README.md

# clean-code-javascript

## Table of Contents
  1. [Introduction](#introduction)
  2. [Variables](#variables)
  3. [Functions](#functions)
  4. [Objects and Data Structures](#objects-and-data-structures)
  5. [Classes](#classes)
  6. [Testing](#testing)
  7. [Concurrency](#concurrency)
  8. [Error Handling](#error-handling)
  9. [Formatting](#formatting)
  10. [Comments](#comments)

## Introduction
![Humorous image of software quality estimation as a count of how many expletives
you shout when reading code](http://www.osnews.com/images/comics/wtfm.jpg)

Software engineering principles, from Robert C. Martin's book
[*Clean Code*](https://www.amazon.com/Clean-Code-Handbook-Software-Craftsmanship/dp/0132350882),
adapted for JavaScript. This is not a style guide. It's a guide to producing
readable, reusable, and refactorable software in JavaScript.

Not every principle herein has to be strictly followed, and even less will be
universally agreed upon. These are guidelines and nothing more, but they are
ones codified over many years of collective experience by the authors of
*Clean Code*.

Our craft of software engineering is just a bit over 50 years old, and we are
still learning a lot. When software architecture is as old as architecture
itself, maybe then we will have harder rules to follow. For now, let these
guidelines serve as a touchstone by which to assess the quality of the
JavaScript code that you and your team produce.

One more thing: knowing these won't immediately make you a better software
developer, and working with them for many years doesn't mean you won't make
mistakes. Every piece of code starts as a first draft, like wet clay getting
shaped into its final form. Finally, we chisel away the imperfections when
we review it with our peers. Don't beat yourself up for first drafts that need
improvement. Beat up the code instead!

## **Variables**
### Use meaningful and pronounceable variable names

**Bad:**
```javascript
const yyyymmdstr = moment().format('YYYY/MM/DD');
```

**Good**:
```javascript
const yearMonthDay = moment().format('YYYY/MM/DD');
```
**[⬆ back to top](#table-of-contents)**

### Use the same vocabulary for the same type of variable

**Bad:**
```javascript
getUserInfo();
getClientData();
getCustomerRecord();
```

**Good**:
```javascript
getUser();
```
**[⬆ back to top](#table-of-contents)**

### Use searchable names
We will read more code than we will ever write. It's important that the code we
do write is readable and searchable. By *not* naming variables that end up
being meaningful for understanding our program, we hurt our readers.
Make your names searchable.

**Bad:**
```javascript
// What the heck is 525600 for?
for (let i = 0; i < 525600; i++) {
  runCronJob();
}
```

**Good**:
```javascript
// Declare them as capitalized `const` globals.
const MINUTES_IN_A_YEAR = 525600;
for (let i = 0; i < MINUTES_IN_A_YEAR; i++) {
  runCronJob();
}
```
**[⬆ back to top](#table-of-contents)**

### Use explanatory variables
**Bad:**
```javascript
const cityStateRegex = /^(.+)[,\\s]+(.+?)\s*(\d{5})?$/;
saveCityState(cityStateRegex.match(cityStateRegex)[1], cityStateRegex.match(cityStateRegex)[2]);
```

**Good**:
```javascript
const cityStateRegex = /^(.+)[,\\s]+(.+?)\s*(\d{5})?$/;
const match = cityStateRegex.match(cityStateRegex)
const city = match[1];
const state = match[2];
saveCityState(city, state);
```
**[⬆ back to top](#table-of-contents)**

### Avoid Mental Mapping
Explicit is better than implicit.

**Bad:**
```javascript
const locations = ['Austin', 'New York', 'San Francisco'];
locations.forEach((l) => {
  doStuff();
  doSomeOtherStuff();
  ...
  ...
  ...
  // Wait, what is `l` for again?
  dispatch(l);
});
```

**Good**:
```javascript
const locations = ['Austin', 'New York', 'San Francisco'];
locations.forEach((location) => {
  doStuff();
  doSomeOtherStuff();
  ...
  ...
  ...
  dispatch(location);
});
```
**[⬆ back to top](#table-of-contents)**

### Don't add unneeded context
If your class/object name tells you something, don't repeat that in your
variable name.

**Bad:**
```javascript
const Car = {
  carMake: 'Honda',
  carModel: 'Accord',
  carColor: 'Blue'
};

function paintCar(car) {
  car.carColor = 'Red';
}
```

**Good**:
```javascript
const Car = {
  make: 'Honda',
  model: 'Accord',
  color: 'Blue'
};

function paintCar(car) {
  car.color = 'Red';
}
```
**[⬆ back to top](#table-of-contents)**

### Short-circuiting is cleaner than conditionals

**Bad:**
```javascript
function createMicrobrewery(name) {
  let breweryName;
  if (name) {
    breweryName = name;
  } else {
    breweryName = 'Hipster Brew Co.';
  }
}
```

**Good**:
```javascript
function createMicrobrewery(name) {
  const breweryName = name || 'Hipster Brew Co.'
}
```
**[⬆ back to top](#table-of-contents)**

## **Functions**
### Function arguments (2 or less ideally)
Limiting the amount of function parameters is incredibly important because it
makes testing your function easier. Having more than three leads to a
combinatorial explosion where you have to test tons of different cases with
each separate argument.

Zero arguments is the ideal case. One or two arguments is ok, and three should
be avoided. Anything more than that should be consolidated. Usually, if you have
more than two arguments then your function is trying to do too much. In cases
where it's not, most of the time a higher-level object will suffice as an
argument.

Since JavaScript allows us to make objects on the fly, without a lot of class
boilerplate, you can use an object if you are finding yourself needing a
lot of arguments.

**Bad:**
```javascript
function createMenu(title, body, buttonText, cancellable) {
  ...
}
```

**Good**:
```javascript
const menuConfig = {
  title: 'Foo',
  body: 'Bar',
  buttonText: 'Baz',
  cancellable: true
}

function createMenu(menuConfig) {
  ...
}

```
**[⬆ back to top](#table-of-contents)**


### Functions should do one thing
This is by far the most important rule in software engineering. When functions
do more than one thing, they are harder to compose, test, and reason about.
When you can isolate a function to just one action, they can be refactored
easily and your code will read much cleaner. If you take nothing else away from
this guide other than this, you'll be ahead of many developers.

