@mswjs/interceptors

Low-level network interception library.

This library supports intercepting the following protocols:

• HTTP (via the http module, XMLHttpRequest, or globalThis.fetch);
• WebSocket (the WebSocket class in Undici and in the browser).

Motivation

While there are a lot of network communication mocking libraries, they tend to use request interception as an implementation detail, giving you a high-level API that includes request matching, timeouts, retries, and so forth.

This library is a strip-to-bone implementation that provides as little abstraction as possible to execute arbitrary logic upon any request. It's primarily designed as an underlying component for high-level API mocking solutions such as Mock Service Worker.

How is this library different?

A traditional API mocking implementation in Node.js looks roughly like this:

import http from 'http'

function applyMock() {
  // Store the original request module.
  const originalHttpRequest = http.request

  // Rewrite the request module entirely.
  http.request = function (...args) {
    // Decide whether to handle this request before
    // the actual request happens.
    if (shouldMock(args)) {
      // If so, never create a request, respond to it
      // using the mocked response from this blackbox.
      return coerceToResponse.bind(this, mock)
    }

    // Otherwise, construct the original request
    // and perform it as-is (receives the original response).
    return originalHttpRequest(...args)
  }
}

This library deviates from such implementation and uses class extensions instead of module rewrites. Such deviation is necessary because, unlike other solutions that include request matching and can determine whether to mock requests before they actually happen, this library is not opinionated about the mocked/bypassed nature of the requests. Instead, it intercepts all requests and delegates the decision of mocking to the end consumer.

class NodeClientRequest extends ClientRequest {
  async end(...args) {
    // Check if there's a mocked response for this request.
    // You control this in the "resolver" function.
    const mockedResponse = await resolver(request)

    // If there is a mocked response, use it to respond to this
    // request, finalizing it afterward as if it received that
    // response from the actual server it connected to.
    if (mockedResponse) {
      this.respondWith(mockedResponse)
      this.finish()
      return
    }

    // Otherwise, perform the original "ClientRequest.prototype.end" call.
    return super.end(...args)
  }
}

By extending the native modules, this library actually constructs requests as soon as they are constructed by the consumer. This enables all the request input validation and transformations done natively by Node.js—something that traditional solutions simply cannot do (they replace http.ClientRequest entirely). The class extension allows to fully utilize Node.js internals instead of polyfilling them, which results in more resilient mocks.

What this library does

This library extends (or patches, where applicable) the following native modules:

• http.get/http.request
• https.get/https.request
• XMLHttpRequest
• fetch

Once extended, it intercepts and normalizes all requests to the Fetch API Request instances. This way, no matter the request source (http.ClientRequest, XMLHttpRequest, window.Request, etc), you always get a specification-compliant request instance to work with.

You can respond to the intercepted request by constructing a Fetch API Response instance. Instead of designing custom abstractions, this library respects the Fetch API specification and takes the responsibility to coerce a single response declaration to the appropriate response formats based on the request-issuing modules (like http.OutgoingMessage to respond to http.ClientRequest, or updating XMLHttpRequest response-related properties).

What this library doesn't do

• Does not provide any request matching logic;
• Does not decide how to handle requests.

Getting started

npm install @mswjs/interceptors

Interceptors

To use this library you need to choose one or multiple interceptors to apply. There are different interceptors exported by this library to spy on respective request-issuing modules:

• ClientRequestInterceptor to spy on http.ClientRequest (http.get/http.request);
• XMLHttpRequestInterceptor to spy on XMLHttpRequest;
• FetchInterceptor to spy on fetch.

Use an interceptor by constructing it and attaching request/response listeners:

import { ClientRequestInterceptor } from '@mswjs/interceptors/ClientRequest'

const interceptor = new ClientRequestInterceptor()

// Enable the interception of requests.
interceptor.apply()

// Listen to any "http.ClientRequest" being dispatched,
// and log its method and full URL.
interceptor.on('request', ({ request, requestId }) => {
  console.log(request.method, request.url)
})

// Listen to any responses sent to "http.ClientRequest".
// Note that this listener is read-only and cannot affect responses.
interceptor.on(
  'response',
  ({ response, isMockedResponse, request, requestId }) => {
    console.log('response to %s %s was:', request.method, request.url, response)
  }
)

All HTTP request interceptors implement the same events:

• request, emitted whenever a request has been dispatched;
• response, emitted whenever any request receives a response.

Using multiple interceptors

You can combine multiple interceptors to capture requests from different request-issuing modules at once.

import { BatchInterceptor } from '@mswjs/interceptors'
import { ClientRequestInterceptor } from '@mswjs/interceptors/ClientRequest'
import { XMLHttpRequestInterceptor } from '@mswjs/interceptors/XMLHttpRequest'

const interceptor = new BatchInterceptor({
  name: 'my-interceptor',
  interceptors: [
    new ClientRequestInterceptor(),
    new XMLHttpRequestInterceptor(),
  ],
})

interceptor.apply()

// This "request" listener will be called on both
// "http.ClientRequest" and "XMLHttpRequest" being dispatched.
interceptor.on('request', listener)

Note that you can use pre-defined presets that cover all the request sources for a given environment type.

