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https://github.com/mswjs/interceptors

Low-level network interception library.
https://github.com/mswjs/interceptors

fetch http https intercept interceptor mock node nodejs request request-interception xhr

Last synced: 3 months ago
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Low-level network interception library.

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README 

        
[![Latest version](https://img.shields.io/npm/v/@mswjs/interceptors.svg)](https://www.npmjs.com/package/@mswjs/interceptors)



# `@mswjs/interceptors`



Low-level network interception library.



This library supports intercepting the following protocols:



- HTTP (via the `http` module, `XMLHttpRequest`, or `globalThis.fetch`);

- [WebSocket](#websocket-interception) (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](https://github.com/mswjs/msw).



### How is this library different?



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



```js

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.



```js

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



```bash

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:



```js

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.



```js

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](#presets) that cover all the request sources for a given environment type.



## Presets



When using [`BatchInterceptor`](#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.



```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.



```js

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](https://developer.mozilla.org/en-US/docs/Web/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.



```js

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:



```js

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:



```js

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:



```js

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](https://github.com/mswjs/msw) 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.



```js

// 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.



```js

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.



```js

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.



```js

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.



```js

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.



```js

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](https://websockets.spec.whatwg.org/) 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.



```js

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.



```js

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.



```js

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()`.



```js

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](https://www.rfc-editor.org/rfc/rfc6455#section-7.4.1) and [Close reason](https://www.rfc-editor.org/rfc/rfc6455#section-7.1.6) to the `client.close()` to mock different connection close scenarios.



```js

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.



```js

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.



```ts

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](#interceptors) instead**.



### `BatchInterceptor`



Applies multiple request interceptors at the same time.



```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', ({ 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.



```js

// 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.



```js

// 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`](https://github.com/nodejs/node)

- [`nock`](https://github.com/nock/nock)

- [`mock-xmlhttprequest`](https://github.com/berniegp/mock-xmlhttprequest)