**Bad:**
```javascript
function emailClients(clients) {
  clients.forEach(client => {
    const clientRecord = database.lookup(client);
    if (clientRecord.isActive()) {
      email(client);
    }
  });
}
```

**Good**:
```javascript
function emailClients(clients) {
  clients
    .filter(isClientActive)
    .forEach(email);
}

function isClientActive(client) {
  const clientRecord = database.lookup(client);
  return clientRecord.isActive();
}
```
**[⬆ back to top](#table-of-contents)**

### Function names should say what they do

**Bad:**
```javascript
function dateAdd(date, month) {
  // ...
}

const date = new Date();

// It's hard to to tell from the function name what is added
dateAdd(date, 1);
```

**Good**:
```javascript
function dateAddMonth(date, month) {
  // ...
}

const date = new Date();
dateAddMonth(date, 1);
```
**[⬆ back to top](#table-of-contents)**

### Functions should only be one level of abstraction
When you have more than one level of abstraction your function is usually
doing too much. Splitting up functions leads to reusability and easier
testing.

**Bad:**
```javascript
function parseBetterJSAlternative(code) {
  const REGEXES = [
    // ...
  ];

  const statements = code.split(' ');
  const tokens = [];
  REGEXES.forEach((REGEX) => {
    statements.forEach((statement) => {
      // ...
    })
  });

  const ast = [];
  tokens.forEach((token) => {
    // lex...
  });

  ast.forEach((node) => {
    // parse...
  })
}
```

**Good**:
```javascript
function tokenize(code) {
  const REGEXES = [
    // ...
  ];

  const statements = code.split(' ');
  const tokens = [];
  REGEXES.forEach((REGEX) => {
    statements.forEach((statement) => {
      tokens.push( // ... );
    })
  });

  return tokens;
}

function lexer(tokens) {
  const ast = [];
  tokens.forEach((token) => {
    ast.push( // ... );
  });

  return ast;
}

function parseBetterJSAlternative(code) {
  const tokens = tokenize(code);
  const ast = lexer(tokens);
  ast.forEach((node) => {
    // parse...
  })
}
```
**[⬆ back to top](#table-of-contents)**

### Remove duplicate code
Never ever, ever, under any circumstance, have duplicate code. There's no reason
for it and it's quite possibly the worst sin you can commit as a professional
developer. Duplicate code means there's more than one place to alter something
if you need to change some logic. JavaScript is untyped, so it makes having
generic functions quite easy. Take advantage of that! Tools like
[jsinpect](https://github.com/danielstjules/jsinspect) can help you find duplicate
code eligible for refactoring.

**Bad:**
```javascript
function showDeveloperList(developers) {
  developers.forEach(developers => {
    const expectedSalary = developer.calculateExpectedSalary();
    const experience = developer.getExperience();
    const githubLink = developer.getGithubLink();
    const data = {
      expectedSalary: expectedSalary,
      experience: experience,
      githubLink: githubLink
    };

    render(data);
  });
}

function showManagerList(managers) {
  managers.forEach(manager => {
    const expectedSalary = manager.calculateExpectedSalary();
    const experience = manager.getExperience();
    const portfolio = manager.getMBAProjects();
    const data = {
      expectedSalary: expectedSalary,
      experience: experience,
      portfolio: portfolio
    };

    render(data);
  });
}
```

**Good**:
```javascript
function showList(employees) {
  employees.forEach(employee => {
    const expectedSalary = employee.calculateExpectedSalary();
    const experience = employee.getExperience();

    let portfolio = employee.getGithubLink();

    if (employee.type === 'manager') {
      portfolio = employee.getMBAProjects();
    }

    const data = {
      expectedSalary: expectedSalary,
      experience: experience,
      portfolio: portfolio
    };

    render(data);
  });
}
```
**[⬆ back to top](#table-of-contents)**

### Use default arguments instead of short circuiting
**Bad:**
```javascript
function writeForumComment(subject, body) {
  subject = subject || 'No Subject';
  body = body || 'No text';
}

```

**Good**:
```javascript
function writeForumComment(subject = 'No subject', body = 'No text') {
  ...
}

```
**[⬆ back to top](#table-of-contents)**

### Set default objects with Object.assign

**Bad:**
```javascript
const menuConfig = {
  title: null,
  body: 'Bar',
  buttonText: null,
  cancellable: true
}

function createMenu(config) {
  config.title = config.title || 'Foo'
  config.body = config.body || 'Bar'
  config.buttonText = config.buttonText || 'Baz'
  config.cancellable = config.cancellable === undefined ? config.cancellable : true;

}

createMenu(menuConfig);
```

**Good**:
```javascript
const menuConfig = {
  title: 'Order',
  // User did not include 'body' key
  buttonText: 'Send',
  cancellable: true
}

function createMenu(config) {
  config = Object.assign({
    title: 'Foo',
    body: 'Bar',
    buttonText: 'Baz',
    cancellable: true
  }, config);

  // config now equals: {title: "Order", body: "Bar", buttonText: "Send", cancellable: true}
  // ...
}

createMenu(menuConfig);
```
**[⬆ back to top](#table-of-contents)**


### Don't use flags as function parameters
Flags tell your user that this function does more than one thing. Functions should do one thing. Split out your functions if they are following different code paths based on a boolean.

**Bad:**
```javascript
function createFile(name, temp) {
  if (temp) {
    fs.create('./temp/' + name);
  } else {
    fs.create(name);
  }
}
```

**Good**:
```javascript
function createFile(name) {
  fs.create(name);
}

function createTempFile(name) {
  createFile('./temp/' + name);
}
```
**[⬆ back to top](#table-of-contents)**

### Avoid Side Effects
A function produces a side effect if it does anything other than take a value in
and return another value or values. A side effect could be writing to a file,
modifying some global variable, or accidentally wiring all your money to a
stranger.

