Dynamic mock scenarios

When developing or presenting your work, you may need to switch between different mock scenarios on runtime. Going to the source code in the middle of a demo isn’t a good experience for your audience.

One of the ways to achieve this is to declare a set of scenarios (handler overrides) and conditionally apply them based on some runtime criteria, like query parameters.

// mocks/scenarios.js
import { http, HttpResponse } from 'msw'
export const scenarios = {
  success: [
    http.get('/user', () => {
      return HttpResponse.json({ name: 'John Maverick' })
  error: [
    http.get('/user', () => {
      return new HttpResponse(null, { status: 500 })
// mocks/browser.js
import { handlers } from './handlers'
import { scenarios } from './scenarios'
// Since the worker is registered on the client, you can read
// the page's query parameters before assigning request handlers.
const scenarioName = new URLSearchParams(window.location.search).get('scenario')
const runtimeScenarios = scenarios[scenarioName] || []
const worker = setupWorker(...runtimeScenarios, ...handlers)

Note that the order of handlers is important: the left-most handlers take precedence during request resolution.

Now, if you wish to see how your application behaves if the GET /user endpoint returns a 500 Internal Server Error response, simply navigate to the page you need with the proper scenario query parameter: