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:white_check_mark: A brief introduction to Test Driven Development (TDD) in JavaScript (Complete Beginner's Step-by-Step Tutorial)

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최종 버전 다운로드 (.zip)

- - ci.yml
- dependabot.yml
- anatomy-of-a-unit-test.png
- build-passing-summary.png
- build-passing.png
- enable-travis-ci.png
- first-failing-test.png
- first-test-passes.png
- gbp-coins.jpg
- initial-index-html-showing-failing-test.png
- jsdoc-html.png
- server-side-command-line-test-run-with-istanbul-100-percent-coverage.png
- server-side-command-line-test-run-with-istanbul.png
- server-side-test-istanbul-coverage-highlights-rogue-code.png
- server-side-test-istanbul-coverage-report.png
- showing-all-passing-tests.png
- showing-coverage.png
- showing-rogue-code-not-covered.png
- showing-rogue-code-on-one-line-goes-un-detected.png
.gitignore
change-functional.js
change.js
CONTRIBUTING.md
index.html
LICENSE
package-lock.json
package.json
README.md
test.js

# 설치 가이드

1. 코드 내려받기

git clone https://github.com/dwyl/learn-tdd

깃허브에서 프로젝트 코드 전체를 내 컴퓨터로 내려받습니다.

cd learn-tdd

방금 내려받은 프로젝트 폴더 안으로 이동합니다.

2. Node.js

쉬움 추천

사전 준비물

Git GitHub에서 프로젝트 코드를 내려받으려면 필요합니다.
Node.js npm 명령어를 쓰려면 Node.js가 설치되어 있어야 합니다. LTS(장기지원) 버전을 추천합니다.

npm init -f && npm install live-server --save-dev && node_modules/.bin/live-server --port=8000

package.json에 명시된 라이브러리들을 내려받아 설치합니다.

✅ 명령어 실행 후 터미널에 나타나는 주소(보통 http://localhost:3000 형태)를 브라우저에서 열어보세요.

이 레포의 README에 적힌 실제 명령어를 그대로 가져왔습니다.

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# Learn **T**est **D**riven **D**evelopment (**TDD**)

A brief introduction to **T**est **D**riven **D**evelopment (**TDD**)
in JavaScript for people who want to write _**more reliable code**_.

[![GitHub Workflow Status](https://img.shields.io/github/actions/workflow/status/dwyl/learn-tdd/ci.yml?label=build&style=flat-square&branch=main)](https://github.com/dwyl/learn-tdd/actions/workflows/ci.yml)
[![codecov.io](https://img.shields.io/codecov/c/github/dwyl/learn-tdd/master.svg?style=flat-square)](https://codecov.io/github/dwyl/learn-tdd?branch=master)
[![Dependencies: None](https://img.shields.io/badge/dependencies-none-brightgreen.svg?style=flat-square)](https://github.com/dwyl/learn-tdd/blob/main/package.json#L35 "Zero Dependencies")
[![contributions welcome](https://img.shields.io/badge/contributions-welcome-brightgreen.svg?style=flat-square)](https://github.com/dwyl/learn-tdd/issues)
[![HitCount](https://hits.dwyl.com/dwyl/learn-tdd.svg)](https://hits.dwyl.com/dwyl/learn-tdd)

</div>

## Why?

_Project(s) without tests_ often end up looking like they are stuck together with _**duct tape**_ ...

![duct tape car fail](https://i.imgur.com/9cNriGK.jpg)

Change _one_ part and _another_ stops working? "_Fixing_" one bug, creates another?

Wouldn't you *prefer* it if everything was
***consistent*** and beautifully integrated? <br />
What if _everyone_ on your team worked _like **clock-work**_ in a disciplined order... like a _**Formula 1 Crew**_ ...

![formula 1 pit stop](https://i.imgur.com/0NDbaam.jpg)

Test Driven Development (TDD) makes your team a well-oiled machine which means you can go _**faster**_.

Once you have a ***suite*** of tests that run on every change, you will
begin to develop a whole other level of ***confidence*** in your codebase
and will discover a new freedom to be ***creative*** without fear of
"*breaking*" anything unexpectedly; truly *game-changing*.

## What?

This tutorial will help you get started with
**T**est **D**riven **D**evelopment (**TDD**) *today*! <br />
In the next ***30 minutes*** you will learn _everything_<sup>1</sup>
you need to write tests for your web project!

### Pre-Requisites

+ **A computer** with a web browser
+ **Internet access** to download the starter files
+ **30 minutes** of your time
+ **_Basic_ Programming Skills** (HTML & JavaScript)
+ (_**Optional**_) _**Bonus Levels requires**_ you to
[_install_ **Node.js**](https://nodejs.org/download/)

### What is Software Testing?
> Software testing is the process of evaluating a software item to detect differences between the expected output and the actual output. Testing assesses the quality of the product. Software testing is a process that should be done during the development process. In other words software testing is a verification and validation process.

### What is TDD?

> Test-driven development (TDD) is an evolutionary approach to development
which combines test-first development, where you write a test before you write
just enough production code to fulfil that test, and refactoring. In other words,
it’s one way to think through your requirements
or design before you write your functional code.

*From [Introduction to Test Driven Development (TDD)](https://agiledata.org/essays/tdd.html)*

#### Further resources
- Software Testing - https://en.wikipedia.org/wiki/Software_testing
- "What is Software Testing" video (from 5:56 onwards) - https://youtu.be/UZy1Dj9JIg4?t=356
- Video intro to Software Development Lifecycle (from 0:52 onwards): https://youtu.be/qMkV_TDdDeA?t=52
- How to Write Clean, Testable Code -  https://youtu.be/XcT4yYu_TTs (ignore the Java code focus on the general principles)
+ [What is software testing?](https://www.codeproject.com/Tips/351122/What-is-software-testing-What-are-the-different-ty) by _Rehman Zafar_

## How?

The *first* thing you need to *understand*
is that writing code following TDD (*discipline*)
is a (*slightly*) different approach from simply
diving into solving the problem (*without a test*).

When reading about TDD you will usually see the expression:
"***Red, Green, Refactor***":

![TDD Cycle: Red, Green, Refactor](https://i.imgur.com/RQe2NQT.jpg)

What this means is that TDD follows a **3-step process**:

1. ***Write a Failing Test*** - Understand the (user)
requirements/story well enough to write a test for what you expect.
(_the test should **fail** initially - hence it being "Red"_)

2. ***Make the (failing) Test Pass*** - Write (*only*) the code you need
to make the (*failing*) test pass, while ensuring your existing/previous tests
all still pass (*no regressions*).

3. ***Refactor the code you wrote*** take the time to tidy up the code
*you* wrote to make it simpler
(*for your future self or colleagues to understand*)
before you need to ship the current feature, do it.

> Thankfully, because you will have good tests,
you don't _need_ to do any refactoring up-front,
you can always do refactoring _later_
if performance bottlenecks are discovered.
Most programming languages have very efficient compilers/interpreters
that remove much of the need for refactoring.
And if you use a linter your code will be naturally "tidy".

To develop the *habit(s)* you will need to be successful with TDD
(*and software engineering in general*)
we need to ***write*** a ***test first*** (*and watch it fail*)
and *then* write the code required to make the test pass.

Writing a _**failing test**_,
before writing the code may seem *counter-intuitive*,
*time consuming* or even "*tedious*" at _**first**_.
But we _urge_ you to think of it this way:

> The ***test*** is the ***question*** you are asking <br />
> your code is the ***answer*** to the question. <br />
> By having a _clear_ question, you can always check
> that your code is working, <br />
> because it _**consistently**_
> gives you the same answer(s) ... _no surprises_,
even when you're working with a large, inter-dependent code base!

## Practical

> _**Note**: This tutorial is meant to be a beginner-friendly introduction to TDD.
The Vending Machine example is _intentionally_ simple
so you can focus on the principles of testing.
Once you understand the basics,
we encourage you to follow our _complete_ Todo List Tutorial 
([https://github.com/dwyl/**todo-list-javascript-tutorial**](https://github.com/dwyl/todo-list-javascript-tutorial)),
which is a step-by-step guide to building an App
following testing and documentation-first best practices._

### Scenario: Vending Machine _Change Calculator_ Micro-Project

![vending machine](https://i.imgur.com/HbwMqDa.jpg)

Imagine you are building a **Vending Machine**
that allows people to buy any item it contains.
The machine accepts coins and calculates the change
to be returned to the customer, given the item **price**
and the **cash** received.

### Single *File* App

We can build the _entire_ "project" in a _**single file**_: `index.html`

> _**Note**: In practice you want to split your JavaScript,
CSS and HTML (Templates) into **separate** files,
but for this example we are keeping everything in `index.html` for simplicity.
If you make it to the "Bonus Levels" you will split things out!_

Create a directory on your computer called **vending-machine**:

In your **terminal** type this command:
```sh
mkdir vending-machine && cd vending-machine
```
(_This will create the directory and move you into it_)

Next create a file called **index.html** e.g: `atom index.html`
(which creates and opens the file in the [Atom text editor](https://atom.io/)
if you have it installed)

(_**Note**: The "atom" command is not installed by default.
In the Atom menu bar there is a command named “Install Shell Commands”
which installs a new command in your Terminal called "atom"._)

Now copy-paste the following *sample code* into the newly created `index.html` file to get started:

```html
<html>
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>Vending Machine Change Calculator TDD Tutorial</title>
    
    <link rel="stylesheet" href="https://code.jquery.com/qunit/qunit-1.18.0.css">
    
    <link rel="stylesheet" href="http://yui.yahooapis.com/pure/0.6.0/pure-min.css">
  </head>
  <body style='margin: 0 1em;'>
    <div id='main'>
      <h1>Vending Machine <em>Change Calculator</em></h1>
      <h2>Calculate the change (<em>coins</em>) to return to a customer when they buy something.</h2>
    </div>

<div id="qunit"></div>
    <div id="qunit-fixture"></div>
    
    <script src="https://code.jquery.com/qunit/qunit-1.18.0.js"></script>
    <script>
      // This is what a simple unit test looks like:
      test('This sample test should always pass!', function(assert) {
        var result = 1 + 1;
        assert.equal(result, 2); // just so we know everything loaded ok
      });
      // A failing test will be RED:
      test('This is what a failing test looks like!', function(assert) {
        var result = [1,2,3].indexOf(1);  // this should be 0
        assert.equal(result, -1); // we *expect* this to fail
      });
    </script>
  </body>
</html>
```