Now, you do need to have side effects in a program on occasion. Like the previous
example, you might need to write to a file. What you want to do is to
centralize where you are doing this. Don't have several functions and classes
that write to a particular file. Have one service that does it. One and only one.

The main point is to avoid common pitfalls like sharing state between objects
without any structure, using mutable data types that can be written to by anything,
and not centralizing where your side effects occur. If you can do this, you will
be happier than the vast majority of other programmers.

**Bad:**
```javascript
// Global variable referenced by following function.
// If we had another function that used this name, now it'd be an array and it could break it.
let name = 'Ryan McDermott';

function splitIntoFirstAndLastName() {
  name = name.split(' ');
}

splitIntoFirstAndLastName();

console.log(name); // ['Ryan', 'McDermott'];
```

**Good**:
```javascript
function splitIntoFirstAndLastName(name) {
  return name.split(' ');
}

const name = 'Ryan McDermott'
const newName = splitIntoFirstAndLastName(name);

console.log(name); // 'Ryan McDermott';
console.log(newName); // ['Ryan', 'McDermott'];
```
**[⬆ back to top](#table-of-contents)**

### Don't write to global functions
Polluting globals is a bad practice in JavaScript because you could clash with another
library and the user of your API would be none-the-wiser until they get an
exception in production. Let's think about an example: what if you wanted to
extend JavaScript's native Array method to have a `diff` method that could
show the difference between two arrays? You could write your new function
to the `Array.prototype`, but it could clash with another library that tried
to do the same thing. What if that other library was just using `diff` to find
the difference between the first and last elements of an array? This is why it
would be much better to just use ES2015/ES6 classes and simply extend the `Array` global.

**Bad:**
```javascript
Array.prototype.diff = function(comparisonArray) {
  const values = [];
  const hash = {};

  for (let i of comparisonArray) {
    hash[i] = true;
  }

  for (let i of this) {
    if (!hash[i]) {
      values.push(i);
    }
  }

  return values;
}
```

**Good:**
```javascript
class SuperArray extends Array {
  constructor(...args) {
    super(...args);
  }

  diff(comparisonArray) {
    const values = [];
    const hash = {};

    for (let i of comparisonArray) {
      hash[i] = true;
    }

    for (let i of this) {
      if (!hash[i]) {
        values.push(i);
      }
    }

    return values;
  }
}
```
**[⬆ back to top](#table-of-contents)**

### Favor functional programming over imperative programming
JavaScript isn't a functional language in the way that Haskell is, but it has
a functional flavor to it. Functional languages are cleaner and easier to test.
Favor this style of programming when you can.

**Bad:**
```javascript
const programmerOutput = [
  {
    name: 'Uncle Bobby',
    linesOfCode: 500
  }, {
    name: 'Suzie Q',
    linesOfCode: 1500
  }, {
    name: 'Jimmy Gosling',
    linesOfCode: 150
  }, {
    name: 'Gracie Hopper',
    linesOfCode: 1000
  }
];

let totalOutput = 0;

for (let i = 0; i < programmerOutput.length; i++) {
  totalOutput += programmerOutput[i].linesOfCode;
}
```

**Good**:
```javascript
const programmerOutput = [
  {
    name: 'Uncle Bobby',
    linesOfCode: 500
  }, {
    name: 'Suzie Q',
    linesOfCode: 1500
  }, {
    name: 'Jimmy Gosling',
    linesOfCode: 150
  }, {
    name: 'Gracie Hopper',
    linesOfCode: 1000
  }
];

const totalOutput = programmerOutput
  .map((programmer) => programmer.linesOfCode)
  .reduce((acc, linesOfCode) => acc + linesOfCode, 0);
```
**[⬆ back to top](#table-of-contents)**

### Encapsulate conditionals

**Bad:**
```javascript
if (fsm.state === 'fetching' && isEmpty(listNode)) {
  /// ...
}
```

**Good**:
```javascript
function shouldShowSpinner(fsm, listNode) {
  return fsm.state === 'fetching' && isEmpty(listNode);
}

if (shouldShowSpinner(fsmInstance, listNodeInstance)) {
  // ...
}
```
**[⬆ back to top](#table-of-contents)**

### Avoid negative conditionals

**Bad:**
```javascript
function isDOMNodeNotPresent(node) {
  // ...
}

if (!isDOMNodeNotPresent(node)) {
  // ...
}
```

**Good**:
```javascript
function isDOMNodePresent(node) {
  // ...
}

if (isDOMNodePresent(node)) {
  // ...
}
```
**[⬆ back to top](#table-of-contents)**

### Avoid conditionals
This seems like an impossible task. Upon first hearing this, most people say,
"how am I supposed to do anything without an `if` statement?" The answer is that
you can use polymorphism to achieve the same task in many cases. The second
question is usually, "well that's great but why would I want to do that?" The
answer is a previous clean code concept we learned: a function should only do
one thing. When you have classes and functions that have `if` statements, you
are telling your user that your function does more than one thing. Remember,
just do one thing.

**Bad:**
```javascript
class Airplane {
  //...
  getCruisingAltitude() {
    switch (this.type) {
      case '777':
        return getMaxAltitude() - getPassengerCount();
      case 'Air Force One':
        return getMaxAltitude();
      case 'Cessna':
        return getMaxAltitude() - getFuelExpenditure();
    }
  }
}
```

**Good**:
```javascript
class Airplane {
  //...
}

class Boeing777 extends Airplane {
  //...
  getCruisingAltitude() {
    return getMaxAltitude() - getPassengerCount();
  }
}

class AirForceOne extends Airplane {
  //...
  getCruisingAltitude() {
    return getMaxAltitude();
  }
}

class Cessna extends Airplane {
  //...
  getCruisingAltitude() {
    return getMaxAltitude() - getFuelExpenditure();
  }
}
```
**[⬆ back to top](#table-of-contents)**

### Avoid type-checking (part 1)
JavaScript is untyped, which means your functions can take any type of argument.
Sometimes you are bitten by this freedom and it becomes tempting to do
type-checking in your functions. There are many ways to avoid having to do this.
The first thing to consider is consistent APIs.

