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Node server for dynamic, fake JSON.

npm install dyson

See installation notes. Check out some demo services.

Build Status npm package dependencies npm version


Dyson allows you to define JSON endpoints based on a simple template object:


When developing client-side applications, for data usually either static JSON files are used, or an actual server, backend, datastore, API, you name it. Sometimes static files are too static, and sometimes an actual server is not available, not accessible, or too tedious to setup.

This is where dyson comes in. Get a full fake server for your application up and running in minutes.


  • Easy configuration, extensive options
  • Dynamic responses
    • Responses can use request data (e.g. to simulate different login scenarios based on username):
      • Request path
      • GET/POST parameters
      • Cookies
    • Respond with different status code for specific requests (e.g. 404 for ?id=999)
    • Includes random data generators
  • Supports to proxy non-configured endpoints to actual services
  • Supports CORS
  • Supports delayed responses
  • Includes dummy image generator
    • Use any external or local image service (included)
    • Supports base64 encoded image strings
  • Supports required parameter validation

Endpoint Configuration

Configure endpoints using simple objects:

module.exports = {
  path: '/user/:id',
  method: 'GET',
  template: {
    id: (params, query, body) =>,
    address: {
      zip: g.zipUS,

The path string is the usual argument provided to Express, as in app.get(path, callback);.

The template object may contain properties of the following types:

  • function: the function will be invoked with arguments (params, query, body, cookies, headers)
  • string, boolean, number, array: returned as-is
  • object: will be recursively iterated
  • promise: if the function is a promise, it will be replaced with the resolving value

Note: the template can also be a function returning the actual data. The template function itself is also invoked with arguments (params, query, body, cookies, headers).


In addition to configured endpoints, dyson registers a dummy image service at /image. E.g. requesting /image/300x200 serves an image with given dimensions.

This service is a proxy to Dynamic Dummy Image Generator by Russell Heimlich.


The default values for the configuration objects:

module.exports = {
  cache: false,
  delay: false,
  proxy: false,
  size: () => _.random(2,10),
  collection: false,
  callback: response.generate,
  render: response.render
  • cache:true means that multiple requests to the same path will result in the same response
  • delay:number will delay the response with number milliseconds (or between [n, m] milliseconds)
  • proxy:false means that requests to this file can be skipped and sent to the configured proxy
  • size:function is the number of objects in the collection
  • collection:true will return a collection
  • callback:function
    • the provided default function is doing the hard work (but can be overridden)
    • used as middleware in Express
    • must set res.body and call next() to render response
  • render:function
    • the default function to render the response (basically res.send(200, res.body);)
    • used as middleware in Express

Fake data generators

You can use anything to generate data. Here are some suggestions:

Just install the generator(s) in your project to use them in your templates:

npm install dyson-generators --save-dev

Please refer to the project pages for usage and examples (here's one using dyson-generators).


Containers can help if you need to send along some meta data, or wrap the response data in a specific way. Just use the container object, and return the data where you want it. Functions in the container object are invoked with arguments (params, query, data):

module.exports = {
  path: '/users',
  template: user.template,
  container: {
    meta: (params, query, data) => ({
      userCount: data.length
    data: {
      all: [],
      the: {
        way: {
          here: (params, query, data) => data

And an example response:

  "meta": {
    "userCount": 2
  "data": {
    "all": [],
    "the": {
      "way": {
        "here": [{
          "id": 412,
          "name": "John"
        }, {
          "id": 218,
          "name": "Olivia"

Combined requests

Basic support for "combined" requests is available, by means of a comma separated path fragment.

For example, a request to /user/5,13 will result in an array of the responses from /user/5 and /user/13.

The , delimiter can be configured (or disabled).

Status codes

By default, all responses are sent with a status code 200 (and the Content-Type: application/json header).

This can be overridden with your own status middleware, e.g.:

module.exports = {
  path: '/feature/:foo?',
  status: (req, res, next) => {
    if( === '999') {

Would result in a 404 when requesting /feature/999.


Override the render method of the Express middleware in the endpoint definition. In the example below, depending on the existence of the callback parameter, either raw JSON response is returned or it is wrapped with the provided callback:

module.exports = {
  render: (req, res) => {
    const callback = req.query.callback;
    if (callback) {
      res.append('Content-Type', 'application/javascript');
    } else {


If you want to run dyson over https:// you have to provide a self-signed (or authority-signed) certificate into the options.https the same way it's required for NodeJS HTTPS to work:

const fs = require('fs');
  configDir: `${__dirname}/dummy`,
  port: 3001,
  https: {
    key: fs.readFileSync(`${__dirname}'/certs/sample.key`),
    crt: fs.readFileSync(`${__dirname}/certs/sample.crt`)

Note: if running HTTPS on port 443, it will require sudo privileges.

Custom middleware

If you need some custom middleware before or after the endpoints are registered, dyson can be initialized programmatically. Then you can use the Express server instance (appBefore or appAfter in the example below) to install middleware before or after the dyson services are registered. An example:

const dyson = require('dyson');
const path = require('path');
const options = {
  configDir: path.join(__dirname, 'services'),
  port: 8765
const configs = dyson.getConfigurations(options);
const appBefore = dyson.createServer(options);
const appAfter = dyson.registerServices(appBefore, options, configs);
console.log(`Dyson listening at port ${options.port}`);

Dyson configuration can also be installed into any Express server:

const express = require('express');
const dyson = require('./lib/dyson');
const path = require('path');
const options = {
  configDir: path.join(__dirname, 'services')
const myApp = express();
const configs = dyson.getConfigurations(options);
dyson.registerServices(myApp, options, configs);


The recommended way to install dyson is to install it locally and put it in your package.json:

npm install dyson

Then you can use it from an npm-script in package.json using e.g. npm run mocks:

  "name": "my-package",
  "version": "1.0.0",
  "scripts": {
    "mocks": "dyson stubs"

You can also install dyson globally to start it from anywhere:

npm install -g dyson


You can put your configuration files anywhere, but either the configuration must have the method property set or the configuration file must be inside a directory representing the method (e.g. stubs/get/sub/endpoint.js). Then start the server:

dyson [dir]

This starts the services configured in [dir] at localhost:3000.

You can also provide an alternative port number by just adding it as a second argument (e.g. dyson path/ 8181).


For a demo project, see webpro/dyson-demo. This demo was also installed with to

Project Configuration

Optionally, you can put a dyson.json file next to the configuration folders (inside [dir]). It enables to configure some behavior of dyson:

  "multiRequest": ",",
  "proxy": true,
  "proxyHost": "",
  "proxyPort": 8080,
  "proxyDelay": [200, 800]
  • Setting multiRequest to false disables the combined requests feature.
  • Setting bodyParserJsonLimit or bodyParserUrlencodedLimit to 1mb increases the limit to 1mb from the bodyParser's default of 100kb.
  • By default, the proxy is set to false


If you want to automatically restart dyson when you change your configuration objects, you can add nodemon as a devDependency. Say your configuration files are in the ./api folder, you can put this in your package.json:

"scripts": {
  "api": "dyson api",
  "watch": "nodemon --watch api --exec dyson api"

Development & run tests

git clone
cd dyson
npm install
npm test

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