Presets

When using BatchInterceptor, you can provide a pre-defined preset to its "interceptors" option to capture all request for that environment.

Node.js preset

This preset combines ClientRequestInterceptor, XMLHttpRequestInterceptor and is meant to be used in Node.js.

import { BatchInterceptor } from '@mswjs/interceptors'
import nodeInterceptors from '@mswjs/interceptors/presets/node'

const interceptor = new BatchInterceptor({
  name: 'my-interceptor',
  interceptors: nodeInterceptors,
})

interceptor.apply()

interceptor.on('request', listener)

Browser preset

This preset combines XMLHttpRequestInterceptor and FetchInterceptor and is meant to be used in a browser.

import { BatchInterceptor } from '@mswjs/interceptors'
import browserInterceptors from '@mswjs/interceptors/presets/browser'

const interceptor = new BatchInterceptor({
  name: 'my-interceptor',
  interceptors: browserInterceptors,
})

interceptor.on('request', listener)

Introspecting requests

All HTTP request interceptors emit a "request" event. In the listener to this event, they expose a request reference, which is a Fetch API Request instance.

There are many ways to describe a request in Node.js but this library coerces different request definitions to a single specification-compliant Request instance to make the handling consistent.

interceptor.on('request', ({ request, requestId }) => {
  console.log(request.method, request.url)
})

Since the exposed request instance implements the Fetch API specification, you can operate with it just as you do with the regular browser request. For example, this is how you would read the request body as JSON:

interceptor.on('request', async ({ request, requestId }) => {
  const json = await request.clone().json()
})

Do not forget to clone the request before reading its body!

Modifying requests

Request representations are readonly. You can, however, mutate the intercepted request's headers in the "request" listener:

interceptor.on('request', ({ request }) => {
  request.headers.set('X-My-Header', 'true')
})

This restriction is done so that the library wouldn't have to unnecessarily synchronize the actual request instance and its Fetch API request representation. As of now, this library is not meant to be used as a full-scale proxy.

Mocking responses

Although this library can be used purely for request introspection purposes, you can also affect request resolution by responding to any intercepted request within the "request" event.

Use the request.respondWith() method to respond to a request with a mocked response:

interceptor.on('request', ({ request, requestId }) => {
  request.respondWith(
    new Response(
      JSON.stringify({
        firstName: 'John',
        lastName: 'Maverick',
      }),
      {
        status: 201,
        statusText: 'Created',
        headers: {
          'Content-Type': 'application/json',
        },
      }
    )
  )
})

We use Fetch API Response class as the middle-ground for mocked response definition. This library then coerces the response instance to the appropriate response format (e.g. to http.OutgoingMessage in the case of http.ClientRequest).

The Response class is built-in in since Node.js 18. Use a Fetch API-compatible polyfill, like node-fetch, for older versions of Node.js.`

Note that a single request can only be handled once. You may want to introduce conditional logic, like routing, in your request listener but it's generally advised to use a higher-level library like Mock Service Worker that does request matching for you.

Requests must be responded to within the same tick as the request listener. This means you cannot respond to a request using setTimeout, as this will delegate the callback to the next tick. If you wish to introduce asynchronous side-effects in the listener, consider making it an async function, awaiting any side-effects you need.

// Respond to all requests with a 500 response
// delayed by 500ms.
interceptor.on('request', async ({ request, requestId }) => {
  await sleep(500)
  request.respondWith(new Response(null, { status: 500 }))
})

Observing responses

You can use the "response" event to transparently observe any incoming responses in your Node.js process.

interceptor.on(
  'response',
  ({ response, isMockedResponse, request, requestId }) => {
    // react to the incoming response...
  }
)

Note that the isMockedResponse property will only be set to true if you resolved this request in the "request" event listener using the request.respondWith() method and providing a mocked Response instance.

WebSocket interception

You can intercept a WebSocket communication using the WebSocketInterceptor class.

Warning

In practice, WebSocket connections can use different mechanisms to work (called "transports"). At this moment, the WebSocket interceptor only supports connections established using the globalThis.WebSocket class. Supporting third-party transports is challenging because they are non-standard and specific to individual WebSocket client libraries.

import { WebSocketInterceptor } from '@mswjs/interceptors/WebSocket'

const interceptor = new WebSocketInterceptor()

Unlike the HTTP-based interceptors that share the same request/response events, the WebSocket interceptor only emits the connection event and let's you handle the incoming/outgoing events in its listener.

WebSocket connection

Whenever a WebSocket instance is constructed, the connection event is emitted on the WebSocket interceptor.

intereceptor.on('connection', ({ client }) => {
  console.log(client.url)
})

The connection event exposes the following arguments:

Name	Type	Description
client	object	An object representing a connected WebSocket client instance.
server	object	An object representing the original WebSocket server connection.