**Bad:**
```javascript
function travelToTexas(vehicle) {
  if (vehicle instanceof Bicycle) {
    vehicle.peddle(this.currentLocation, new Location('texas'));
  } else if (vehicle instanceof Car) {
    vehicle.drive(this.currentLocation, new Location('texas'));
  }
}
```

**Good**:
```javascript
function travelToTexas(vehicle) {
  vehicle.move(this.currentLocation, new Location('texas'));
}
```
**[⬆ back to top](#table-of-contents)**

### Avoid type-checking (part 2)
If you are working with basic primitive values like strings, integers, and arrays,
and you can't use polymorphism but you still feel the need to type-check,
you should consider using TypeScript. It is an excellent alternative to normal
JavaScript, as it provides you with static typing on top of standard JavaScript
syntax. The problem with manually type-checking normal JavaScript is that
doing it well requires so much extra verbiage that the faux "type-safety" you get
doesn't make up for the lost readability. Keep your JavaScript clean, write
good tests, and have good code reviews. Otherwise, do all of that but with
TypeScript (which, like I said, is a great alternative!).

**Bad:**
```javascript
function combine(val1, val2) {
  if (typeof val1 == "number" && typeof val2 == "number" ||
      typeof val1 == "string" && typeof val2 == "string") {
    return val1 + val2;
  } else {
    throw new Error('Must be of type String or Number');
  }
}
```

**Good**:
```javascript
function combine(val1, val2) {
  return val1 + val2;
}
```
**[⬆ back to top](#table-of-contents)**

### Don't over-optimize
Modern browsers do a lot of optimization under-the-hood at runtime. A lot of
times, if you are optimizing then you are just wasting your time. [There are good
resources](https://github.com/petkaantonov/bluebird/wiki/Optimization-killers)
for seeing where optimization is lacking. Target those in the meantime, until
they are fixed if they can be.

**Bad:**
```javascript

// On old browsers, each iteration with uncached `list.length` would be costly
// because of `list.length` recomputation. In modern browsers, this is optimized.
for (let i = 0, len = list.length; i < len; i++) {
  // ...
}
```

**Good**:
```javascript
for (let i = 0; i < list.length; i++) {
  // ...
}
```
**[⬆ back to top](#table-of-contents)**

### Remove dead code
Dead code is just as bad as duplicate code. There's no reason to keep it in
your codebase. If it's not being called, get rid of it! It will still be safe
in your version history if you still need it.

**Bad:**
```javascript
function oldRequestModule(url) {
  // ...
}

function newRequestModule(url) {
  // ...
}

const req = newRequestModule;
inventoryTracker('apples', req, 'www.inventory-awesome.io');

```

**Good**:
```javascript
function newRequestModule(url) {
  // ...
}

const req = newRequestModule;
inventoryTracker('apples', req, 'www.inventory-awesome.io');
```
**[⬆ back to top](#table-of-contents)**

## **Objects and Data Structures**
### Use getters and setters
JavaScript doesn't have interfaces or types so it is very hard to enforce this
pattern, because we don't have keywords like `public` and `private`. As it is,
using getters and setters to access data on objects is far better than simply
looking for a property on an object. "Why?" you might ask. Well, here's an
unorganized list of reasons why:

1. When you want to do more beyond getting an object property, you don't have
to look up and change every accessor in your codebase.
2. Makes adding validation simple when doing a `set`.
3. Encapsulates the internal representation.
4. Easy to add logging and error handling when getting and setting.
5. Inheriting this class, you can override default functionality.
6. You can lazy load your object's properties, let's say getting it from a
server.


**Bad:**
```javascript
class BankAccount {
  constructor() {
	   this.balance = 1000;
  }
}

const bankAccount = new BankAccount();

// Buy shoes...
bankAccount.balance = bankAccount.balance - 100;
```

**Good**:
```javascript
class BankAccount {
  constructor() {
	   this.balance = 1000;
  }

  // It doesn't have to be prefixed with `get` or `set` to be a getter/setter
  withdraw(amount) {
  	if (verifyAmountCanBeDeducted(amount)) {
  	  this.balance -= amount;
  	}
  }
}

const bankAccount = new BankAccount();

// Buy shoes...
bankAccount.withdraw(100);
```
**[⬆ back to top](#table-of-contents)**


### Make objects have private members
This can be accomplished through closures (for ES5 and below).

**Bad:**
```javascript

const Employee = function(name) {
  this.name = name;
}

Employee.prototype.getName = function() {
  return this.name;
}

const employee = new Employee('John Doe');
console.log('Employee name: ' + employee.getName()); // Employee name: John Doe
delete employee.name;
console.log('Employee name: ' + employee.getName()); // Employee name: undefined
```

**Good**:
```javascript
const Employee = (function() {
  function Employee(name) {
    this.getName = function() {
      return name;
    };
  }

  return Employee;
}());

const employee = new Employee('John Doe');
console.log('Employee name: ' + employee.getName()); // Employee name: John Doe
delete employee.name;
console.log('Employee name: ' + employee.getName()); // Employee name: John Doe
```
**[⬆ back to top](#table-of-contents)**


## **Classes**
### Single Responsibility Principle (SRP)
As stated in Clean Code, "There should never be more than one reason for a class
to change". It's tempting to jam-pack a class with a lot of functionality, like
when you can only take one suitcase on your flight. The issue with this is
that your class won't be conceptually cohesive and it will give it many reasons
to change. Minimizing the amount of times you need to change a class is important.
It's important because if too much functionality is in one class and you modify a piece of it,
it can be difficult to understand how that will affect other dependent modules in
your codebase.

