Introducing DeliveryGuy

I like the Fetch API. It’s supported by all modern browsers, easy to use and has some really good polyfills for older devices. But Fetch has one major flaw: it will only throw errors if there is a network problem.

That’s a really odd decision from my point of view. Nearly any HTTP library out there throws errors or rejects the promise in case of a HTTP error.

A Fetch response object has the property ok to determine if the server responded with an error, but that’s not very comfortable to use.

Since my team and I have decided to use Fetch in a Vue-based web app, I decided to create a little wrapper for much more convenience. Say hello to DeliveryGuy.


Well, surprise, it’s a Node module, so just use any package manager you like. My personal choice is yarn.

yarn install delivery-guy


import { deliverJson } from 'delivery-guy'

const getItems = async () => {
  try {
    const items = await deliverJson('/api/items')
  } catch (e) {
    console.log('HTTP Status', e.response.status)
    console.log('Response Body'. e.responseBody)

What’s going on here?

DeliveryGuy exports two main functions:

  • deliver() will return a response promise like fetch() does.
  • deliverJson() presumes your response body contains JSON. It’s basically a shortcut and returns the promise of Response.json().

Both will accept the same two parameters as fetch() does and pass them along.

If the server responds with a HTTP error, DeliveryGuy will throw an error.

Due to the inheritance limitations of built-in classes with ES5 I mentioned in my last post, it’s only possible to set custom properties of a custom error class.

DeliveryGuy provides additional two properties on an error object:

  • response has the original response object of a Fetch call.
  • responseBody contains the response body and will try to parse it as JSON. If JSON.parse fails, it will return the response body in its original state.


DeliveryGuy allows you comfortably call the Fetch API without a hassle on HTTP errors. Just use try/catch and you’re done.

Please let me know on GitHub if you have feedback, a feature request or found a bug. Thank you!

Why custom errors in JavaScript with Babel are broken

Have you ever tried to write a custom error class in JavaScript? Well, it does work to a certain extend. But if you want to add custom methods or call instanceof to determine the error type it will not work properly.

Here is a little example of a custom error class:

class MyError extends Error {
  constructor(foo = 'bar', ...params) {
    if (Error.captureStackTrace) {
      Error.captureStackTrace(this, MyError)
    this.foo = foo
  getFoo() {
    return this.foo

try {
  throw new MyError('myBar')
} catch(e) {
  console.log(e instanceof MyError) // -> false
  console.log(e.getFoo()) // -> Uncaught TypeError: e.getFoo is not a function

Works fine in any browser with ES6/ES2015 support, but if you transpile the example with Babel to ES5 and execute the code, you will get the results shown in the comments.


Due to limitations of ES5 it’s not possible to inherit from built-in classes like Error, see the Babel docs.

Possible solution

The docs mention a plug-in called babel-plugin-transform-builtin-extend to resolve this issue, but if you have to support older browsers it may not help. In order to work the plug-in needs support for __proto__. Take a guess which browser does not support __proto__ … and of course, it’s the web developers best friend aka Internet Explorer. Thankfully it affects only version 10 and below.


If it’s not feasable to use the plug-in, you can at least access properties set in the constructor. A call to e.foo in the example is possible, but e instanceof MyError will return false, since you will always get an instance of Error.


Nothing of this ideal. We have to wait until it’s possible to use ES6/ES2015 directly. Yes, we all could set our transpile targets to ES6/ES2015 today, but our clients usually won’t allow it. Some customer is always browsing the web with an ancient device/browser.

Awesome tests with Vue and Jest

Jest is a very neat JavaScript testing library from Facebook. It’s mostly syntax-compatible with Jasmine and needs zero or very less configuration. Code coverage reports are there out-of-the-box and with sandboxed tests and snapshot testing it has some unique features.

Set-up Jest


You are using Vue CLI? Consider yourself lucky, the set-up of Jest could not be simpler:

yarn add --dev jest @vue/cli-plugin-unit-jest
vue invoke unit-jest

Vue CLI will do the rest and also create an example spec for the HelloWorld component.


Install all necessary dependencies:

yarn add --dev @vue/test-utils babel-jest jest jest-serializer-vue vue-jest

Create jest.config.js in your project root directory:

module.exports = {
  moduleFileExtensions: ['js', 'json', 'vue'],
  transform: {
    '^.+\\.vue$': 'vue-jest',
    '^.+\\.js?$': 'babel-jest'
  moduleNameMapper: {
    '^@/(.*)$': '<rootDir>/src/$1'
  snapshotSerializers: ['jest-serializer-vue'],
  testMatch: ['<rootDir>/src/tests/**/*.spec.js']

Please adjust the paths in moduleNameMapper and testMatch to your project.

You should also add a modern JavaScript preset to your .babelrc file:

  "presets": ["es2015"]

Optional step

Add the following line to your .gitignore file:


The set-up is now complete. Let’s write a test file!

Writing tests

Here is a litte example Vue component:

  <ul class="demo">
    <li class="demo__item" v-for="value in values" :key="value">{{ value }}</li>

export default {
  name: 'demo',
  data () {
    return {
      values: []
  created() {
  methods: {
    async fetchValues() {
      const response = await fetch('/api/demo/values')
      this.values = await response.json()

The component Demo will fetch some values after its creation and display them in an unordered list. This is example is quite simple, but testing is a little more complex due to the usage of fetch, async and await.

