@mswjs/interceptors

Low-level HTTP/HTTPS/XHR/fetch request interception library.

Intercepts any requests issued by:

• http.get/http.request
• https.get/https.request
• XMLHttpRequest
• fetch
• Any third-party libraries that use the modules above (i.e. request, node-fetch, etc.)

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?

As interception is often combined with request route matching, some libraries can determine whether a request should be mocked before it actually happens. This approach is not suitable for this library, as it rather intercepts all requests and then let's you decide which ones should be mocked. This affects the level at which interception happens, and also the way mocked/original responses are constructed, in comparison to other solutions.

What this library does

This library monkey-patches the following native modules:

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

Once patched, it provisions the interception of requests and normalizes them to something called isomorphic request instances. That normalization ensures the same request handling for the consumer of the library, while requests originating from different modules may differ internally.

In its mocking phase, this library accepts an isomorphic response instance that describes a module-agnostic mocked response. This allows you to respond to requests issued by different modules using the same response instance.

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

API

createInterceptor(options: CreateInterceptorOptions)

Enables request interception in the current process.

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

const interceptor = createInterceptor({
  modules: nodeInterceptors,
  resolver(request, ref) {
    // 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.

createRemoteInterceptor(options: CreateRemoteInterceptorOptions)

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.

import { createRemoteInterceptor } from '@mswjs/interceptors'

const interceptor = createRemoteInterceptor({
  modules: nodeInterceptors,
})

interceptor.apply()

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

createRemoteResolver(options: CreateRemoteResolverOptions)

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

import { spawn } from 'child_process'
import { createRemoteResolver } from '@mswjs/interceptors'

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

createRemoteResolver({
  process: appProcess,
  resolver(request) {
    // Optionally, return a mocked response
    // for a request that occurred in the "appProcess".
  },
})

Interceptors

This library utilizes a concept of interceptors–functions that patch necessary modules, handle mocked responses, and restore patched modules.

List of interceptors:

• /interceptors/ClientRequest
• /interceptors/XMLHttpRequest
• /interceptors/fetch

To use a single, or multiple interceptors, import and provide them to the RequestInterceptor constructor.

import { createInterceptor } from '@mswjs/interceptors'
import { interceptXMLHttpRequest } from '@mswjs/interceptors/lib/interceptors/XMLHttpRequest'

// This `interceptor` instance would handle only XMLHttpRequest,
// ignoring requests issued via `http`/`https` modules.
const interceptor = new createInterceptor({
  modules: [interceptXMLHttpRequest],
})

Interceptors are crucial in leveraging environment-specific module overrides. Certain environments (i.e. React Native) do not have access to native Node.js modules (like http). Importing such modules raises an exception, and must be avoided.

Methods

.apply(): void

Applies module patches and enables interception of the requests.

interceptor.apply()

.on(event, listener): boolean

Adds an event listener to one of the following supported events:

• request, whenever a new request happens.
• response, whenever a request library responds to a request.

interceptor.on('request', (request) => {
  console.log('[%s] %s', request.method, request.url.toString())
})

.restore(): void

Restores all patched modules and stops intercepting future requests.

interceptor.restore()

Special mention

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

• node
• nock
• mock-xmlhttprequest