clean-code-javascript

Table of Contents

1. Introduction
2. Variables
3. Functions
4. Objects and Data Structures
5. Classes
6. Testing
7. Concurrency
8. Formatting
9. Comments

Introduction

Software engineering principles, from Robert C. Martin's book Clean Code, 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:

var yyyymmdstr = moment().format('YYYY/MM/DD');

Good:

var yearMonthDay = moment().format('YYYY/MM/DD');

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Use the same vocabulary for the same type of variable

Bad:

getUserInfo();
getClientData();
getCustomerRecord();

Good:

getUser();

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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:

// What the heck is 525600 for?
for (var i = 0; i < 525600; i++) {
  runCronJob();
}

Good:

// Declare them as capitalized `var` globals.
var MINUTES_IN_A_YEAR = 525600;
for (var i = 0; i < MINUTES_IN_A_YEAR; i++) {
  runCronJob();
}

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Use explanatory variables

Bad:

let cityStateRegex = /^(.+)[,\\s]+(.+?)\s*(\d{5})?$/;
saveCityState(cityStateRegex.match(cityStateRegex)[1], cityStateRegex.match(cityStateRegex)[2]);

Good:

let cityStateRegex = /^(.+)[,\\s]+(.+?)\s*(\d{5})?$/;
let match = cityStateRegex.match(cityStateRegex)
let city = match[1];
let state = match[2];
saveCityState(city, state);

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Avoid Mental Mapping

Explicit is better than implicit.

Bad:

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

Good:

var locations = ['Austin', 'New York', 'San Francisco'];
locations.forEach((location) => {
  doStuff();
  doSomeOtherStuff();
  ...
  ...
  ...
  dispatch(location);
});

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Don't add unneeded context

If your class/object name tells you something, don't repeat that in your variable name.

Bad:

var Car = {
  carMake: 'Honda',
  carModel: 'Accord',
  carColor: 'Blue'
};

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

Good:

var Car = {
  make: 'Honda',
  model: 'Accord',
  color: 'Blue'
};

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

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Short-circuiting is cleaner than conditionals

Bad:

function createMicrobrewery(name) {
  var breweryName;
  if (name) {
    breweryName = name;
  } else {
    breweryName = 'Hipster Brew Co.';
  }
}

Good:

function createMicrobrewery(name) {
  var breweryName = name || 'Hipster Brew Co.'
}

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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:

function createMenu(title, body, buttonText, cancellable) {
  ...
}

Good:

var menuConfig = {
  title: 'Foo',
  body: 'Bar',
  buttonText: 'Baz',
  cancellable: true
}

function createMenu(menuConfig) {
  ...
}

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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:

function emailClients(clients) {
  clients.forEach(client => {
    let clientRecord = database.lookup(client);
    if (clientRecord.isActive()) {
      email(client);
    }
  });
}

Good:

function emailClients(clients) {
  clients.forEach(client => {
    emailClientIfNeeded(client);
  });
}

function emailClientIfNeeded(client) {
  if (isClientActive(client)) {
    email(client);
  }
}

function isClientActive(client) {
  let clientRecord = database.lookup(client);
  return clientRecord.isActive();
}

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Function names should say what they do

Bad:

function dateAdd(date, month) {
  // ...
}

let date = new Date();

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

Good:

function dateAddMonth(date, month) {
  // ...
}

let date = new Date();
dateAddMonth(date, 1);

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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:

function parseBetterJSAlternative(code) {
  let REGEXES = [
    // ...
  ];

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

  let ast;
  tokens.forEach((token) => {
    // lex...
  });

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

Good:

function tokenize(code) {
  let REGEXES = [
    // ...
  ];

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

  return tokens;
}

function lexer(tokens) {
  let ast;
  tokens.forEach((token) => {
    // lex...
  });

  return ast;
}

function parseBetterJSAlternative(code) {
  let tokens = tokenize(code);
  let ast = lexer(tokens);
  ast.forEach((node) => {
    // parse...
  })  
}

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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!