#### Open index.html in your Browser

When you ***open*** `index.html` in your ***web browser***
you should expect to see something like this: (_without the annotation pointing out the qunit div, and the green and red annotations pointing out the Passing and Failing tests_)

![learn-tdd-initial-index-html-showing-failing-test](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/initial-index-html-showing-failing-test.png)

##### Explanation

There is quite a lot of code in the **index.html** you just created,
let's step through it to understand the parts:

The first part of **index.html** is a standard HTML head and body:

```html
<html>
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>Vending Machine Change Calculator TDD</title>
    
    <link rel="stylesheet" href="https://code.jquery.com/qunit/qunit-1.18.0.css">
    
    <link rel="stylesheet" href="http://yui.yahooapis.com/pure/0.6.0/pure-min.css">
  </head>
  <body>
    <div id='main'>
      <h1>Vending Machine Change Calculator</h1>
      <h2>Calculate the Change for a Given Price and Cash Received</h2>
    </div>
```
Nothing special here, we are simply setting up the page
and loading the CSS files.

Next we see the **qunit divs**
(_where the **test results** will be **displayed**_)
and load the JQuery and QUnit Libraries from CDN:

```html
    <div id="qunit"></div>
    <div id="qunit-fixture"></div>
    
    <script src="https://code.jquery.com/qunit/qunit-1.18.0.js"></script>
```

Finally we see our test(s) - the interesting part of the file:

```html
    <script>
      // This is what a simple unit test looks like:
      test('This sample test should always pass!', function(assert) {
        var result = 1 + 1;
        assert.equal(result, 2);
      });

// A failing test will be RED:
      test('This is what a failing test looks like!', function(assert) {
        var result = [1,2,3].indexOf(1);  // this should be 0
        assert.equal(result, -1); // we *expect* this to fail
      });

</script>
  </body>
</html>
```

If you are new to writing ***automated tests***, don't worry - 
they are really simple. There are **3 parts**:

1. **Description** - usually the *first* parameter to QUnit's test() method, describing what is expected to happen in the test
2. **Computation** - executes a function/method
(*which invokes the method you will write to make your test pass*)
3. **Assertion** - verifies that the result of your computation
is what you ***expect*** it to be.

![anatomy-of-a-unit-test](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/anatomy-of-a-unit-test.png)

In the above screenshot, the assertion is `assert.equal(result, 2)` <br />
We are giving the `equal` method two arguments; the `result` of our computation
and our expected value - in this case **2**. _That's it_.

_**Note**_:
The latest version of QUnit uses the `QUnit.test()` function to run tests.
Later in this workshop we use [blanket.js](https://blanketjs.org/)
which is not compatible with the latest
version of QUnit. It is for this reason
that we are calling `test()` to run the tests in this workshop.

##### Further Reading:

+ Test assertion: https://en.wikipedia.org/wiki/Test_assertion
+ What are Test Assertions and how do they work?:
https://www.thoughtworks.com/insights/blog/test-assertions-how-do-they-work

## Requirements

As a customer, I want to buy a selected item from the **vending machine**
and see what my change is as a **result** into the various **coins**
so that I can select one of the options and receive my change.

Acceptance criteria:
- A successful call of a function `getChange` should return
the change value in the various **coins** available
- Unit Tests should exist when the function is ready
- The selection of the desired return is out of scope

##### Complementary User Story view
> Given a **Price** and an amount of **Cash** from the Customer
> Return: **Change** to the customer (*in notes and coins*).

### Understand what is needed

+ Create a `function` called `getChange` that accepts _**two parameters**_:
`totalPayable` and `cashPaid`
+ For a given `totalPayable`
(the total amount an item in the vending machine costs)
and `cashPaid` (the amount of cash the customer paid into the vending machine),
`getChange` should _**calculate**_ the _**change**_
the machine should _return_ to the customer
+ `getChange` should _**return**_ change as an `array` of coins (largest to smallest)
that the vending machine will need to _dispense_ to the customer.

#### _Example_

If a customer buys an item costing £2.15
(_we represent this as **215 pennies**_ `totalPayable`)
and pays £3 (3 x £1 or _**300 pennies**_ `cashPaid`)
into the vending machine, the _**change**_ will be **85p**. <br />
To dispense the 85p of change we should _return_
**four coins** to the person: 50p, 20p, 10p and 5p. <br />
An **array** of these coins would look like: `[50, 20, 10, 5]`

#### Coins

In the UK we have the following Coins:

![GBP Coins](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/gbp-coins.jpg "GBP Coins")

If we use the penny as the unit (i.e. 100 pennies in a pound)
the coins can be represented as:

-  200 (£2)
-  100 (£1)
-   50 (50p)
-   20 (20p)
-   10 (10p)
-    5 (5p)
-    2 (2p)
-    1 (1p)

this can be stored as an Array:

```javascript
var coins = [200, 100, 50, 20, 10, 5, 2, 1];
```

_**Note**_: The same can be done for any other cash system ($ ¥ €)
simply use the cent, sen or rin as the unit and scale up notes.

