Learn Test Driven Development (TDD)

A brief introduction to Test Driven Development (TDD) in JavaScript for people who want to write more reliable code.

Why?

Project(s) without tests often end up looking like they are stuck together with duct tape ...

Change one part and the other stops working? "Fixing" one bug, creates another?

Wouldn't you prefer if everything was consistent and beautifully integrated?
What if everyone on your team worked like clock-work in a disciplined order... like a Formula 1 Crew ...

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 else; truly game-changing.

What?

This tutorial will help you get started with Test Driven Development (TDD) today!
In the next 30 minutes you will learn everything¹ you need to know 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 time
• Basic Programming Skills (HTML & JavaScript)
• (Optional) Bonus Levels require you to install Node.js

What is Software Testing?

> Software testing is the process of evaluation a software item to detect differences between given input and expected 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.

Rehman Zafar at What is software testing?

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 fulfill that test and refactoring. In other words, it’s one way to think through your requirements or design before your write your functional code.

From Introduction to Test Driven Development (TDD)

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 principals)
• Practical Full-Stack JavaScript Web Application Test Driven Development - https://github.com/nelsonic/practical-js-tdd

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 see the expression: "Red, Green, Refactor":

What this means is that there's 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 - if you have 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.

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
> your code is the answer to the question.
> By having a clear question, you can always check > that your code is working,
> because it consistently > gives you the same answer(s) ... no surprises, even when you're working with a large, inter-dependent code base!

Practical

Scenario: Vending Machine Change Calculator Micro-Project

Imagine you are building a Vending Machine that allows people to buy any item. The machine accepts coins and calculates the change to be returned to the customer given the price of the Item 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:

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 if you have it installed)

(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 to get started:

  
    
    <title>Vending Machine Change Calculator TDD Tutorial</title>
    
    
    
    
  
  
    <div id="main">
      <h1>Vending Machine <em>Change Calculator</em></h1>
      <h3>Calculate the change (<em>coins</em>) to return to a customer when they buy something.</h3>
    </div>

    <div id="qunit"></div>
    <div id="qunit-fixture"></div>
    
    
    
  

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 annotations)

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:

  
    
    <title>Vending Machine Change Calculator TDD</title>
    
    
    
    
  
  
    <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 JQuery and the QUnit Library from CDN:

    <div id="qunit"></div>
    <div id="qunit-fixture"></div>
    
    

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

If you are completely new to writing test assertions, don't worry, they are really simple, there are 3 parts:

1. Description - usually the first parameter to QUnit's test() method.
2. Computation - execute a function/method (exercising the method you will write to solve your test)
3. Assertion - check that the result of your calculation is what you expect it to be.

In the above screenshot, the assertion is assert.equal(result, 2)
We are giving the equal method two arguments; the result of our computation and our expected value in this case 2. That's it.

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

> Given a Price and amount of Cash from Customer > Return: Change to customer (in notes and coins).

Essentially we are building a simple calculator that only does subtraction (Price - Cash = Change), but also splits the result into the various 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 person paid into the vending machine), getChange should calculate the change we need to return to the customer
• return change as an array of coins (largest to smallest) that the vending machine will need to dispense to the customer.

Example

If a person 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.
To make up this 85p of change we would return four coins to the person: 50p, 20p, 10p and 5p.
An array of these coins would look like: [50, 20, 10, 5]

Coins

In the UK we have the following 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:

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 intro article by Scott Ambler (especially the diagrams) otherwise this (test-fail-code-pass) process may seem strange ...

In Test First Development (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:

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/

Watch it Fail

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

> Q: Why deliberately write a test we know is going to fail...?
> A: To get used to the idea of only writing the code required to pass > the current (failing) test.
> Read: "The Importance of Test Failure: https://www.sustainabletdd.com/2012/03/importance-of-test-failure.html

Create the getChange function

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

your index.html should now look like this:

  
    
    <title>Vending Machine Change Calculator TDD</title>
    
    
  
  
    <div style="padding: 2em;" id="main">
      <h1>Vending Machine Change Calculator</h1>
      <h2>Calculate the Change for a Given Price and Cash Received</h2>
      
    </div>

    <div id="qunit"></div>
    <div id="qunit-fixture"></div>
    

    

    
  

Refresh index.html in the Browser

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 return an array containing the coins equal to the difference:

e.g:

totalPayable = 215          // £2.15
cashPaid     = 300          // £3.00
dfference    =  85          // 85p
change       = [50,20,10,5] // 50p, 20p, 10p, 5p

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

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?

function getChange (totalPayable, cashPaid) {
    return [50, 20, 10, 5];    // just "enough to pass the failing test"
};

This will pass, but you have hard coded the result (not exactly 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:

totalPayable = 486           // £4.86
cashPaid     = 600           // £6.00
dfference    = 114           // £1.14
change       = [100,10,2,2]  // £1, 10p, 2p, 2p

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

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:

function getChange (totalPayable, cashPaid) {
    if(totalPayable == 486 && cashPaid == 600)
        return [100, 10, 2, 2];
    else if(totalPayable == 215 && cashPaid == 300)
        return [50, 20, 10, 5];
};

The Arthur Andersen Approach gets results in the short run ...

