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1.0.6 • Public • Published


Add highly configurable API mocks to your express server

⚠️ Only supports ES Modules

This library only supports ES Modules, so your project and your mock files should as well. For more information on how to configure your project as such, check this post


This is the first iteration of this module, inspired by express-mock-api-middleware, and converted to ES Modules.

Future versions will receive these additional features:

  • [ ] Load get routes directly from json files.
  • [ ] Add config options globally or to "globbed" routes
    • [ ] enable these rules "randomly" to simulate an unpredictable behavior
    • [ ] status code (e.g. easily return 404 for a set of routes)
    • [ ] error responses (with 50x status code and error response body)
    • [ ] server delays to fake slow responses
  • [ ] Configure request conditions (.e.g. certain headers or query string in the request)
  • [ ] Add helpers around the default express API to make using the request and sending responses easier


Install this module

yarn add -D @mediamonks/monck


Use the standalone server

import path from 'path';
import { createServer } from '@mediamonks/monck';

// minimum
const app = createServer();

// all options (with defaults)
const app = createServer({
  mountPath: '/api/',
  host: 'localhost',
  port: '9002',
  useUnixSocket: false,
  socketPath: path.resolve(__dirname, './socket'), // default = path.resolve(process.cwd(), './socket')
  mockDir: path.resolve(__dirname, './mocks'), // default = path.resolve(process.cwd(), './mocks')
  ignore: ['*.sample.js'], // default = []

Or use the middleware in your existing server

import express from 'express'

import { createMockMiddleWare } from '@mediamonks/monck';

const app = express();

// add middleware
app.use('/api/', createMockMiddleWare());

// with options
app.use('/api/', createMockMiddleWare('/api/', {
  ignore: ['*.sample.js']

app.listen(9002, 'localhost', () => {


Server options:

  • mountPath?: string - On what path the mock API should be mounted. All configured mock endpoints will be prefixed by this path.
  • useUnixSocket?: boolean - Whether to use a unix socket to start the server instead of the default host:port.
  • socketPath?: string - Where to create the unix socket.
  • host?: string - What port to use.
  • port?: number - What host to use.

Middleware options:

  • mockDir?: string - Where the mock config files can be found.
  • ignore?: string | Array<string> - Add a pattern or an array of glob patterns to exclude matches. Note: ignore patterns are always in dot:true mode, regardless of any other settings - (See how to use ignore in options options).

CLI usage

Monck also provides a CLI that allows you to start the mock server without any node scripts required.

Usage: monck [options]

Server options:
  -m, --mount-path   On what path the mock API should be mounted. All configured mock endpoints will be prefixed by this
                                                                                             [string] [default: "/api/"]
  -h, --host         The host that will be used to run the server, passed to `app.listen`[string] [default: "localhost"]
  -p, --port         The port that will be used to run the server, passed to `app.listen`       [number] [default: 9002]
  -u, --unix-socket  Whether to use a unix socket to start the server instead of the default `host:port`.      [boolean]
  -s, --socket-path  Where to create the unix socket. Only needed when `unix-socket` is true.                   [string]

Middleware options:
  -d, --mock-dir  Where the mock config files can be found                                 [string] [default: "./mocks"]
  -i, --ignore    Add a glob pattern to exclude matches. Note: ignore patterns are always in `dot:true` mode, regardless
                   of any other settings                                                                        [string]

      --help  Show help                                                                                        [boolean]

  monck.js                       Start a server on default host and port.
  monck.js -h localhost -p 9002  Start a server on a specific host and port
  monck.js -u -s ./monck-socket  Start a server connected to the socket at that location
  monck.js -m api                Make all mock routes available on the "api/" path.
  monck.js -d ./mocks            Specify a folder where the mock files are located.
  monck.js -i "*.sample.js"      Ignore sample files in the mock folder.

For more information about the parameters, please visit https://github.com/mediamonks/monck

Mock configuration

All files in the configured mock directly will be required and watched for changes. The exported object keys will be added as routes to the server/middleware. Each route key is defined as "[method] [path]".

The value of each key is either an object - which is directly returned as-is, or an express RequestHandler that is executed and allows you to fabricate your own response using res.send().

TypeScript and ESM support

Since monck needs to load and execute your files at runtime, it's your responsibility to make sure these files can be loaded the correct way.

If you want to load TypeScript files, you need to transpile those files first, or make use of something like ts-node or tsm to use them during development.

Otherwise, you'd have to make use of ES modules, you need to make sure that your package.json or your file extensions are properly set up.

A combination of the both requires that both configurations are perfectly in sync.

  • tsconfig
    • "module": "ESNext"
    • "moduleResolution": "node"
    • "esModuleInterop": true
  • package.json
    • "type": "module"
    • or, your file extension set to .mjs
  • running
    • for TS/commonjs, using ts-node file.ts should be enough
    • For TS/ESM, use node with experimental features; node --experimental-specifier-resolution=node --loader ts-node/esm file.ts

Example file

// mocks/user.ts

import path from 'path';
import { existsSync } from 'fs';
import type { RequestConfig } from '@mediamonks/monck';

export default {
  // return static object
  'GET /user/info': {
    id: '123',
    userName: 'john123',
    email: 'john.doe@provider.org',
    firstName: 'John',
    lastName: 'Doe',
  // execute logic
  'POST /user/login': (req, res) => {
    const { userName, password } = req.body;
    if (userName === 'john' && password === 'password') {
        success: true,
    } else {
        success: false,
  // do more custom logic
  'GET /product/:id': (req, res) => {
    const { id } = req.params;
    const productPath = path.join(__dirname, `products/${id}.json`);
    if (existsSync(productPath)) {
    } else {
      res.sendFile(path.join(__dirname, `products/default.json`));
} as RequestConfig;


Set the DEBUG environment variable to see debug logs from this module.



The default setup is using esm/typescript, and can be seen by running yarn dev.



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