Of course, there are some tools to help us:

yarn add --dev fetch-mock flush-promises

Now, let’s write a test:

import { shallow } from '@vue/test-utils'
import fetchMock from 'fetch-mock'
import flushPromises from 'flush-promises'
import Demo from '@/components/Demo.vue'

const values = [

describe('Demo.vue', () => {
  beforeEach(() => {
    fetchMock.get('/api/demo/values', values)

  it('renders component', async () => {
    const wrapper = shallow(Demo)
    await flushPromises()


  afterEach(() => {

What’s going on?

  1. shallow from Vue Test Utils creates a wrapper of the rendered and mounted component, any child components will be stubs. If you need child components in your test, please use mount instead of shallow.

  2. fetchMock will create a mocked version of the Fetch API. In this case it will return the defined values for a GET request to /api/demo/values. If you send a request that’s not defined in fetchMock, it will throw an exception and break your tests.

  3. The test itself is defined as async to use await for flushPromises(). It will wait until the mocked request is finished and the values are stored in the component’s data.

  4. You can now access the data property values and compare the content to the response of the mocked HTTP request.


Setting up Jest for Vue is easy, even if you have to do it manually.

A little warning: setting up Jest for an existing app can be tedious. The current AngularJS app of our customer can’t be tested with Jest, at least for now. The AngularJS HTTP mock does not work and I haven’t figured out the problem yet.

But enough of Angular: the real deal comes with the testing itself. Async/await is a nice and simple way for testing asynchronous behaviour. I don’t think this could be easier and it’s a reliable method with the power of modern JavaScript. Try to imagine what the demo test would look like in ES5 …

Vue Loader Setup in Webpack

Since it’s no option to use Vue CLI in my current project, I had to manually add vue-loader to the Webpack config. Well, I was positively surprised how simple it was. Especially compared to a manual set-up of Angular 2 shortly after its launch in late 2016, while Angular CLI was an alpha version and buggy as hell.

Shall we begin?


Let’s add the necessary modules to the package:

yarn add --dev vue-loader vue-template-compiler
yarn add vue

If you want to use a template engine like Pug or prefer TypeScript over JavaScript, you can add the respective Webpack loader package, pug-loader for example. Webpack will also tell you in detail, if modules are missing.


Just add the following rule to your Webpack config:

  test: /\.vue$/,
  use: [
      loader: 'vue-loader'

Webpack will now be able to use import statements with Vue single file components.

If you want to have a separate JavaScript file with your Vue application, you can add a new entry point:

  entry: {,
    'current-application': [
      path.resolve(__dirname, 'js/current-application.js')
    'vue-application': [
      path.resolve(__dirname, 'js/vue-application.js')

Not fancy enough? How about vendor chunks?

  optimization: {
    splitChunks: {
      chunks: 'all',
      cacheGroups: {
        vendor: {
          chunks: 'initial',
          test: /node_modules\/(?!(vue|vue-resource)\/).*/,
          name: 'vendor',
          enforce: true
        'vue-vendor': {
          chunks: 'initial',
          test: /node_modules\/(vue|vue-resource)\/.*/,
          name: 'vue-vendor',
          enforce: true

This will separate your vendor files from node_modules into vendor.js and vue-vendor.js. The property test contains a regex to determine which modules should go into the vendor chunks.

Of course, this comes not even close to what Vue CLI can do. I highly recommend to use Vue CLI when it’s feasible. It’s quite easy to configure while not being dumbed down, has an excellent documentation and is very well maintained. At least for our customer’s set-up it would be difficult to use Vue CLI at the moment, but we are eager to migrate to Vue CLI asap.

Pimp Your Visual Studio Code

Like VS Code? Yeah, me too. Here are some very useful extensions and optical enhancements to get an even better experience with VS Code.


Settings Sync

I love my Mac, but sometimes I have to do stuff (besides gaming) on my Windows PC as well. So, if I want to use the same config on both devices, I have to install my extensions and copy my config back and forth. That sucks. Settings Sync to the rescue! It syncs your settings, extensions etc. through the power of Gists on GitHub.



It’s quite simple: just bookmark certain lines of code you need often.


EditorConfig for VS Code

Share your coding styles between editors, IDEs etc. on a per project basis. It’s really nice for teams as well.


Bracket Pair Colorizer

Bracket madness? No more!


Markdown All in One

Do you write a lot of Markdown? Here are some really useful helpers.




If you like darker and bluish color themes, give Cobalt2 a try. The color choices with a contrast between yellow and blue can reduce strain on your eyes.


Material Icon Theme

Matches nicely with Cobalt2 or other darker themes. There are a ton of icons for almost any file type and even for many special folders like node_modules etc.


But that’s not the end. VS Code can do much more, you can find some other very useful tips and tricks on vscodecandothat.com.

Dear GitHub

Look, I want to like Atom, but there are too many caveats. Atom is slow and resource hungry as hell, which is especially bad on a mobile device. It literally eats my MacBook’s battery away. VS Code is not perfect either, but the general experience is much better. So, please, get your shit together and improve Atom. We need more awesome editors on the market!