But its arguably more work than simply solving the problem. Let's do that instead.

Try It Yourself (before looking at the solution!)

> Try to create your own getChange method that passes the two tests > before you look at the solution...

to re-cap these are our two tests:

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 coins:

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

The sum of the (array containing one of each) coins is: 388p

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!)

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

Note: feel free to suggest a more compact algorithm

function getChange(cost, paid){
  var possibleCoins = [200, 100, 50, 20, 10, 5, 2, 1]; // Must be in decending order
  var changeToMake = paid - cost;
  var coinsToReturn = [];// Array we will fill with coins to return to the user

  while(changeToMake){ // while there is sill money left to give loop! same as "while(changeToMake > 0)"
    for (var i = 0; i < possibleCoins.length; i++) {
      if (changeToMake >= possibleCoins[i]) { // if changeToGive is larger than the current coin use that coin
        changeToMake -= possibleCoins[i]; // remove this coins value from the total change
        coinsToReturn.push(possibleCoins[i]); // add the coin to the return list of coins
        i--; // Move back one to try the same coin again
      }
    }
  }
  return coinsToReturn;
}

####Alternative Solution

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

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

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

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

If you see this:

Congratulations! You can do Test Driven Development (TDD).
Give yourself a pat on the back! Tweet your success!
Take a break, grab some water and come back for the #BonusLevel

- - -

Bonus Level 1: Code Coverage (10 mins)

What is Code Coverage?

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:

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! (and 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

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);
});

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.

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

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

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

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

Static Server

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

> Note: You don't need to understand Hapi.js to > run a simple (static) node.js server. > But, if you want to learn hapi.js, visit: > https://github.com/nelsonic/learn-hapi > (after you've finished this tutorial...!)

Create a file called static-server.js and paste the code from learn-tdd/static-server.js into the file.

Next, open your terminal and run this command to install the node modules and start the server:

npm install hapi inert nodemon --s && ./node_modules/.bin/nodemon static-server.js

It will take a a minute to install hapi.js and nodemon, but once that's done your static server will start up.

That starts a node.js (hapi.js) HTTP server on port 8000.

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

You should expect to see:

Click change.js to expand the code coverage view

Here we can clearly see which lines are not being covered by our 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?

> The (sad?) fact is: > Blanket.js Code Coverage analysis will not detect all bugs or rogue code. > you still need a human to do code review!

But ... if you use Istanbul to check coverage on the server, Istanbul is much better at spotting un-tested code! (see: Bonus Level 2!)

Bonus Level 2: Node.js (server-side) Tests (10 mins)

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

/* 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

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

Also you need to add this to the change.js file you created in Bonus Level 1

/* 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 node.js following modules by runnin 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:

node test.js

And to see code server-side coverage:

istanbul cover test.js

You should expect to see something like this in your terminal:

To view the detailed coverage report, open ./coverage/lcov-report/index.html you should expect to see:

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

Let's remove the "rogue" code lines and re-run the tests:

Refresh the Code Coverage report in your browser:

> Boom! You know how to run your QUnit-based Unit Tests server-side!

Top Tip: Use Codecov.io 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 to your readme file
e.g: 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
2) Enable Travis for your project (Note: the project will need to be hosted on GitHub)

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

language: node_js
node_js:
  - 0.12

4) Ensure that you have a package.json file with test script.
(if in doubt, just copy-paste the package.json from this project!)

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

Done.

Bonus Level 4: Documentation with JSDoc (5 mins)

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:

/**
 * 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: npm install jsdoc --save-dev

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

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

This clearly documents the functionality of the getChange method.

---

Conclusion

In the last 90 minutes you learned how to:

• Write code following Test Driven Developement (TDD) discipline
• 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 document your code using simple comment blocks above your functions.

> Please Star this repository and share it with your coder friends/colleagues.
> If you have any questions please (don't hesitate) ask: > https://github.com/dwyl/learn-tdd/issues

---

¹Ok, 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: https://github.com/dwyl/learn-tdd/issues