#### The First Test

If you are *totally* new to TDD I recommend reading this
[introductory article](https://www.agiledata.org/essays/tdd.html) by Scott Ambler
(especially the diagrams) otherwise this (test-fail-code-pass) process
may seem *strange* ...

In **T**est **F**irst **D**evelopment (TFD) we write a test *first* and *then*
write the code that makes the test pass.

#### First Requirement

So, back in our **index.html** file ***remove the dummy tests***
and add the following lines:

```js
test('getChange(1,1) should equal [] - an empty array', function(assert) {
  var result = getChange(1, 1); //no change/coins just an empty array
  var expected = [];
  assert.deepEqual(result, expected);
}); // use deepEqual for arrays see: https://api.qunitjs.com/deepEqual/
```
We use QUnit's `deepEqual` (_assert_) method to check that all the _elements_
in the two arrays are _**identical**_. see: https://api.qunitjs.com/deepEqual/

At this point, your `index.html` file should look like this:

```html
<html>
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>Vending Machine Change Calculator TDD</title>
    <link rel="stylesheet" href="https://code.jquery.com/qunit/qunit-1.18.0.css">
    <link rel="stylesheet" href="http://yui.yahooapis.com/pure/0.6.0/pure-min.css">
  </head>
  <body>
    <div id='main' style='padding: 2em;'>
      <h1>Vending Machine Change Calculator</h1>
      <h2>Calculate the Change for a Given Price and Cash Received</h2>
    </div>

</script>
  </body>
</html>
```

#### Watch it _Fail_

Back in your browser window, _refresh_ the browser and watch it *fail*:

![first failing test](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/first-failing-test.png)

> **Q**: Why *deliberately* write a test we *know* is going to *fail*...? <br />
> **A**: To get used to the idea of *only* writing the code required to *pass*
>    the *current* (*failing*) *test*. <br />
> *Read*: "***The Importance of Test Failure***:
https://www.sustainabletdd.com/2012/03/importance-of-test-failure.html<br />
**Note**: This also proves the test **will** fail if the code doesn't behave as expected.

#### Create the getChange `function`

In your `index.html` file add the following code (*above the tests*)

```js
<script>
function getChange (totalPayable, cashPaid) {
    var change = [];
    // your code goes here

return change;
};
</script>
```

Your `index.html` should now look something like this:

var change = [];

return change
    };
    </script>

</script>
  </body>
</html>
```

#### Refresh `index.html` in the Browser

![first test passes](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/first-test-passes.png)

It Passed!!

#### Now Let's Write A *Real* Test

Going back to the requirements, we need our `getChange` method to accept
two arguments/parameters (`totalPayable` and `cashPaid`), and to `return` an
`array` containing the coins equal to the difference between them:

e.g:
```js
totalPayable = 215          // £2.15
cashPaid     = 300          // £3.00
difference   =  85          // 85p
change       = [50,20,10,5] // 50p, 20p, 10p, 5p
```

Add the following test to tests section of `index.html`:

```javascript
test('getChange(215, 300) should return [50, 20, 10, 5]', function(assert) {
  var result = getChange(215, 300); // expect an array containing [50,20,10,5]
  var expected = [50, 20, 10, 5];
  assert.deepEqual(result, expected);
})
```

#### Write the Method to Pass the Test

What if I _**cheat**_ and make `getChange` return the expected result?

```javascript
function getChange (totalPayable, cashPaid) {
  'use strict';

var change = [50, 20, 10, 5]; // just "enough to pass the failing test"

return change;
};
```

This will _pass_ the new test, but it also introduces a regression. The original
test `getChange(1,1) should equal [] - an empty array` is now failing.

Step 2 of the **TDD** process requires that *all* tests should pass, not just the
newly added one.

The `getChange` function needs to cater for two scenarios; when change should be returned and when it shouldn't. A new implementation of `getChange` that
handles both scenarios could be:

```javascript
function getChange (totalPayable, cashPaid) {
  'use strict';

var change = [];

if((cashPaid - totalPayable) != 0) { // Is any change required?
    change = [50, 20, 10, 5]; // just "enough to pass the failing test"
  }

return change;
};
```

The regression has been fixed and all tests _pass_, but you have *hard coded*
the result (*not exactly useful for a calculator...*)

This only works *once*. When the Spec (Test) Writer writes the next test,
the method will need to be re-written to satisfy it.

Let's try it.  Work out what you expect so you can write your test:
```js
totalPayable = 486           // £4.86
cashPaid     = 600           // £6.00
difference   = 114           // £1.14
change       = [100,10,2,2]  // £1, 10p, 2p, 2p
```

Add the following test to `index.html` and refresh your browser:

```javascript
test('getChange(486, 600) should equal [100, 10, 2, 2]', function(assert) {
  var result = getChange(486, 600);
  var expected = [100, 10, 2, 2];
  assert.deepEqual(result, expected);
})
```

#### Should We _Keep Cheating or Solve the Problem_?

We could _keep cheating_ by writing a series of if statements:

```javascript
function getChange (totalPayable, cashPaid) {
  'use strict';

var change = [];

if((cashPaid - totalPayable) != 0) { // Is any change required?
    if(totalPayable == 486 && cashPaid == 600)
        change = [100, 10, 2, 2];
    else if(totalPayable == 215 && cashPaid == 300)
        change = [50, 20, 10, 5];
  }

return change;
};
```
The _**Arthur Andersen Approach**_ gets results in the *short run* ...

But it's arguably *more work* than simply *solving* the problem.
So let's do that instead.

# Try It Yourself (_before looking at the solution_!)

> Try to create your own `getChange` method that passes the three tests
> _before_ you look at the solution...

To re-cap, these are our three tests:
```js
test('getChange(1,1) should equal [] - an empty array', function(assert) {
  var result = getChange(1, 1); //no change/coins just an empty array
  var expected = [];
  assert.deepEqual(result, expected);
});

test('getChange(215, 300) should return [50, 20, 10, 5]', function(assert) {
  var result = getChange(215, 300); // expect an array containing [50,20,10,5]
  var expected = [50, 20, 10, 5];
  assert.deepEqual(result, expected);
});

test('getChange(486, 600) should equal [100, 10, 2, 2]', function(assert) {
  var result = getChange(486, 600);
  var expected = [100, 10, 2, 2];
  assert.deepEqual(result, expected);
});
```

#### One More Test to be _Sure_ it Works?

Let's invent a test that will return one of each of the coins ...

Recall that we have 8 types of coins:

```javascript
var coins = [200, 100, 50, 20, 10, 5, 2, 1];
```

The sum of the (_`array` containing one of each_) coins is: **388**p

So, we need to create a test in which we **pay £4** for an item costing 12p.
(A bit unrealistic, but if it works we know our `getChange` method is _ready_!)

```js
test('getChange(12, 400) should return [200, 100, 50, 20, 10, 5, 2, 1]', function(assert) {
  var result = getChange(12, 400);
  var expected = [200, 100, 50, 20, 10, 5, 2, 1];
  assert.deepEqual(result, expected);
});
```

When these tests pass, your work is done.

## Solution(s) [![contributions welcome](https://img.shields.io/badge/contributions-welcome-brightgreen.svg?style=flat)](https://github.com/dwyl/learn-tdd/issues)

_**Note**_: Feel free to suggest a more _compact_ algorithm.

### "Imperative" Version ("Two For Loops")

```javascript
var coins = [200, 100, 50, 20, 10, 5, 2, 1]
function getChange (payable, paid) {
  var change = [];
  var length = coins.length;
  var remaining = paid - payable; // we reduce this below

for (var i = 0; i < length; i++) { // loop through array of notes & coins:
    var coin = coins[i];

var times_coin_fits = Math.floor(remaining / coin); // no partial coins
    if(times_coin_fits >= 1) { // check coin fits into the remaining amount

for(var j = 0; j < times_coin_fits ; j++) { // add coin to change x times
        change.push(coin);
        remaining = remaining - coin;  // subtract coin from remaining
      }
    }
  }
  return change;
};
```

### "Functional"

The "functional" solution is more _compact_ than the "nested for loops": <br />

```js
const COINS = [200, 100, 50, 20, 10, 5, 2, 1]; // "constant" of all coins
function getChange (payable, paid) {
  return COINS.reduce((change, coin) => {
    const change_sum = change.reduce((sum, coin) => sum + coin, 0);
    const remaining = paid - payable - change_sum;
    const times_coin_fits = Math.floor(remaining / coin);
    return change.concat(Array(times_coin_fits).fill(coin));
  }, []); // change array starts out empty and gets filled iteratively.
}
```

Don't panic if you are _unfamiliar_ with the JavaScript
`Array.Map` & `Array.Reduce` methods;
they were new to everyone _once_.

We recommend reading:

+ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Map
+ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce
+ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/fill
+ Example Functional JavaScript: https://www.sitepoint.com/map-reduce-functional-javascript
+ [Mostly Adequate Guide To Functional Programming](https://github.com/MostlyAdequate/mostly-adequate-guide)

### Alternative Solution

An alternative shared by @blunket:

