msw/src/browser/setupWorker/start/utils/getWorkerInstance.ts

Seamless REST/GraphQL API mocking library for browser and Node.js.

import { until } from 'until-async'
import { devUtils } from '~/core/utils/internal/devUtils'
import { getAbsoluteWorkerUrl } from '../../../utils/getAbsoluteWorkerUrl'
import { getWorkerByRegistration } from './getWorkerByRegistration'
import { ServiceWorkerInstanceTuple, FindWorker } from '../../glossary'

/**
 * Returns an active Service Worker instance.
 * When not found, registers a new Service Worker.
 */
export const getWorkerInstance = async (
  url: string,
  options: RegistrationOptions = {},
  findWorker: FindWorker,
): Promise<ServiceWorkerInstanceTuple> => {
  // Resolve the absolute Service Worker URL.
  const absoluteWorkerUrl = getAbsoluteWorkerUrl(url)

  const mockRegistrations = await navigator.serviceWorker
    .getRegistrations()
    .then((registrations) =>
      registrations.filter((registration) =>
        getWorkerByRegistration(registration, absoluteWorkerUrl, findWorker),
      ),
    )
  if (!navigator.serviceWorker.controller && mockRegistrations.length > 0) {
    // Reload the page when it has associated workers, but no active controller.
    // The absence of a controller can mean either:
    // - page has no Service Worker associated with it
    // - page has been hard-reloaded and its workers won't be used until the next reload.
    // Since we've checked that there are registrations associated with this page,
    // at this point we are sure it's hard reload that falls into this clause.
    location.reload()
  }

  const [existingRegistration] = mockRegistrations

  if (existingRegistration) {
    // Schedule the worker update in the background.
    // Update ensures the existing worker is up-to-date.
    existingRegistration.update()

    // Return the worker reference immediately.
    return [
      getWorkerByRegistration(
        existingRegistration,
        absoluteWorkerUrl,
        findWorker,
      ),
      existingRegistration,
    ]
  }

  // When the Service Worker wasn't found, register it anew and return the reference.
  const [registrationError, registrationResult] = await until<
    Error,
    ServiceWorkerInstanceTuple
  >(async () => {
    const registration = await navigator.serviceWorker.register(url, options)
    return [
      // Compare existing worker registration by its worker URL,
      // to prevent irrelevant workers to resolve here (such as Codesandbox worker).
      getWorkerByRegistration(registration, absoluteWorkerUrl, findWorker),
      registration,
    ]
  })

  // Handle Service Worker registration errors.
  if (registrationError) {
    const isWorkerMissing = registrationError.message.includes('(404)')

    // Produce a custom error message when given a non-existing Service Worker url.
    // Suggest developers to check their setup.
    if (isWorkerMissing) {
      const scopeUrl = new URL(options?.scope || '/', location.href)

      throw new Error(
        devUtils.formatMessage(`\
Failed to register a Service Worker for scope ('${scopeUrl.href}') with script ('${absoluteWorkerUrl}'): Service Worker script does not exist at the given path.

Did you forget to run "npx msw init <PUBLIC_DIR>"?

Learn more about creating the Service Worker script: https://mswjs.io/docs/cli/init`),
      )
    }

    // Fallback error message for any other registration errors.
    throw new Error(
      devUtils.formatMessage(
        'Failed to register the Service Worker:\n\n%s',
        registrationError.message,
      ),
    )
  }

  return registrationResult
}