Mocking? Why When & How to Mock in your Tests

## Why?

**_Situation**_: You want to use a service (_API_) made by someone else in your project, but you don't want to call that API inside your tests because the network is slow/unreliable or simply because the service requires multi-step (_user-input_) authentication (e.g: OAuth2) and you don't want to go through that in your tests.
## What?

In unit testing a **_Mock**_ is used to _simulate_ an object or response (_from an API_) in a controlled way.
## How?

Lets say you are building an app called "**_BroLee?**_" that gives people recommendations on when they should carry an umbrella to wear based on the weather conditions in their location.
### Example:

``` js

```

**_nock**_ is made by @pgte and is the **_gold standard**_ in simplified libraries.
## Notes
- Sinon.js is the Grandaddy of Mocking in JavaScript: http://sinonjs.org (_I've tried to teach it to complete beginners and most found it confusing, stick to nock and keep it simple_)
- The wikipedia article is quite good: https://en.wikipedia.org/wiki/Mock_object

No file or test is identified in the issue. Review the existing book structure and the draft’s mocking section first, then determine the expected scope for a useful JavaScript example; done should be a complete, beginner-oriented explanation of why and how to mock, with a working example and accurate references.

Written by the indexing model from the issue text.