**Bad:**
```javascript
class UserSettings {
  constructor(user) {
    this.user = user;
  }

  changeSettings(settings) {
    if (this.verifyCredentials(user)) {
      // ...
    }
  }

  verifyCredentials(user) {
    // ...
  }
}
```

**Good**:
```javascript
class UserAuth {
  constructor(user) {
    this.user = user;
  }

  verifyCredentials() {
    // ...
  }
}


class UserSettings {
  constructor(user) {
    this.user = user;
    this.auth = new UserAuth(user)
  }

  changeSettings(settings) {
    if (this.auth.verifyCredentials()) {
      // ...
    }
  }
}
```
**[⬆ back to top](#table-of-contents)**

### Open/Closed Principle (OCP)
As stated by Bertrand Meyer, "software entities (classes, modules, functions,
etc.) should be open for extension, but closed for modification." What does that
mean though? This principle basically states that you should allow users to
extend the functionality of your module without having to open up the `.js`
source code file and manually manipulate it.

**Bad:**
```javascript
class AjaxRequester {
  constructor() {
    // What if we wanted another HTTP Method, like DELETE? We would have to
    // open this file up and modify this and put it in manually.
    this.HTTP_METHODS = ['POST', 'PUT', 'GET'];
  }

  get(url) {
    // ...
  }

}
```

**Good**:
```javascript
class AjaxRequester {
  constructor() {
    this.HTTP_METHODS = ['POST', 'PUT', 'GET'];
  }

  get(url) {
    // ...
  }

  addHTTPMethod(method) {
    this.HTTP_METHODS.push(method);
  }
}
```
**[⬆ back to top](#table-of-contents)**


### Liskov Substitution Principle (LSP)
This is a scary term for a very simple concept. It's formally defined as "If S
is a subtype of T, then objects of type T may be replaced with objects of type S
(i.e., objects of type S may substitute objects of type T) without altering any
of the desirable properties of that program (correctness, task performed,
etc.)." That's an even scarier definition.

The best explanation for this is if you have a parent class and a child class,
then the base class and child class can be used interchangeably without getting
incorrect results. This might still be confusing, so let's take a look at the
classic Square-Rectangle example. Mathematically, a square is a rectangle, but
if you model it using the "is-a" relationship via inheritance, you quickly
get into trouble.

**Bad:**
```javascript
class Rectangle {
  constructor() {
    this.width = 0;
    this.height = 0;
  }

  setColor(color) {
    // ...
  }

  render(area) {
    // ...
  }

  setWidth(width) {
    this.width = width;
  }

  setHeight(height) {
    this.height = height;
  }

  getArea() {
    return this.width * this.height;
  }
}

class Square extends Rectangle {
  constructor() {
    super();
  }

  setWidth(width) {
    this.width = width;
    this.height = width;
  }

  setHeight(height) {
    this.width = height;
    this.height = height;
  }
}

function renderLargeRectangles(rectangles) {
  rectangles.forEach((rectangle) => {
    rectangle.setWidth(4);
    rectangle.setHeight(5);
    let area = rectangle.getArea(); // BAD: Will return 25 for Square. Should be 20.
    rectangle.render(area);
  })
}

let rectangles = [new Rectangle(), new Rectangle(), new Square()];
renderLargeRectangles(rectangles);
```

**Good**:
```javascript
class Shape {
  constructor() {}

  setColor(color) {
    // ...
  }

  render(area) {
    // ...
  }
}

class Rectangle extends Shape {
  constructor() {
    super();
    this.width = 0;
    this.height = 0;
  }

  setWidth(width) {
    this.width = width;
  }

  setHeight(height) {
    this.height = height;
  }

  getArea() {
    return this.width * this.height;
  }
}

class Square extends Shape {
  constructor() {
    super();
    this.length = 0;
  }

  setLength(length) {
    this.length = length;
  }

  getArea() {
    return this.length * this.length;
  }
}

function renderLargeShapes(shapes) {
  shapes.forEach((shape) => {
    switch (shape.constructor.name) {
      case 'Square':
        shape.setLength(5);
      case 'Rectangle':
        shape.setWidth(4);
        shape.setHeight(5);
    }

    const area = shape.getArea();
    shape.render(area);
  })
}

const shapes = [new Rectangle(), new Rectangle(), new Square()];
renderLargeShapes(shapes);
```
**[⬆ back to top](#table-of-contents)**

### Interface Segregation Principle (ISP)
JavaScript doesn't have interfaces so this principle doesn't apply as strictly
as others. However, it's important and relevant even with JavaScript's lack of
type system.

ISP states that "Clients should not be forced to depend upon interfaces that
they do not use." Interfaces are implicit contracts in JavaScript because of
duck typing.

A good example to look at that demonstrates this principle in JavaScript is for
classes that require large settings objects. Not requiring clients to setup
huge amounts of options is beneficial, because most of the time they won't need
all of the settings. Making them optional helps prevent having a "fat interface".

**Bad:**
```javascript
class DOMTraverser {
  constructor(settings) {
    this.settings = settings;
    this.setup();
  }

  setup() {
    this.rootNode = this.settings.rootNode;
    this.animationModule.setup();
  }

  traverse() {
    // ...
  }
}

const $ = new DOMTraverser({
  rootNode: document.getElementsByTagName('body'),
  animationModule: function() {} // Most of the time, we won't need to animate when traversing.
  // ...
});

```

**Good**:
```javascript
class DOMTraverser {
  constructor(settings) {
    this.settings = settings;
    this.options = settings.options;
    this.setup();
  }

  setup() {
    this.rootNode = this.settings.rootNode;
    this.setupOptions();
  }

  setupOptions() {
    if (this.options.animationModule) {
      // ...
    }
  }

  traverse() {
    // ...
  }
}

const $ = new DOMTraverser({
  rootNode: document.getElementsByTagName('body'),
  options: {
    animationModule: function() {}
  }
});
```
**[⬆ back to top](#table-of-contents)**

### Dependency Inversion Principle (DIP)
This principle states two essential things:
1. High-level modules should not depend on low-level modules. Both should
depend on abstractions.
2. Abstractions should not depend upon details. Details should depend on
abstractions.

This can be hard to understand at first, but if you've worked with Angular.js,
you've seen an implementation of this principle in the form of Dependency
Injection (DI). While they are not identical concepts, DIP keeps high-level
modules from knowing the details of its low-level modules and setting them up.
It can accomplish this through DI. A huge benefit of this is that it reduces
the coupling between modules. Coupling is a very bad development pattern because
it makes your code hard to refactor.

