of the time we can only create or run a handful of these tests to make sure all or part of the critical paths are covered.

When the behavior of the other system is changed, you have to go through some manual process to make actual calls, adjust the test doubles you have, and fix the code.

That's a lot of manual work and the worst of all it reduces the level of trust you have on your test suite.

The solution

komondor can turn your stubbed unit test to system integration test by a simple switch. It also makes writing these tests systematic and simple.

When writing a test that needs to access a remote system across a boundary, you will do a three steps test-waltz:

  • write a test and making it pass while making actual remote calls
  • find out and record the data you recevied from the remote calls
  • use the recorded data to create a test double and use the test double in the test

Using komondor, these three steps becomes very straight forward.

The following example will create a test that needs to communicate to GitHub api.

step 1: writing a passing test with actual remote calls

Step 1 is writing a passing test. Your logic should be unit testable, i.e. you can create a test double to mock out the remote calls. It should be the same as what you have been doing.

import { test } from 'ava'

// test subject
function getFollowers(github: GitHub, username: string) {
  return new Promise((a, r) => {
    }, (err, res) => {
      if (err) r(err)
      // massage the response in some way that make sense to your application.
      const response = messege(res)

test('get follower of a user', t => {
  const github = new GitHub()

  const followers = await getFollowers(github, 'someRealUser')

  // assert `followers` is correct.

step 2: find out and record the data

The test is passing. Now it is time to figure out the shape of the data (which you probably already know as you consumed it), and tell komondor to save the data.

First thing to do is to use komondor to spy on the data. You can do this in step 1, but I'm doing it here to make the steps more concrete for demostration purpose.

Unchange (and uninterested) lines from the code above are omitted for clarity.

import { spec } from 'komondor'

// test subject

test('get follower of a user', async t => {
  const github = new GitHub()
  const getFollowersSpec = await spec(github.users.getFollowersForUser)

  // do `specs.subject.bind(github.users)` when needed.
  github.users.getFollowersForUser = specs.subject

  const followers = await getFollowers(github, 'someRealUser')

  // (optional) get the actual actions recorded by `komondor` for inspection

  // (required) tells spec that it is ready to save the record (in save mode).
  await getFollowersSpec.done()

The code above uses komondor to spy on the call and make sure the data received meet your expectation.

getFollowersSpec.satisfy() uses satisfier to validate the data. Please check it out to see how to define your expectation.

Once the test pass again (meaning the spy is working correctly and you have setup the right expectation), you can now tell komondor to save the result.

To do that, all you need to do is changing the call from spec() to spec.save() and provide a name to it.:

  const getFollowersSpec = await spec.save(
    'github getFollowersForUser',

When you run the test, the result will be saved.

step 3: replay the recorded data

The last step is to tell komondor to use the recorded data in the test.

The way to do it is extremely simple. All you need is to change the call from spec.save() to spec.simulate():

  const getFollowersSpec = await spec.simulate(
    'github getFollowersForUser',

That's it! Now your test will be ran using the saved result and not making actual remote calls.


given is a construct to declare and verify the fixture of the spec to run on. It is a mean to communicate between engineer, QA, and possibility IT. It can consist of spec, which is used to configure or validate if the environment is valid. These specs provide the necessary information for the QA and IT to re-create the fixture.

Alternatively, a service can be provided to get the fixture information at real time, so that the fixture can be decoupled from the tests.

For example, when a test ask for a "version 1, normal load" environment, the service can spawn up a new docker container and return the endpoint information to the test. That means each test can be run in its own environment.



function spec<T>(subject: T): Promise<Spec<T>>
function spec<T>(name: string, subject: T): Promise<Spec<T>>
function spec.save<T>(name: string, subject: T): Promise<Spec<T>>
function spec.simulate<T>(name: string, subject: T): Promise<Spec<T>>


function given<T>(clause: string, localHandler?: (context: GivenContext) => any): Promise<Given<T>>
function given.save<T>(clause: string, localHandler?: (context: GivenContext) => any): Promise<Given<T>>
function given.simulate<T>(clause: string, localHandler?: (context: GivenContext) => any): Promise<Given<T>>

function onGiven(clause: string | RegExp, handler: (context: GivenContext) => any): void


komondor supports plugins to spec on subjects other than basic function, class, and promise.

To create a plugin, please check komondor-plugin


Since komondor will record the calls, if they contain sensitive information you don't want to keep in the record, you can remove them in the spec.actions before you make the spec.done() call.


You can use this method instead of speced.done() if you want to validate the call has been performed correctly.

It is generally better than speced.done(); however, since there are still come changes need to be made to the spec record before it is stable, using speced.done() saves you some tendious work in updating the expectations before that happens.


Nothing happen when I change to save mode

Check if you have wait for speced.done().

i.e., you should have await yourSpec.done(...) in your test.

The record will be save when done() resolves.

Enable komondor log

komondor provides some logs for debugging purpose. Since they are chatty, they are turned off by default.

To enable them, do the following:

import { addAppender, getLogger, logLevel } from '@unional/logging'
import { ColorAppender } from 'aurelia-logging-color'

// Skip if you have already done this for other purpose, or
// you are already using a different appender.
addAppender(new ColorAppender())


Using komondor with node-fetch

node-fetch and some other libraries provides more features then just handling simple data structure. For example they support streaming and web-socket.

komondor cannot work with them directly because it needs to be able to serialize and deserialize the result.

The response object from node-fetch is not serializable:

import fetch = require('node-fetch')

test('response is complex', async t => {
  const response = await fetch('some url')
  // it contains `.text()` method and others,
  // so it is not serializable.
  const json = JSON.parse(response.text())

One way to get around this is create a wraping function that returns DTO. komondor can then work on that function.

import fetch = require('node-fetch')

async function fetchJson(url, options) {
  const response = await fetch(url, options)
  return JSON.parse(response.text())

test('komondor can work with fetchJson', async t => {
  const yourSpec = await spec(fetchJson)
  const json = await yourSpec.fn('someUri')

  yourSpec.satisfy({ ... })

Creating fetchJson() is actually a good practice because it creates a separation between the layer. Your application code uses fetchJson() instead of fetch(), meaning it does not know the information comes from the network.

(yes, for that you should give it a more netural name and remove the HTTP specific options)

Test fails after upgrade

Since the libary is still in unstable stage, when you upgrade to a version, your test may break even if there is no breaking change.

It is due to the under lying record change and features being added.

You should able to get them pass again by running the spec and scenario in save mode to update the record. After you do that the test should pass again.

The easiest way to do that is using config:

import { config } from 'komondor'


Depends on your test environment, you may use the filter to run only a subset of test in save mode, fitting the actual environment you can configure at a particular time.

Wallaby config

Since komondor will write files to the file system, if you use wallaby you need configure it as follow so that the changed files will be written correctly:

module.exports = () => {
  return {
    'files': [
      // load spec records
      { pattern: '__komondor__/**/*', instrument: false },
    setup(wallaby) {
      const fs = require('fs');
      if (fs.patched) return;
      const path = require('path');

      const writeFile = fs.writeFileSync;
      fs.writeFileSync = function(file, content) {
        if (/__komondor__/.test(file)) {
          writeFile(path.join(wallaby.localProjectDir, file.replace(wallaby.projectCacheDir, '')), content);
        return writeFile.apply(this, arguments);
      const mkdirSync = fs.mkdirSync;
      fs.mkdirSync = function (dir, mode) {
        if (/__komondor__/.test(dir)) {
          mkdirSync(path.join(wallaby.localProjectDir, dir.replace(wallaby.projectCacheDir, '')), mode);
        return mkdirSync.apply(this, arguments);
      fs.patched = true;


# right after fork
npm install

# begin making changes
git checkout -b <branch>
npm run watch

# edit `webpack.config.dev.js` to exclude dependencies for the global build.

# after making change(s)
git commit -m "<commit message>"
git push

# create PR

Npm Commands

There are a few useful commands you can use during development.

# Run tests (and lint) automatically whenever you save a file.
npm run watch

# Run tests with coverage stats (but won't fail you if coverage does not meet criteria)
npm run test

# Manually verify the project.
# This will be ran during 'npm preversion' so you normally don't need to run this yourself.
npm run verify

# Build the project.
# You normally don't need to do this.
npm run build

# Run tslint
# You normally don't need to do this as `npm run watch` and `npm version` will automatically run lint for you.
npm run lint