mswMock Service Worker
  1. Recipes
  2. Debugging uncaught requests

Debugging uncaught requests

Whenever you are faced with a request that is not being intercepted, follow the suggestions on this page to debug the issue.

Enable onUnhandledRequest option

  • Applicable to: setupWorker/setupServer

The library comes with a built-in mechanism to react to unhandled requests. You can configure the onUnhandledRequest option of your worker/server to warn or error whenever there's a request that does not have a corresponding request handler.

Learn more about how to configure this option for your worker or server:

start (setupWorker)/docs/api/setup-worker/start#onunhandledrequest
listen (setupServer)/docs/api/setup-server/listen#onunhandledrequest

Examine the handlers

  • Applicable to: setupWorker/setupServer

You can print out the list of current request handlers at any point of your application runtime or during a test. Browsing the list of handlers can help you determine if a request handler that is not appended on runtime with .use(), or if the handler you thought is there is actually missing.

printHandlers (setupWorker)/docs/api/setup-worker/print-handlers
printHandlers (setupServer)/docs/api/setup-server/print-handlers

Verify the worker's scope

  • Applicable to: setupWorker

A Service Worker can only intercept the requests under its scope. The maximum scope of the worker is its file location.

For example, if you are registering the MSW Service Worker at a custom path:

2 serviceWorker: {
3 url: '/assets/mockServiceWorker.js',
4 },

Then MSW will only intercept the requests under /assets/* path. A request to /user or /posts will not be intercepted as they lie outside of the worker's scope (/assets).

The Service Worker distributed by MSW is a regular Service Worker like any other. While not mandatory, the more familiar you are with how Service Workers function, the fewer issues you will experience. Learn more about Using Service Worker (MDN).

If you've confirmed that the worker's scope cannot capture the requests your application is making, there are a few ways to address that.

Register the worker at a higher scope

We recommend registering the Service Worker at the root of your application. That is the default behavior of calling worker.start() without the custom serviceWorker.url option.

You can verify that your Service Worker is registered at the root by accessing it:

  • <SITE_URL>/mockServiceWorker.js, where SITE_URL is the local URL of your application.

By registering the worker at the root, it can intercept all requests that your application makes, which is often what you want.

Set the Service-Worker-Allowed response header

This option is the last resort and is not generally recommended.

If you are in control over the configuration of your development server, you can configure it to set the Service-Worker-Allowed header on all responses of your application. Doing that will permit the worker to capture requests that lie outside of its scope.

Here's an example of how to set the Service-Worker-Allowed header on an Express server:

1const express = require('express')
3const app = express()
5app.use((req, res, next) => {
6 res.setHeader('Service-Worker-Allowed', '/')
7 next()

Run in debug mode

  • Applicable to: setupServer

Mock Service Worker supports debug when run in NodeJS. You can leverage this to look into the internals of how requests are intercepted, as well as provide the output logs to the library maintainers for more details.

Append the DEBUG environmental variable to the command that runs your tests:

1$ DEBUG=* npm test

With that variable set, you will see many messages in your terminal indicating various stages of request interception that can give you useful hints as to what may have gone wrong.