As stated previously, JavaScript doesn't have interfaces so the abstractions
that are depended upon are implicit contracts. That is to say, the methods
and properties that an object/class exposes to another object/class. In the
example below, the implicit contract is that any Request module for an
`InventoryTracker` will have a `requestItems` method.

**Bad:**
```javascript
class InventoryTracker {
  constructor(items) {
    this.items = items;

    // BAD: We have created a dependency on a specific request implementation.
    // We should just have requestItems depend on a request method: `request`
    this.requester = new InventoryRequester();
  }

  requestItems() {
    this.items.forEach((item) => {
      this.requester.requestItem(item);
    });
  }
}

class InventoryRequester {
  constructor() {
    this.REQ_METHODS = ['HTTP'];
  }

  requestItem(item) {
    // ...
  }
}

const inventoryTracker = new InventoryTracker(['apples', 'bananas']);
inventoryTracker.requestItems();
```

**Good**:
```javascript
class InventoryTracker {
  constructor(items, requester) {
    this.items = items;
    this.requester = requester;
  }

  requestItems() {
    this.items.forEach((item) => {
      this.requester.requestItem(item);
    });
  }
}

class InventoryRequesterV1 {
  constructor() {
    this.REQ_METHODS = ['HTTP'];
  }

  requestItem(item) {
    // ...
  }
}

class InventoryRequesterV2 {
  constructor() {
    this.REQ_METHODS = ['WS'];
  }

  requestItem(item) {
    // ...
  }
}

// By constructing our dependencies externally and injecting them, we can easily
// substitute our request module for a fancy new one that uses WebSockets.
const inventoryTracker = new InventoryTracker(['apples', 'bananas'], new InventoryRequesterV2());
inventoryTracker.requestItems();
```
**[⬆ back to top](#table-of-contents)**

### Prefer ES2015/ES6 classes over ES5 plain functions
It's very difficult to get readable class inheritance, construction, and method
definitions for classical ES5 classes. If you need inheritance (and be aware
that you might not), then prefer classes. However, prefer small functions over
classes until you find yourself needing larger and more complex objects.

**Bad:**
```javascript
const Animal = function(age) {
    if (!(this instanceof Animal)) {
        throw new Error("Instantiate Animal with `new`");
    }

    this.age = age;
};

Animal.prototype.move = function() {};

const Mammal = function(age, furColor) {
    if (!(this instanceof Mammal)) {
        throw new Error("Instantiate Mammal with `new`");
    }

    Animal.call(this, age);
    this.furColor = furColor;
};

Mammal.prototype = Object.create(Animal.prototype);
Mammal.prototype.constructor = Mammal;
Mammal.prototype.liveBirth = function() {};

const Human = function(age, furColor, languageSpoken) {
    if (!(this instanceof Human)) {
        throw new Error("Instantiate Human with `new`");
    }

    Mammal.call(this, age, furColor);
    this.languageSpoken = languageSpoken;
};

Human.prototype = Object.create(Mammal.prototype);
Human.prototype.constructor = Human;
Human.prototype.speak = function() {};
```

**Good:**
```javascript
class Animal {
    constructor(age) {
        this.age = age;
    }

    move() {}
}

class Mammal extends Animal {
    constructor(age, furColor) {
        super(age);
        this.furColor = furColor;
    }

    liveBirth() {}
}

class Human extends Mammal {
    constructor(age, furColor, languageSpoken) {
        super(age, furColor);
        this.languageSpoken = languageSpoken;
    }

    speak() {}
}
```
**[⬆ back to top](#table-of-contents)**


### Use method chaining
Against the advice of Clean Code, this is one place where we will have to differ.
It has been argued that method chaining is unclean and violates the [Law of Demeter](https://en.wikipedia.org/wiki/Law_of_Demeter).
Maybe it's true, but this pattern is very useful in JavaScript and you see it in
many libraries such as jQuery and Lodash. It allows your code to be expressive,
and less verbose. For that reason, I say, use method chaining and take a look at
how clean your code will be. In your class functions, simply return `this` at
the end of every function, and you can chain further class methods onto it.

**Bad:**
```javascript
class Car {
  constructor() {
    this.make = 'Honda';
    this.model = 'Accord';
    this.color = 'white';
  }

  setMake(make) {
    this.make = make;
  }

  setModel(model) {
    this.model = model;
  }

  setColor(color) {
    this.color = color;
  }

  save() {
    console.log(this.make, this.model, this.color);
  }
}

const car = new Car();
car.setColor('pink');
car.setMake('Ford');
car.setModel('F-150')
car.save();
```

**Good**:
```javascript
class Car {
  constructor() {
    this.make = 'Honda';
    this.model = 'Accord';
    this.color = 'white';
  }

  setMake(make) {
    this.make = make;
    // NOTE: Returning this for chaining
    return this;
  }

  setModel(model) {
    this.model = model;
    // NOTE: Returning this for chaining
    return this;
  }

  setColor(color) {
    this.color = color;
    // NOTE: Returning this for chaining
    return this;
  }

  save() {
    console.log(this.make, this.model, this.color);
    // NOTE: Returning this for chaining
    return this;
  }
}

const car = new Car()
  .setColor('pink')
  .setMake('Ford')
  .setModel('F-150')
  .save();
```
**[⬆ back to top](#table-of-contents)**

### Prefer composition over inheritance
As stated famously in [*Design Patterns*](https://en.wikipedia.org/wiki/Design_Patterns) by the Gang of Four,
you should prefer composition over inheritance where you can. There are lots of
good reasons to use inheritance and lots of good reasons to use composition.
The main point for this maxim is that if your mind instinctively goes for
inheritance, try to think if composition could model your problem better. In some
cases it can.

You might be wondering then, "when should I use inheritance?" It
depends on your problem at hand, but this is a decent list of when inheritance
makes more sense than composition:

1. Your inheritance represents an "is-a" relationship and not a "has-a"
relationship (Animal->Human vs. User->UserDetails).
2. You can reuse code from the base classes (Humans can move like all animals).
3. You want to make global changes to derived classes by changing a base class.
(Change the caloric expenditure of all animals when they move).