Bad:

function showDeveloperList(developers) {
  developers.forEach(developers => {
    var expectedSalary = developer.calculateExpectedSalary();
    var experience = developer.getExperience();
    var githubLink = developer.getGithubLink();
    var data = {
      expectedSalary: expectedSalary,
      experience: experience,
      githubLink: githubLink
    };

    render(data);
  });
}

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

    render(data);
  });
}

Good:

function showList(employees) {
  employees.forEach(employee => {
    var expectedSalary = employee.calculateExpectedSalary();
    var experience = employee.getExperience();
    var portfolio;

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

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

    render(data);
  });
}

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Use default arguments instead of short circuiting

Bad:

function writeForumComment(subject, body) {
  subject = subject || 'No Subject';
  body = body || 'No text';
}

Good:

function writeForumComment(subject = 'No subject', body = 'No text') {
  ...
}

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Set default objects with Object.assign

Bad:

var 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:

var 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);

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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:

function createFile(name, temp) {
  if (temp) {
    fs.create('./temp/' + name);
  } else {
    fs.create(name);
  }
}

Good:

function createTempFile(name) {
  fs.create('./temp/' + name);
}

function createFile(name) {
  fs.create(name);
}

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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 Nigerian prince.

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:

// 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.
var name = 'Ryan McDermott';

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

splitIntoFirstAndLastName();

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

Good:

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

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

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

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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 ES6 classes and simply extend the Array global.

Bad:

Array.prototype.diff = function(comparisonArray) {
  var values = [];
  var hash = {};

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

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

  return values;
}

Good:

class SuperArray extends Array {
  constructor(...args) {
    super(...args);
  }

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

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

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

    return values;
  }
}

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Favor functional programming over imperative programming

If Haskell were an IPA then JavaScript would be an O'Douls. That is to say, 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:

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

var totalOutput = 0;

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

Good:

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

var totalOutput = programmerOutput
  .map((programmer) => programmer.linesOfCode)
  .reduce((acc, linesOfCode) => acc + linesOfCode, 0);

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Encapsulate conditionals

Bad:

if (fsm.state === 'fetching' && isEmpty(listNode)) {
  /// ...
}

Good:

function shouldShowSpinner(fsm, listNode) {
  return fsm.state === 'fetching' && isEmpty(listNode);
}

if (shouldShowSpinner(fsmInstance, listNodeInstance)) {
  // ...
}

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Avoid negative conditionals

Bad:

function isDOMNodeNotPresent(node) {
  // ...
}

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

Good:

function isDOMNodePresent(node) {
  // ...
}

if (isDOMNodePresent(node)) {
  // ...
}

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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:

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

Good:

class Airplane {
  //...
}

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

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

class Cesna extends Airplane {
  //...
  getCruisingAltitude() {
    return getMaxAltitude() - getFuelExpenditure();
  }
}

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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:

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:

function travelToTexas(vehicle) {
  vehicle.move(this.currentLocation, new Location('texas'));
}

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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:

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:

function combine(val1, val2) {
  return val1 + val2;
}

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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 for seeing where optimization is lacking. Target those in the meantime, until they are fixed if they can be.

Bad:

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

Good:

for (var i = 0; i < list.length; i++) {
  // ...
}

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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:

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

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

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

Good:

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

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

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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:

class BankAccount {
  constructor() {
	   this.balance = 1000;
  }
}

let bankAccount = new BankAccount();

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

Good:

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;
  	}
  }
}

let bankAccount = new BankAccount();

// Buy shoes...
bankAccount.withdraw(100);

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Make objects have private members

This can be accomplished through closures (for ES5 and below).

