DIY DI

class: center, middle

# DIY DI

@killtheliterate

---

# code with a side-effecting dependency

```javascript
import axios from 'axios'

// Arbitrary example of something that looks real-world-ish
export function ClientMaker (baseURL) {
  const client = axios.create({
    baseURL,
    headers: { 'A-Header-That-I-Need': 'yep' }
  })

return {
    get: endpoint => client.get(endpoint),
    post: (endpoint, payload) => client.post(endpoint, payload)
    // ...etc
  }
}
```

---

# a test that relies on network connectivity

```javascript
import { ClientMaker } from './http'

const client = ClientMaker('https://postman-echo.com')

test('can Post', () => {
  return client.post('/post', { foo: 'bar' })
    .then(response => {
      expect(response.data.data).toEqual({ foo: 'bar' })
    })
})
```

* test will fail if there is not network connection (a side-effect)
* leads to "flaky tests" (the worst)

```sh
$ npm test

FAIL  ./_broken.test.js
  ✕ can Post (24ms)

● can Post

getaddrinfo ENOTFOUND postman-echo.com postman-echo.com:443
```

---

# a solution: mocking libraries

* Sinon
* Jest dependency mocking
* Proxyquire
* etc...

power-tools. like bringing a bazooka to a butter knife fight.

---

# dependency injection with a closure

```javascript
import axios from 'axios'

// by adding a closure, we can inject axios...
const _ClientMaker = (axios) => (baseURL) {
  const client = axios.create({
    baseURL,
    headers: { 'A-Header-That-I-Need': 'yep' }
  })

return {
    get: endpoint => client.get(endpoint),
    post: (endpoint, payload) => client.post(endpoint, payload)
    // ...etc
  }
}

// ...here
export const ClientMaker = _ClientMaker(axios)

export const Test = {
  _ClientMaker
}
```

---

# a test with ez-mode dependency injection

```javascript
const Test = require('./http').Test

// create a predictable dependency
const AxiosMock = {
  create: () => ({
    post: jest.fn(/* ...truncated 'cause slides */)
  })
}

const client = Test._ClientMakerP(AxiosMock)('https://postman-echo.com')

test('can POST', () => {
  return client.post('/post', { foo: 'bar' })
    .then(response => {
      expect(response.data.data).toEqual({ foo: 'bar' })
    })
})
```

---

# that's all, folks

[https://thedrearlight.com/blog/di-for-easy.html](https://thedrearlight.com/blog/di-for-easy.html)
[https://github.com/killtheliterate/diy-di/tree/master/example](https://github.com/killtheliterate/diy-di/tree/master/example)