**Bad:**
```javascript
class Employee {
  constructor(name, email) {
    this.name = name;
    this.email = email;
  }

  // ...
}

// Bad because Employees "have" tax data. EmployeeTaxData is not a type of Employee
class EmployeeTaxData extends Employee {
  constructor(ssn, salary) {
    super();
    this.ssn = ssn;
    this.salary = salary;
  }

  // ...
}
```

**Good**:
```javascript
class Employee {
  constructor(name, email) {
    this.name = name;
    this.email = email;

  }

  setTaxData(ssn, salary) {
    this.taxData = new EmployeeTaxData(ssn, salary);
  }
  // ...
}

class EmployeeTaxData {
  constructor(ssn, salary) {
    this.ssn = ssn;
    this.salary = salary;
  }

  // ...
}
```
**[⬆ back to top](#table-of-contents)**

## **Testing**
Testing is more important than shipping. If you have have no tests or an
inadequate amount, then every time you ship code you won't be sure that you
didn't break anything. Deciding on what constitutes an adequate amount is up
to your team, but having 100% coverage (all statements and branches) is how
you achieve very high confidence and developer peace of mind. This means that
in addition to having a great testing framework, you also need to use a
[good coverage tool](http://gotwarlost.github.io/istanbul/).

There's no excuse to not write tests. There's [plenty of good JS test frameworks]
(http://jstherightway.org/#testing-tools), so find one that your team prefers.
When you find one that works for your team, then aim to always write tests
for every new feature/module you introduce. If your preferred method is
Test Driven Development (TDD), that is great, but the main point is to just
make sure you are reaching your coverage goals before launching any feature,
or refactoring an existing one.

### Single concept per test

**Bad:**
```javascript
const assert = require('assert');

describe('MakeMomentJSGreatAgain', function() {
  it('handles date boundaries', function() {
    let date;

    date = new MakeMomentJSGreatAgain('1/1/2015');
    date.addDays(30);
    date.shouldEqual('1/31/2015');

    date = new MakeMomentJSGreatAgain('2/1/2016');
    date.addDays(28);
    assert.equal('02/29/2016', date);

    date = new MakeMomentJSGreatAgain('2/1/2015');
    date.addDays(28);
    assert.equal('03/01/2015', date);
  });
});
```

**Good**:
```javascript
const assert = require('assert');

describe('MakeMomentJSGreatAgain', function() {
  it('handles 30-day months', function() {
    const date = new MakeMomentJSGreatAgain('1/1/2015');
    date.addDays(30);
    date.shouldEqual('1/31/2015');
  });

  it('handles leap year', function() {
    const date = new MakeMomentJSGreatAgain('2/1/2016');
    date.addDays(28);
    assert.equal('02/29/2016', date);
  });

  it('handles non-leap year', function() {
    const date = new MakeMomentJSGreatAgain('2/1/2015');
    date.addDays(28);
    assert.equal('03/01/2015', date);
  });
});
```
**[⬆ back to top](#table-of-contents)**

## **Concurrency**
### Use Promises, not callbacks
Callbacks aren't clean, and they cause excessive amounts of nesting. With ES2015/ES6,
Promises are a built-in global type. Use them!

**Bad:**
```javascript
require('request').get('https://en.wikipedia.org/wiki/Robert_Cecil_Martin', function(err, response) {
  if (err) {
    console.error(err);
  }
  else {
    require('fs').writeFile('article.html', response.body, function(err) {
      if (err) {
        console.error(err);
      } else {
        console.log('File written');
      }
    })
  }
})

```

**Good**:
```javascript
require('request-promise').get('https://en.wikipedia.org/wiki/Robert_Cecil_Martin')
  .then(function(response) {
    return require('fs-promise').writeFile('article.html', response);
  })
  .then(function() {
    console.log('File written');
  })
  .catch(function(err) {
    console.error(err);
  })

```
**[⬆ back to top](#table-of-contents)**

### Async/Await are even cleaner than Promises
Promises are a very clean alternative to callbacks, but ES2017/ES8 brings async and await
which offer an even cleaner solution. All you need is a function that is prefixed
in an `async` keyword, and then you can write your logic imperatively without
a `then` chain of functions. Use this if you can take advantage of ES2017/ES8 features
today!

**Bad:**
```javascript
require('request-promise').get('https://en.wikipedia.org/wiki/Robert_Cecil_Martin')
  .then(function(response) {
    return require('fs-promise').writeFile('article.html', response);
  })
  .then(function() {
    console.log('File written');
  })
  .catch(function(err) {
    console.error(err);
  })

```

**Good**:
```javascript
async function getCleanCodeArticle() {
  try {
    const request = await require('request-promise')
    const response = await request.get('https://en.wikipedia.org/wiki/Robert_Cecil_Martin');
    const fileHandle = await require('fs-promise');

    await fileHandle.writeFile('article.html', response);
    console.log('File written');
  } catch(err) {
    console.log(err);
  }
}
```
**[⬆ back to top](#table-of-contents)**


## **Error Handling**
Thrown errors are a good thing! They mean the runtime has successfully
identified when something in your program has gone wrong and it's letting
you know by stopping function execution on the current stack, killing the
process (in Node), and notifying you in the console with a stack trace.

### Don't ignore caught errors
Doing nothing with a caught error doesn't give you the ability to ever fix
or react to said error. Logging the error to the console (`console.log`)
isn't much better as often times it can get lost in a sea of things printed
to the console. If you wrap any bit of code in a `try/catch` it means you
think an error may occur there and therefore you should have a plan,
or create a code path, for when it occurs.

