userscript.js

// ==UserScript==
// @name        Request Mocker
// @description Edit this script to mock requests on you site (also edit @match)
// @match       *://*/*
// @grant       none
// @version     1.0
// @author      Mateusz Kasprzak
// @require     https://unpkg.com/msw@0.20.4/lib/umd/index.js
// ==/UserScript==

const {rest, setupWorker} = MockServiceWorker;

const handlers = [
  rest.post('/login', (req, res, ctx) => {
    // Persist user's authentication in the session
    sessionStorage.setItem('is-authenticated', true)
    return res(
      // Respond with a 200 status code
      ctx.status(200),
    )
  }),
  rest.get('/user', (req, res, ctx) => {
    // Check if the user is authenticated in this session
    const isAuthenticated = sessionStorage.getItem('is-authenticated')
    if (!isAuthenticated) {
      // If not authenticated, respond with a 403 error
      return res(
        ctx.status(403),
        ctx.json({
          errorMessage: 'Not authorized',
        }),
      )
    }
    // If authenticated, return a mocked user details
    return res(
      ctx.status(200),
      ctx.json({
        username: 'admin',
      }),
    )
  }),
]

setupWorker(...handlers).start({
  serviceWorker: {
    // Points to the custom location of the Service Worker file.
    url: 'https://unpkg.com/msw@0.20.4/lib/umd/mockServiceWorker.js'
  }
})