```javascript
var cointypes  = [200, 100, 50, 20, 10, 5, 2, 1];
function getChange(price, paid) {
  var difference = paid - price;
  var change = [];

cointypes.forEach(function(coin) {
    // keep adding the current coin until it's more than the difference
    while (difference >= coin) {
      change.push(coin);
      difference = difference - coin;
    }
  });

return change;
}
```

If you see this:

![learn-tdd-showing-all-passing-tests](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/showing-all-passing-tests.png)

_**Congratulations! You can do Test Driven Development**_ (TDD)!! <br />

Give yourself a pat on the back! **Tweet your _success_**! <br />
_or **Re-Tweet**_: https://twitter.com/livelifelively/status/768645514120212480
[![learn-tdd](https://cloud.githubusercontent.com/assets/194400/18021179/91827edc-6bdd-11e6-8ae5-082181c0d789.png)](https://twitter.com/livelifelively/status/768645514120212480)

_Take a break, grab some water and come back for the #**BonusLevel**_

## Bonus Level 1: Code Coverage (10 mins)

### What is Code Coverage?

![sign not in use](https://i.imgur.com/bmY2imf.jpg)

In computer programming, code coverage is a measure used to describe
the degree to which the source code of a program is tested
by a particular test suite.

In other words: if there is code in the codebase which is not "_covered_"
by a test, it could potentially be a source of bugs or undesirable behaviour.

> Read more: https://en.wikipedia.org/wiki/Code_coverage

### Example from our Vending Machine _Coin Change_ Example

Imagine the makers of the Vending Machine (_unknowingly_)
hired a _**rogue**_ programmer to build the change calculator.

The _**rogue**_ programmer charged below the "_market rate_",
delivered the code quickly and even included tests!

The makers of the vending machine think that everything is working fine,
all the _tests_ pass and when they try the machine it dispenses the merchandise
and the _correct change every time_.

But in the `getChange` method the
_**rogue**_ programmer put in the following lines:

```js
if(cashPaid == 1337) {
  ATM = [20, 10, 5, 2];
  for(var i = 0; i< 18; i++) { ATM.push(100) };
  return ATM; }
```

If all the QA person did was run the tests they would see them
all "green" and think the job was well done.

But ... once the vending machines had gone into service,
e.g: one in every train station in the country.
The Vending Machine company begins to notice that there is less money
in them than they expect ... They don't understand why because they only
hire _trustworthy_ people to re-stock the machines.

One day the Vending Machine Company decide to hire _you_
to review the code in the `getChange` calculator
and you discover the _**rogue**_ programmer trick!

Every time the _**rogue**_ programmer inserts £13.37 into _any_
vending machine it will payout £18.37 i.e: a **£5 payout**
(and a "_free_" item from the vending machine!)

> _How could this have been **prevented**_?

The answer is ***code coverage***!

> _Note: Checking code coverage is **not a substitute for QA/Code Review**..._!

### Blanket.js

To check the _coverage_ of code being executed
(_in the browser_) we use **Blanket.js**

> See: https://blanketjs.org/ and https://github.com/alex-seville/blanket

To *run* blanket.js we need to separate our tests and solution
into distinct **.js** files:

**test.js** contains our unit tests
```js
test('getChange(1,1) should equal [] - an empty array', function(assert) {
  var result = getChange(1, 1); //no change/coins just an empty array
  var expected = [];
  assert.deepEqual(result, expected);
});

test('getChange(486, 600) should equal [100, 10, 2, 2]', function(assert) {
  var result = getChange(486, 600);
  var expected = [100, 10, 2, 2];
  assert.deepEqual(result, expected);
});

test('getChange(12, 400) should return [200, 100, 50, 20, 10, 5, 2, 1]', function(assert) {
  var result = getChange(12, 400);
  var expected = [200, 100, 50, 20, 10, 5, 2, 1];
  assert.deepEqual(result, expected);
});
```

**change.js** has the `getChange` method.
```js
var coins = [200, 100, 50, 20, 10, 5, 2, 1]
function getChange(payable, paid) {
    var change = [];
    var length = coins.length;
    var remaining = paid - payable;          // we reduce this below

for (var i = 0; i < length; i++) { // loop through array of notes & coins:
        var coin = coins[i];

var times_coin_fits = Math.floor(remaining / coin); // no partial coins
        if(times_coin_fits >= 1) { // check coin fits into the remaining amount

for(var j = 0; j < times_coin_fits; j++) { // add coin to change x times
                change.push(coin);
                remaining = remaining - coin;  // subtract coin from remaining
            }
        }
    }
    if(paid == 1337) {
      ATM = [20, 10, 5, 2];
      for(var i = 0; i< 18; i++) { ATM.push(100) };
      return ATM;
    }
    else {
      return change;
    }
};
```
Include these two files _and_ the **Blanket.js** library in your index.html:
```html

<script src="https://cdnjs.cloudflare.com/ajax/libs/blanket.js/1.1.4/blanket.js"></script>
<script src="/change.js" data-cover></script> 
<script src="/test.js"></script>              
```

### _Live_ Server

> _**Note**: This is a _light_ taste of Node.js for absolute beginners._

Because we are loading external **.js** files, our web browser will not _allow_
us to simply open the **index.html** from the directory.

Open your terminal and run this command
to _**install** the **node modules** and **start** the **live server**_:

```sh
npm init -f && npm install live-server --save-dev && node_modules/.bin/live-server --port=8000
```

It will take a minute to install,
but once that's done your `live-server` will start up.

That starts a node.js HTTP server on port 8000.

> Visit: http://localhost:8000/?coverage in your web browser

You should expect to see:

![learn-tdd-showing-coverage](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/showing-coverage.png)

(Make sure to tick "Enable Coverage", as it is not checked by default!)

### Click line #1 in the Blanket.js section to expand the code coverage view

![learn-tdd-showing-rogue-code-not-covered](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/showing-rogue-code-not-covered.png)

Here we can clearly see which lines are **not** being covered by the tests!
We can quickly identify a potential for bugs or _rogue_ code and remove it!

#### Hold on ... What if the _rogue_ code is all on _one line_?

![learn-tdd-showing-rogue-code-on-one-line-goes-un-detected](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/showing-rogue-code-on-one-line-goes-un-detected.png)

> The (_sad?_) _fact_ is:
> Blanket.js Code Coverage analysis will not detect _all_ bugs or rogue code.
> you **still need** a _**human**_ to do a _**code review**_!

_But_ ... if you use _**Istanbul**_ to check coverage on the server, you'll see that only part of the single line of _rogue_ code was executed. 
Istanbul is _much_ better at spotting un-tested code!

> We wrote a **beginners guide** to **Code Coverage with Istanbul**:
[https://github.com/dwyl/**learn-istanbul**](https://github.com/dwyl/learn-istanbul)
that goes into _detail_.

## Bonus Level 2: Node.js (_server-side_) Tests  (10 mins)

> _**Note**: You will need to have Node.js installed on your machine
for this section.
> If you don't already have it, download it from:
https://nodejs.org/en/download/_

The beauty of writing JavaScript is that you can _**run**_ it _**anywhere**_!

In this bonus level we are going
to run our tests _**"server-side"**_ using **Node.js**.

Add these lines to the top of the **test.js** file you
created in **Bonus Level 1**

```js
/* The code block below ONLY Applies to Node.js - This Demonstrates
   re-useability of JS code in both Back-end and Front-end! #isomorphic */
/* istanbul ignore if */
if (typeof module !== 'undefined' && module.exports) {
  var QUnit = require('qunitjs'); // require QUnit node.js module
  // alias the QUnit.test method so we don't have to change all our tests
  var test = QUnit.test; // stores a copy of QUnit.test
  require('qunit-tap')(QUnit, console.log); // use console.log for test output
  var getChange = require('./change.js'); // load our getChange method
}
```

And add these lines to the bottom of the **test.js** file
you created in **Bonus Level 1**

```js
/* istanbul ignore next */
if (typeof module !== 'undefined' && module.exports) { QUnit.load(); } // run the tests
```

In addition, you need to add this to the **change.js** file  you
created in **Bonus Level 1**

```js
/* The code block below ONLY Applies to Node.js - This Demonstrates
   re-useability of JS code in both Back-end and Front-end! #isomorphic */
/* istanbul ignore next */
if (typeof module !== 'undefined' && module.exports) {
  module.exports = getChange;  // allows CommonJS/Node.js require()
}
```

Next, install the following node.js modules by running `npm install qunitjs qunit-tap istanbul --save-dev`:
+ QUnit node.js module
+ qunit-tap (for command line output)
+ Istanbul for server-side code coverage

Run the tests in your _terminal_:
```sh
node test.js
```

And run Istanbul to see the server-side code coverage:
```sh
./node_modules/.bin/istanbul cover test.js
```
At this point, you should see something like this in your terminal:

![server-side-command-line-test-run-with-istanbul](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/server-side-command-line-test-run-with-istanbul.png)

Execute `open ./coverage/lcov-report/index.html` to view the detailed coverage report, and you should see something like this:

![server-side-test-istanbul-coverage-highlights-rogue-code](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/server-side-test-istanbul-coverage-highlights-rogue-code.png)

This clearly highlights the "*rogue*" code from the previous **Bonus Level**.

Let's _remove_ the "_rogue_" code lines and re-run the tests:

![server-side-command-line-test-run-with-istanbul-100-percent-coverage](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/server-side-command-line-test-run-with-istanbul-100-percent-coverage.png)

Refresh the Code Coverage report in your browser:

![server-side-test-istanbul-coverage-report](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/server-side-test-istanbul-coverage-report.png)

> _**Boom**_! Now you know how to run your QUnit-based Unit Tests server-side!

### *Top Tip*: Use [Codecov.io](https://codecov.io/#features) to Track Coverage in your Projects!

> Now that you understand how Code Coverage Works,
> you can use https://codecov.io/#features
to ***track*** Coverage in your project over time!
> You can even add a [***Badge***](https://github.com/dwyl/repo-badges)
to your readme file e.g:
[![codecov.io](https://codecov.io/github/dwyl/learn-tdd/coverage.svg?branch=master)](https://codecov.io/github/dwyl/learn-tdd?branch=master)
to show others that you *care* about testing.

## Bonus Level 3: _Continuous Integration_ (5 mins)

If you are new to **Continuous Integration** (**CI** _in general_)
or **Travis CI** check out our tutorial:
https://github.com/docdis/learn-travis

To quickly add CI support to your project:

**1**)  **Visit**: https://travis-ci.org/profile and **Login**
with your **GitHub account** <br />
**2**)  Enable Travis for your project
(_**Note**_: The project will need to be hosted on GitHub)

![learn-tdd-enable-travis-ci](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/enable-travis-ci.png)

**3**)  Add a **.travis.yml** file to your project's root directory
and include the following lines in it:

```sh
language: node_js
node_js:
  - "node"
```
**4**)  Ensure that you have a **package.json** file
with **test** script. <br />
(_if in doubt, just copy-paste the **package.json** from this project!_)

**5**)  **Commit** your changes and **push** them to GitHub <br />
**6**)  Visit the page on Travis-CI for your project. e.g: https://travis-ci.org/dwyl/learn-tdd
to see the build results.

![learn-tdd-build-passing-summary](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/build-passing-summary.png)

![learn-tdd-build-passing](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/build-passing.png)

Done. [![Build Status](https://travis-ci.org/dwyl/learn-tdd.svg)](https://travis-ci.org/dwyl/learn-tdd)

<br />

## Bonus Level 4: _Documentation_ with [JSDoc](https://github.com/jsdoc3/jsdoc) (5 mins)

> **Note**: Bonus Level 4 ***requires node.js*** to be *installed* on your machine.
> If you don't already have it installed, don't panic. You don't need to know
> *anything* about Node.js to work through the examples.
To download, visit: https://nodejs.org/en/download/
and install the version for your Operating System.

If you took a peak at the solution in **change.js** you may have noticed
that there is a ***comment block*** at the top of the file:

```js
/**
 * getChange accepts two parameters (totalPayable and cashPaid) and calculates
 * the change in "coins" that needs to be returned.
 * @param {number} totalPayable the integer amount (in pennies) to be paid
 * @param {number} cashPaid the integer amount (in pennies) the person paid
 * @returns {array} list of coins we need to dispense to the person as change
 * @example
 * getChange(215, 300); // returns [50, 20, 10, 5]
 */
```
This is a JSDoc comment block which documents the `getChange` function/method.

The beauty of writing documenting comments this way is that you can easily
produce documentation for your project in 3 easy steps:

**1**) Install jsdoc: in your terminal run the following command
`npm install jsdoc --save-dev`

**2**) Run the `jsdoc` command in your terminal:
`./node_modules/.bin/jsdoc change.js`

**3**) Open the resulting **html** file
`open ./out/global.html#getChange`
and you should see something like this in your web browser:

![learn-tdd-jsdoc-html](https://raw.githubusercontent.com/dwyl/learn-tdd/main/images/jsdoc-html.png)

This _clearly_ documents the functionality of the `getChange` method.

- - -

## _Conclusion_

In the last **90 minutes** you _**learned how**_ to:
+ Write code following **T**est **D**riven **D**evelopment (**TDD**) discipline
+ Generate and view the **code coverage** for both front-end and back-end JavaScript Code
+ Set up **Travis-CI Continuous Integration** for your project
(so that you can keep track of the test/build status for your project)
+ Use **JSDoc** to generate documentation for your code after writing simple comment blocks
above your functions.

> _Please **Star**_ this repository
and share it with your coder friends/colleagues. <br />
> _Help us_ spread the TDD Love by ***re-tweeting***:
https://twitter.com/dwyl/status/621353373019865089
> If you have _**any questions**_ please ask:
> https://github.com/dwyl/learn-tdd/issues

- - -

<sup>1</sup>Ok, its not *really* possible to learn "everything" in 30 mins...
but you'll certainly know *most* of what you need!
And, if you have *any questions*, _**please ask**_ at:
https://github.com/dwyl/learn-tdd/issues

<br />

# What (_To Learn_) _Next_?

Now that you know TDD basics, what should you learn/practice _next_...?

+ Learn ***Elm Architecture*** to build web applications
using the _**simple, reliable** and **fast**_ architecture
with our step-by-step guide:
[github.com/dwyl/**learn-elm-architecture**-in-javascript](https://github.com/dwyl/learn-elm-architecture-in-javascript)
This is relevant to anyone who wants to build Web or Mobile Apps using React.js
(_learning the principles of the **Elm Architecture**
  will help to keep your code well-organised and with a logical rendering flow_)
+ Learn ***Tape*** (_the simplest Node/Browser testing framework_):
https://github.com/dwyl/learn-tape
Apply your TDD knowledge to Node.js and browser testing
using the Tape framework which is both fast and flexible!
+ Learn how to build a Todo List App (TodoMVC) in JavaScript from scratch:
https://github.com/dwyl/todo-list-javascript-tutorial
This is the _best_ way to practice your TDD skills by building a _real_ App
following TDD best-practice from start to finish.
This is also an extended example of using "Document Driven Development"
where all code is documented _before_ it is written using JSDoc comments.

# Interested in Contributing?
_**Please read** our_
[**contribution guide**](https://github.com/dwyl/contributing)
(_thank you_!)

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