MSW dynamic HTTP middleware

Create dynamic handlers for MSW HTTP middleware

Features

For rapid prototyping or local web application development, you probably want to mock API calls, so that you don't need a running API server.

This can be done using MSW running inside the app using a service worker or by running a separate MSW server to which you proxy API calls.

Both solutions use static mock handlers. The responses are fixed and cannot be changed during runtime of the app or mock server. But how do you quickly test client side error handling when your API always returns success responses? This is not possible without code changes.

Changing handlers on runtime is also essential when you use a mock server in end-to-end testing, for example in Cypress.

This package is an extension on top of @mswjs/http-middleware that adds extra endpoints to change the mock handlers based on scenarios at runtime.

The following endpoints are available:

Endpoint	When to use
GET /scenario	Get all loaded scenarios and activated handlers
PUT /scenario	Activate a scenario
DELETE /scenario	Reset mock server to initial state, optionally with ?resetAll=true to also deactive default scenario handlers

There is also a UI provided that can be statically hosted on for example /ui, so that you can set scenarios from the browser instead of using curl / Postman. Note that GraphQL is not supported (yet).

How to use

First install this package and @msw/http-middleware as devDependency in your project:

npm install -D msw-dynamic-http-middleware @msw/http-middleware

In the example directory you find an example server. Run npm install in this dir and then npm start to run the server on http://localhost:9800.

Basically this is everything you need in your application for a dynamic MSW server:

// server.js
const express = require('express');
const { rest } = require('msw');
const { createMiddleware } = require('@mswjs/http-middleware');
const { createHandlers, middleware } = require('msw-dynamic-http-middleware');

const scenarios = {
  // Scenarios for one endpoint
  'user success': rest.get('/user', (req, res, ctx) => res(ctx.json({ name: 'frank' }))),
  'user error': rest.get('/user', (req, res, ctx) => res(ctx.status(500))),
  'users success': rest.get('/users', (req, res, ctx) => res(ctx.json([{ name: 'frank' }]))),
  'users error': rest.get('/users', (req, res, ctx) => res(ctx.status(500))),

  // Scenarios for multiple endpoints
  success: [
    rest.get('/user', (req, res, ctx) => res(ctx.json({ name: 'frank' }))),
    rest.get('/users', (req, res, ctx) => res(ctx.json([{ name: 'frank' }]))),
  ],
  error: [
    rest.get('/user', (req, res, ctx) => res(ctx.status(500))),
    rest.get('/users', (req, res, ctx) => res(ctx.status(500))),
  ],
};

const handlers = createHandlers(scenarios, 'success');

const app = express();

app.use(express.json());
app.use(createMiddleware(...handlers));
app.use('/ui', middleware); // Run the UI on /ui. Can be any path, but don't use /scenario or one of your scenario endpoints

const port = 9800;
app.listen(port, () => console.info(`MSW server running at http://localhost:${port}`));

Run the server with node server.js.

Then navigate to http://localhost:9800/ui or set scenarios by calling the endpoint:

PUT /scenario
{
  "scenario": "user success"
}

You need scenarios because MSW handlers are not (easily) serializable and a scenario name is.

Check the docs for your favorite client framework on how to proxy API calls to this web server. Here are some examples.

Vue CLI:

// vue.config.js
export default {
  devServer: {
    proxy: {
      '^/api': 'http://localhost:9800',
    },
  },
};

ViteJS:

// vite.config.js
export default {
  server: {
    proxy: {
      '^/api': 'http://localhost:9800',
    },
  },
};

Create React App:

// package.json
"proxy": "http://localhost:4000",

Angular CLI:

// src/proxy.conf.js
{
  "/api": {
    "target": "http://localhost:3000",
    "secure": false
  }
}

// angular.json
...
"architect": {
  "serve": {
    "builder": "@angular-devkit/build-angular:dev-server",
    "options": {
      "browserTarget": "your-application-name:build",
      "proxyConfig": "src/proxy.conf.json"
    },
...

Clearing handlers

Endpoints that don't have a handler assigned (yet), for example when the server starts, return a default response, which is currently an empty body with status 200. However, you could pass a default scenario name to createHandlers to start the server with an initial set of handlers. For example create a scenario 'success' with success responses for all your endpoints to start the server with a "happy flow" set up.

Besides setting scenarios on runtime using the PUT /scenarios endpoint, you could also reset state to the default handler using the DELETE /scenarios endpoint. If the handlers were created with a default scenario, the server resets to this default scenario, unless you pass ?resetAll=true query parameter.

Endpoints that do not exist because there are no scenarios defined for this endpoint, do return the default response from @mswjs/http-middelware, which is { "error": "Mock not found" } with status 404.

Does this package work with MSW only?

This package builds upon @mswjs/http-middleware but if you want to set predefined scenarios for vanilla MSW handlers, consider using MSW UI.

The functionality in this package is loosely based on ng-apimock but that project is pretty old and not maintained anymore. It is also less flexible because it depends on scenarios that are defined in JSON files.