Injecting mock data into applications in 2019

April 30, 2019 • ☕️ 7 min read

Imagine this timeline… it’s 2019 and injecting mock data into web applications sucks. Keeping application state to be deterministic and flexible is incredibly difficult and cumbersome.

Oh wait, this is the timeline we live in 😱

Let’s take a look at that in the most shameless abrasive way possible - with me shilling a library I wrote!

How do we currently get data into our applications for development?

Some of the things that I’m sure we’ve all tried when trying to mash data into our applications:

Connect the application to production services

No. Just don’t.

If you do this, the developer police will come and take you to developer jail 🚓

In developer jail, you have to talk about “which IDE is the best” and Tabs vs. Spaces all day, every day. You don’t want that and neither do I.

This only leads to problems. You’ll accidentally mutate something on one (or all) of your users and everyone will be sad. Plus, you’ll be in jail.

Connect the application to a UAT/Staging services


If you do this, the developer police will also come and take you to developer jail 🚓

Inevitably, you’ll end up having many people connecting to these services and user data will be mutated in weird ways that make it difficult to test new features or reprocuce bugs. As with the above scenario, everyone will be sad (and in jail - which will probably make you more sad).

Connect the application to a local copy of your services

This is the first approach that won’t land you in developer jail.

However, it’s a pain to do well - and if you don’t do it well, you’ll go straight to you-know-where.

You don’t want to have to set your services up manually, so now you’ll probably need to containerise (how do you even spell that word?) things and have ways to reliably reproduce your data stores from scratch and then when things change with your application architecture and implementation etc… you’re probably going to have to do a lot of extra work to keep up with that.

Dave, all of these solutions suck so far. What else can I do?

Yes, yes they do. Let’s continue…

Run a local mock service to expose data

Okay, we’re getting somewhere.

We could write a really simple service (Node/Express servers are easy to set up and is generally familiar to frontend developers) to expose the endpoints and data that our application has. Writing a simple Node service isn’t that hard and it doesn’t put us in a spot where we can be touching actual customer data, nor are we going to be at risk of modifying something that someone else is relying on, since it’s our own personal environment.

The downside here, though, is that we now have quite a lot of code to write and maintain when all we really care about is the data that comes back in response to our requests. We also still don’t have easy ways of specifically and reliably testing happy/unhappy paths in our applications.

Dave, what about something off-the-shelf?

There are loads and loads and loads and loads and loads and loads (and loads) of solutions available to achieve exactly what we’re talking about (did I mention there are loads of them?). Let’s just look at a couple to see what they’re like.

disclaimer: I don’t want it to sound like I’m trash-talking anything that someone has put time and effort to develop. I’m merely pointing out the underlying problems with data injection that I see in web application development today.


json-server is a simple solution that allows you to specify a JSON file describing the endpoints of your application, and provides a HTTP interface to that. Boom 💥. Zero coding and easy to set up. It also has about a billion Github ⭐️’s so obviously it’s doing a lot of things right.

Here’s what a sample config looks like (shamelessly lifted from its Github page):

  "posts": [{ "id": 1, "title": "json-server", "author": "typicode" }],
  "comments": [{ "id": 1, "body": "some comment", "postId": 1 }],
  "profile": { "name": "typicode" }

Super easy to write. Super easy to understand. I’ll let you guess what is returned when you make a GET to /comments (spoiler: you’re correct).

There are some downsides to this:

  • What if I have a tonne of endpoints in my application?
    • This file becomes quite large
  • How do I easily test happy/unhappy paths? (i.e. how do I test that my POST to /login has failed?)
  • How can I simulate delay into individual responses?
    • There is a --delay flag you can pass into the process, but the delay is applied to all responses
  • Writing lots of JSON is a pain. You get little auto-complete and no typings on anything.


I wanted to give some code examples for apimocker but to come up with a self-contained example that demonstrates basic usage is a bit lengthy, so I’ll leave that up to you to check out on its Github page. Essentially, it’s a beefier version of json-server.

apimocker allows for things like:

  • Switch-responses based on URL parameters (this is a term I like to refer to as scenarios - remember this for later)
  • Splitting of responses into files
  • Global response delay, or endpoint-specific response delay

This is all very cool 😎, except that these config files become very large, very quickly and can be quite hard to read. For example, to configure more advanced switch-responses (scenarios - keep remembering this term!) you need to know JSON path, which is like XPath, only more annoying (okay, that’s subjective).

These solutions are still a bit heavy and a bit cumbersome, in my opinion. In these cases, we’re spining up a new process and having that constantly run. My dev laptop is a fairly new Macbook, but it still sounds like it’s about to launch into orbit any time I look at it.

Wouldn’t it just be nice to have some way of not writing these messy, sometimes complicated configs and save on machine resources?

Hmmm 🤔

Enter data-mocks

Here’s the part where I shamelessly plug a library I’ve written. I bet you’ve been looking forward to this!

