A powerful service mocking, recording, and playback utility.

Notice: These docs are for mockyeah 1.x & above. For legacy mockyeah 0.x, please see


If you haven't read Getting Started - Client-Side, you can start there too.

@mockyeah/fetch can monkeypatch or be used in place of the native fetch API to enable some mockyeah features. This requires fetch API support - recommended client-side fetch polyfills are whatwg-fetch or isomorhpic-fetch.

To use, simply:

$ npm add @mockyeah/fetch


import 'isomorhpic-fetch';
import Mockyeah from '@mockyeah/fetch';

new Mockyeah();

Or to add mocks programmatically:

import 'isomorhpic-fetch';
import Mockyeah from '@mockyeah/fetch';

const mockyeah = new Mockyeah();

mockyeah.mock('https://example.local?ok=yes', {
  json: { fake: 'response' }
    url: 'https://example.local?ok=true',
    body: {
      up: 'yes'
    json: () => ({ hello: 'there' })

fetch('https://example.local?ok=yes').then(async res => {
  const data = await res.json();
  console.log('data is now { fake: "response" }', data);


See Configuration for full constructor options.

The mockyeah object has these public properties (see types for more details):

  fetch: (url: string, fetchOptions?: {}, mockyeahFetchOptions?: MockyeahFetchOptions)
    => Promise<Response>,
  mock: (match: MatchObject, options?: ResponseOptions ) => void,
  all: (match: MatchObject, option?s: ResponseOptions ) => void,
  get: (match: MatchObject, options?: ResponseOptions ) => void,
  post: (match: MatchObject, options?: ResponseOptions ) => void,
  put: (match: MatchObject, options?: ResponseOptions ) => void,
  delete: (match: MatchObject, options?: ResponseOptions ) => void,
  options: (match: MatchObject, options?: ResponseOptions ) => void,
  patch: (match: MatchObject, options?: ResponseOptions ) => void

Service Worker

@mockyeah/fetch can be configured to use a Service Worker to echo client-side mocked requests to the Network tab in your browser's Dev Tools, so that they appear like real requests, even though they're not hitting a server.

If your site doesn't have a Service Worker yet, the easiest way is use the pre-configured one we offer at @mockyeah/fetch/dist/serviceWorker.

If you're using Webpack, you could add another build config like:

module.exports = [
  // ...other configs
    entry: '@mockyeah/fetch/dist/serviceWorker',
    output: {
      filename: '__mockyeahServiceWorker.js'

Then configure in your app:

import Mockyeah from '@mockyeah/fetch';

const mockyeah = new Mockyeah({
  serviceWorker: true

...or with overrides for defaults:

import Mockyeah from '@mockyeah/fetch';

const mockyeah = new Mockyeah({
  serviceWorker: true,
  // the path where you'll be hosting the service worker:
  serviceWorkerURL: '/__mockyeahServiceWorker.js',
  // the scope of pages on your site you want it on:
  serviceWorkerScope: '/'

However, if you already have a Service Worker, @mockyeah/fetch exposes functions you can integrate. These were designed with Workbox in mind, so here's how that might look in your custom Service Worker:

import { handlerCb, matchCb } from '@mockyeah/fetch/dist/workbox';
import { registerRoute } from 'workbox-routing';

registerRoute(matchCb, handlerCb);

Note that with a custom Service Worker like the above you'll have to have your own code to register it.

Then you'll want to disable auto-registering in your mockyeah instance with serviceWorkerRegister:

import Mockyeah from '@mockyeah/fetch';

const mockyeah = new Mockyeah({
  serviceWorker: true,
  serviceWorkerRegister: false

© 2020 mockyeah, Built with Gatsby