**Bad:**
```javascript
try {
  functionThatMightThrow();
} catch (error) {
  console.log(error);
}
```

**Good:**
```javascript
try {
  functionThatMightThrow();
} catch (error) {
  // One option (more noisy than console.log):
  console.error(error);
  // Another option:
  notifyUserOfError(error);
  // Another option:
  reportErrorToService(error);
  // OR do all three!
}
```

### Don't ignore rejected promises
For the same reason you shouldn't ignore caught errors
from `try/catch`.

**Bad:**
```javascript
getdata()
.then(data => {
  functionThatMightThrow(data);
})
.catch(error => {
  console.log(error);
});
```

**Good:**
```javascript
getdata()
.then(data => {
  functionThatMightThrow(data);
})
.catch(error => {
  // One option (more noisy than console.log):
  console.error(error);
  // Another option:
  notifyUserOfError(error);
  // Another option:
  reportErrorToService(error);
  // OR do all three!
});
```

**[⬆ back to top](#table-of-contents)**


## **Formatting**
Formatting is subjective. Like many rules herein, there is no hard and fast
rule that you must follow. The main point is DO NOT ARGUE over formatting.
There are [tons of tools](http://standardjs.com/rules.html) to automate this.
Use one! It's a waste of time and money for engineers to argue over formatting.

For things that don't fall under the purview of automatic formatting
(indentation, tabs vs. spaces, double vs. single quotes, etc.) look here
for some guidance.

### Use consistent capitalization
JavaScript is untyped, so capitalization tells you a lot about your variables,
functions, etc. These rules are subjective, so your team can choose whatever
they want. The point is, no matter what you all choose, just be consistent.

**Bad:**
```javascript
const DAYS_IN_WEEK = 7;
const daysInMonth = 30;

const songs = ['Back In Black', 'Stairway to Heaven', 'Hey Jude'];
const Artists = ['ACDC', 'Led Zeppelin', 'The Beatles'];

function eraseDatabase() {}
function restore_database() {}

class animal {}
class Alpaca {}
```

**Good**:
```javascript
const DAYS_IN_WEEK = 7;
const DAYS_IN_MONTH = 30;

const songs = ['Back In Black', 'Stairway to Heaven', 'Hey Jude'];
const artists = ['ACDC', 'Led Zeppelin', 'The Beatles'];

function eraseDatabase() {}
function restoreDatabase() {}

class Animal {}
class Alpaca {}
```
**[⬆ back to top](#table-of-contents)**


### Function callers and callees should be close
If a function calls another, keep those functions vertically close in the source
file. Ideally, keep the caller right above the callee. We tend to read code from
top-to-bottom, like a newspaper. Because of this, make your code read that way.

**Bad:**
```javascript
class PerformanceReview {
  constructor(employee) {
    this.employee = employee;
  }

  lookupPeers() {
    return db.lookup(this.employee, 'peers');
  }

  lookupMananger() {
    return db.lookup(this.employee, 'manager');
  }

  getPeerReviews() {
    const peers = this.lookupPeers();
    // ...
  }

  perfReview() {
      this.getPeerReviews();
      this.getManagerReview();
      this.getSelfReview();
  }

  getManagerReview() {
    const manager = this.lookupManager();
  }

  getSelfReview() {
    // ...
  }
}

let review = new PerformanceReview(user);
review.perfReview();
```

**Good**:
```javascript
class PerformanceReview {
  constructor(employee) {
    this.employee = employee;
  }

  perfReview() {
      this.getPeerReviews();
      this.getManagerReview();
      this.getSelfReview();
  }

  getPeerReviews() {
    const peers = this.lookupPeers();
    // ...
  }

  lookupPeers() {
    return db.lookup(this.employee, 'peers');
  }

  getManagerReview() {
    const manager = this.lookupManager();
  }

  lookupMananger() {
    return db.lookup(this.employee, 'manager');
  }

  getSelfReview() {
    // ...
  }
}

let review = new PerformanceReview(employee);
review.perfReview();
```

**[⬆ back to top](#table-of-contents)**

## **Comments**
### Only comment things that have business logic complexity.
Comments are an apology, not a requirement. Good code *mostly* documents itself.

**Bad:**
```javascript
function hashIt(data) {
  // The hash
  let hash = 0;

  // Length of string
  const length = data.length;

  // Loop through every character in data
  for (let i = 0; i < length; i++) {
    // Get character code.
    const char = data.charCodeAt(i);
    // Make the hash
    hash = ((hash << 5) - hash) + char;
    // Convert to 32-bit integer
    hash = hash & hash;
  }
}
```

**Good**:
```javascript

function hashIt(data) {
  let hash = 0;
  const length = data.length;

  for (let i = 0; i < length; i++) {
    const char = data.charCodeAt(i);
    hash = ((hash << 5) - hash) + char;

    // Convert to 32-bit integer
    hash = hash & hash;
  }
}

```
**[⬆ back to top](#table-of-contents)**

### Don't leave commented out code in your codebase
Version control exists for a reason. Leave old code in your history.

**Bad:**
```javascript
doStuff();
// doOtherStuff();
// doSomeMoreStuff();
// doSoMuchStuff();
```

**Good**:
```javascript
doStuff();
```
**[⬆ back to top](#table-of-contents)**

### Don't have journal comments
Remember, use version control! There's no need for dead code, commented code,
and especially journal comments. Use `git log` to get history!

**Bad:**
```javascript
/**
 * 2016-12-20: Removed monads, didn't understand them (RM)
 * 2016-10-01: Improved using special monads (JP)
 * 2016-02-03: Removed type-checking (LI)
 * 2015-03-14: Added combine with type-checking (JR)
 */
function combine(a, b) {
  return a + b;
}
```

**Good**:
```javascript
function combine(a, b) {
  return a + b;
}
```
**[⬆ back to top](#table-of-contents)**

### Avoid positional markers
They usually just add noise. Let the functions and variable names along with the
proper indentation and formatting give the visual structure to your code.

**Bad:**
```javascript
////////////////////////////////////////////////////////////////////////////////
// Scope Model Instantiation
////////////////////////////////////////////////////////////////////////////////
const $scope.model = {
  menu: 'foo',
  nav: 'bar'
};

////////////////////////////////////////////////////////////////////////////////
// Action setup
////////////////////////////////////////////////////////////////////////////////
const actions = function() {
  // ...
}
```

**Good**:
```javascript
const $scope.model = {
  menu: 'foo',
  nav: 'bar'
};

const actions = function() {
  // ...
}
```
**[⬆ back to top](#table-of-contents)**