I co-authored a library called data-mocks. It’s very rad - but of course I’d say that…

Here’s a brief overview of what it is/does:

  • Uses a code driven config to define endpoints we want to mock, and their respective responses
  • Similar to angular-multimocks, but framework agnostic
    • This means it’ll work with plain JS, Angular, React, React Native etc…
    • This library was actually the inspiration for writing data-mocks
  • Compatible with XHR and Fetch
  • Light and easy to set up
  • Scenario (he said that term again!) support

Under the hood, data-mocks intercepts HTTP requests your application makes, based on a regex performed against the URL that is being requested. No actual HTTP requests get made as they are picked up by fetch-mock for Fetch requests and xhr-mock for XHR requests. The authors of those two libraries are the real MVPs here and deserve massive amounts of respect 🏆

Let’s look at a basic React app that makes a request to an API:


import React from 'react';
import ReactDOM from 'react-dom';

import { SomeComponent } from './SomeComponent';

ReactDOM.render(<SomeComponent />, document.getElementById('app'));


import React, { Component } from 'react';

export class SomeComponent extends React.Component {
  state = {
    thing: undefined,

  componentDidMount() {
    fetch('www.davesapi.com/get-my-thing').then((thing) =>
      this.setState({ thing }),

  render() {
    return (
        <h1>Dave's cool thing</h1>
        Here's a thing: {!this.state.thing ? 'Loading...' : this.state.thing}
        {!this.state.thing === 'badThing' ? null : (
          <h1>OH NO, THAT IS A BAD THING!</h1>

When the component loads, we hit our server and we display our thing once it comes back.

Neato burrito 🌯

But what if something bad (i.e. badThing) comes back from the API? 🤔

We could:

  • Hardcode our server to respond with badThing? ❌
    • That would suck and is way too manual
  • Hardcode the response in our component? ❌
    • That would also suck and is also way too manual
  • Use data-mocks? ✅

Introducing data-mocks into an application is very simple.

All a mock is, is the following:

  • A URL matcher, which is just a regex on the endpoint name
  • A HTTP method
  • A response object
  • A response code
  • A delay, in milliseconds

That’s it. Five things to describe literally any mock. No JSON config. No having to learn a massive API or read a massive readme (although there is one available…).

All we need to do is call the injectMocks function exposed by data-mocks and pass in a list of mocks we want to use. Very neat.


import React from 'react';
import ReactDOM from 'react-dom';

import { SomeComponent } from './SomeComponent';

import { injectMocks } from 'data-mocks';

const mocks = {
  default: [
      url: /get-my-thing/,
      method: 'GET',
      response: { thing: 'this is a thing' },
      responseCode: 200,
      delay: 250,

injectMocks(mocks, 'default'); // This sets the default scenario

ReactDOM.render(<SomeComponent />, document.getElementById('app'));

Sick - we’re good to go. We can now run our app locally and we get the same behaviour as we did when we were connecting to the actual service.

But I promised you better than that - I promised you an easy way to test other paths of use in the app.


We can now talk about them! 🕺💃

A scenario is just a mapping of responses that is given a name (i.e. the scenario you are running). The default scenario is the default set of behaviour exhibited by endpoints in the application. Any non-default scenario will give you the full set of default mocks, plus all mocks defined in the scenario. If a mock has the same URL matcher in the specified scenario, it will override the response in the corresponding default mock.

In simpler terms:

A scenario is a set of API responses you want your application to give in a given scenario (yes, we’ve used the term to define the term).

Let’s see what this looks like:


import React from 'react';
import ReactDOM from 'react-dom';

import { SomeComponent } from './SomeComponent';

import { injectMocks, extractScenarioFromLocation } from 'data-mocks';

const mocks = {
  default: [
      url: /get-my-thing/,
      method: 'GET',
      response: { thing: 'thisIsAThing' },
      responseCode: 200,
      delay: 250,
  badThing: [
      url: /get-my-thing/,
      method: 'GET',
      response: { thing: 'badThing' },
      responseCode: 200,
      delay: 250,

injectMocks(mocks, extractScenarioFromLocation(window.location));

ReactDOM.render(<SomeComponent />, document.getElementById('app'));

You’re probably getting the idea here. However, two things will probably stick out:

  • “When I run this, I still get the default scenario respone. What gives?”
  • “What is this extractScenarioFromLocation(window.location) business?”

To answer both at once: we need to specify which scenario to run. The way we do this is through the URL of the page we’re on. Go on, add ?scenario=badThing to your query string (i.e. localhost:8080?scenario=badThing). If you have another way that you’d like to pass in the scenario name to the injectMocks function, feel free to not use the extractScenarioFromLocation util function.

You’ll now see that we get the unhappy path response from our mocks.

This is especially helpful when running things like Cypress tests - as we don’t need to change anything at all in our code to test different behaviours and make assertions based on what comes back from our APIs!

Other considerations

There are a couple of things to keep in mind with this approach that will make life nice and easy for you:

  • You can avoid having mocks bundled into your application by surrounding the call to injectMocks with a if (process.env === 'development') { injectMocks(...); } block.
    • Alternatively you can define a separate entry point into your application (i.e. App.dev.jsx) and point your dev build to there. This is a bit more work, though.
  • You can move your mock definitions into their own section of the project (i.e. /mocks) to avoid clutter in the entry point of your application

Summary of what this allows us to do

With all of the above, we can now:

  • Easily integrate new mocks into the application
  • Easily toggle between our new best friend, scenarios
    • This allows us to test feature/bug fixes when developing and in automated UI tests. This is the killer feature of data-mocks, in my opinion. Nothing else seems to offer something as easy (in terms of configuring and using the feature)
  • Write frontend code without having to have a single line of backend code (or a mock API server) written
  • Not have to write yet another JSON config

There you go, now we’re data-mock’ing!

If you’ve got any questions about this or want to contribute, please ping me or open a PR/Github issue 😊


The (not so witty) ramblings of an Australian developer.
Written by Dave Cooper