Intercepting outgoing client events

To intercept an event sent by the WebSocket client, add a message listener to the client object.

interceptor.on('connection', ({ client }) => {
  // Intercept all outgoing events from the client.
  client.addEventListener('message', (event) => {
    console.log('received:', event.data)
  })
})

Mocking incoming server events

To mock an event sent from the server to the client, call client.send() with the event payload.

interceptor.on('connection', ({ client }) => {
  // Send a mocked "MessageEvent" to the client
  // with the given "hello" string as the data.
  client.send('hello')
})

The WebSocket interceptor respects the WebSocket WHATWG standard and supports sending all supported data types (string, Blob, ArrayBuffer, etc).

Bypassing events

By default, the WebSocket interceptor prevents all the outgoing client events from hitting the production server. This is a sensible default to support mocking a WebSocket communication when a WebSocket server doesn't exist.

To bypass an event, first establish the actual server connection by calling server.connect(), and then call server.send() with the data you wish to forward to the original server.

interceptor.on('connection', ({ client, server }) => {
  // First, connect to the original server.
  server.connect()

  // Forward all outgoing client events to the original server.
  client.addEventListener('message', (event) => server.send(event.data))
})

Intercepting incoming server events

The WebSocket communication is duplex and the WebSocket interceptor allows you to intercept both outgoing (client) events and incoming (original server) events.

To intercept an incoming event from the original server, first establish the original connection by calling server.connect(), and then add a message listener to the server object.

interceptor.on('connection', ({ server }) => {
  server.connect()

  server.addEventListener('message', (event) => {
    console.log('original server sent:', event.data)
  })
})

Unlike the outgoing client events, incoming server events are automatically forwarded to the client as soon as you call server.connect(). This keeps the original server connection authentic if you ever decide to open one.

If you wish to prevent the automatic forwarding of the server events to the client, call event.preventDefault() on the incoming event you wish to prevent. This can be handy for observing as well as modifying incoming events.

interceptor.on('connection', ({ client, server }) => {
  server.connect()

  server.addEventListener('message', (event) => {
    if (event.data === 'hello from server') {
      // Never forward this event to the client.
      event.preventDefault()

      // Instead, send this mock data.
      client.send('greetings, client')
      return
    }
  })
})

Closing the connection

You can terminate the open WebSocket client connection by calling client.close().

interceptor.on('connection', ({ client }) => {
  client.close()
})

By default, this will close the connection with the 1000 code, meaning a graceful disconnect.

You can provide a custom close Status code and Close reason to the client.close() to mock different connection close scenarios.

interceptor.on('connection', ({ client }) => {
  client.close(3000, 'Close reason')
})

You can also close the connection with the termination status code (1001 - 1015), which are not configurable by the user otherwise.

interceptor.on('connection', ({ client }) => {
  // Terminate the connection because the "server"
  // cannot accept the data sent from the client.
  client.close(1003)
})

API

Interceptor

A generic class implemented by all interceptors. You do not interact with this class directly.

class Interceptor {
  // Applies the interceptor, enabling the interception of requests
  // in the current process.
  apply(): void

  // Listens to the public interceptor events.
  // For HTTP requests, these are "request' and "response" events.
  on(event, listener): void

  // Cleans up any side-effects introduced by the interceptor
  // and disables the interception of requests.
  dispose(): void
}

For public consumption, use interceptors instead.

BatchInterceptor

Applies multiple request interceptors at the same time.

import { BatchInterceptor } from '@mswjs/interceptors'
import nodeInterceptors from '@mswjs/interceptors/presets/node'

const interceptor = new BatchInterceptor({
  name: 'my-interceptor',
  interceptors: nodeInterceptors,
})

interceptor.apply()

interceptor.on('request', ({ request, requestId }) => {
  // Inspect the intercepted "request".
  // Optionally, return a mocked response.
})

Using the /presets/node interceptors preset is the recommended way to ensure all requests get intercepted, regardless of their origin.

RemoteHttpInterceptor

Enables request interception in the current process while delegating the response resolution logic to the parent process. Requires the current process to be a child process. Requires the parent process to establish a resolver by calling the createRemoteResolver function.

// child.js
import { RemoteHttpInterceptor } from '@mswjs/interceptors/RemoteHttpInterceptor'
import { ClientRequestInterceptor } from '@mswjs/interceptors/ClientRequest'

const interceptor = new RemoteHttpInterceptor({
  // Alternatively, you can use presets.
  interceptors: [new ClientRequestInterceptor()],
})

interceptor.apply()

process.on('disconnect', () => {
  interceptor.dispose()
})

You can still listen to and handle any requests in the child process via the request event listener. Keep in mind that a single request can only be responded to once.

RemoteHttpResolver

Resolves an intercepted request in the given child process. Requires for that child process to enable request interception by calling the createRemoteInterceptor function.

// parent.js
import { spawn } from 'child_process'
import { RemoteHttpResolver } from '@mswjs/interceptors/RemoteHttpInterceptor'

const appProcess = spawn('node', ['app.js'], {
  stdio: ['inherit', 'inherit', 'inherit', 'ipc'],
})

const resolver = new RemoteHttpResolver({
  process: appProcess,
})

resolver.on('request', ({ request, requestId }) => {
  // Optionally, return a mocked response
  // for a request that occurred in the "appProcess".
})

Special mention

The following libraries were used as an inspiration to write this low-level API:

• node
• nock
• mock-xmlhttprequest