Mock Service Worker
  1. Api
  2. response


Response (res) is a function that composes a mocked response object.

Unlike the Response object in NodeJS, the res argument in Mock Service Worker is used to create and transform a mocked response. When called, it passes a created response from one response transformer to another, eventually resolving to a mocked response used by the Service Worker.

Mocked response

A call to the res() function creates a mocked response object. Each response transformer accepted by the res() function modifies that mocked response and passes it to the next response transformer, thus, creating a functional composition.


Property nameTypeDescription
statusnumberMocked response HTTP status code.
statusTextstringHTTP status text of the current status code.
bodystringStringified body of the response.
headersHeadersMocked response HTTP headers.
delaynumberDelay duration (ms) before responding with a mocked response.


Standard usage

1rest.get('/users', (req, res, ctx) => {
2 return res(
3 // This response transformer sets a custom status text on the response.
4 ctx.status(301),
5 // While this one appends a "Content-Type" response header.
6 ctx.set('Content-Type', 'application/json'),
7 )

Raw usage

The res() call can accept a function that modifies and returns a created mocked response instance. It's recommended, however, to use standard response transformers for faster reference and reliability.

1rest.get('/users', (req, res) => {
2 return res((res) => {
3 res.status = 301
4 res.headers.set('Content-Type', 'application/json')
5 return res
6 })

Custom response composition

Usually the res function is available as an argument of a request handler. However, in some cases (like building a custom response composition) it's useful to import the res function directly, outside of the request handler's scope.

You can do that by importing the response function from the msw package:

1// src/mocks/res.js
2import { response, context } from 'msw'
4export function res(...transformers) {
5 // A custom response composition chain that embeds
6 // a random realistic server response delay to each `res()` call.
7 return response(...transformers, context.delay())

Such custom response composition can later be used when creating a mocked response in a request handler:

1// src/mocks/handlers.js
2import { setupWorker, rest } from 'msw'
3import { res } from './res'
5const worker = setupWorker(
6 rest.get('/user', (req, _, ctx) => {
7 // Notice how this response resolver is using a custom `res` function
8 // imported at the top of this module.
9 return res(ctx.json({ firstName: 'John', lastName: 'Maverick' }))
10 }),