Bad:

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

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

var 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:

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

  return Employee;
}());

var 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

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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:

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

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

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

Good:

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()) {
      // ...
    }
  }
}

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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:

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:

class AjaxRequester {
  constructor() {
    this.HTTP_METHODS = ['POST', 'PUT', 'GET'];
  }

  get(url) {
    // ...
  }

  addHTTPMethod(method) {
    this.HTTP_METHODS.push(method);
  }
}

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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:

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:

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);
    }

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

let shapes = [new Rectangle(), new Rectangle(), new Square()];
renderLargeShapes(shapes);

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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:

class DOMTraverser {
  constructor(settings) {
    this.settings = settings;
    this.setup();
  }

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

  traverse() {
    // ...
  }
}

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

Good:

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() {
    // ...
  }
}

let $ = new DOMTraverser({
  rootNode: document.getElementsByTagName('body'),
  options: {  
    animationModule: function() {}
  }
});

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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:

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) {
    // ...
  }
}

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

Good:

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.
let inventoryTracker = new InventoryTracker(['apples', 'bananas'], new InventoryRequesterV2());
inventoryTracker.requestItems();

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Prefer 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:

var Animal = function(age) {
    if (!(this instanceof Animal)) {
        throw new Error("Instantiate Animal with `new`");
    }

    this.age = age;
};

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

var 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() {};

var 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:

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() {}
}

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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. 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:

class Car {
  constructor() {
    this.make = 'Honda';
    this.model = 'Accord';
    this.color = 'white';
  }

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

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

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

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

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

Good:

class Car {
  constructor() {
    this.make = 'Honda';
    this.model = 'Accord';
    this.color = 'white';
  }

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

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

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Prefer composition over inheritance

As stated famously in 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:

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:

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;
  }

  // ...
}

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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.

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:

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:

const assert = require('assert');

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

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

  it('handles non-leap year', function() {
    let date = new MakeMomentJSGreatAgain('2/1/2015');
    date.addDays(28);
    assert.equal('03/01/2015', date);
  });
});

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Concurrency

Use Promises, not callbacks

Callbacks aren't clean, and they cause excessive amounts of nesting. With ES6, Promises are a built-in global type. Use them!

Bad:

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:

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.log(err);
  })

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Async/Await are even cleaner than Promises

Promises are a very clean alternative to callbacks, but ES7 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 ES7 features today!

Bad:

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.log(err);
  })

Good:

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

    await fileHandle.writeFile('article.html', response);
    console.log('File written');
  } catch(err) {
      console.log(err);
    }
  }

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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 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:

var DAYS_IN_WEEK = 7;
var daysInMonth = 30;

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

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

class animal {}
class Alpaca {}

Good:

var DAYS_IN_WEEK = 7;
var DAYS_IN_MONTH = 30;

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

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

class Animal {}
class Alpaca {}

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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:

class PerformanceReview {
  constructor(employee) {
    this.employee = employee;
  }

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

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

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

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

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

  getSelfReview() {
    // ...
  }
}

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

Good:

class PerformanceReview {
  constructor(employee) {
    this.employee = employee;
  }

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

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

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

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

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

  getSelfReview() {
    // ...
  }
}

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

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Comments

Only comment things that have business logic complexity.

Comments are an apology, not a requirement. Good code mostly documents itself.

Bad:

function hashIt(data) {
  // The hash
  var hash = 0;

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

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

Good:

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

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

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

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Don't leave commented code in your codebase

Version control exists for a reason. Leave old code in your history.

Bad:

doStuff();
// doOtherStuff();
// doSomeMoreStuff();
// doSoMuchStuff();

Good:

doStuff();

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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:

/**
 * 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:

function combine(a, b) {
  return a + b;
}

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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:

////////////////////////////////////////////////////////////////////////////////
// Scope Model Instantiation
////////////////////////////////////////////////////////////////////////////////
let $scope.model = {
  menu: 'foo',
  nav: 'bar'
};

////////////////////////////////////////////////////////////////////////////////
// Action setup
////////////////////////////////////////////////////////////////////////////////
let actions = function() {
  // ...
}

Good:

let $scope.model = {
  menu: 'foo',
  nav: 'bar'
};

let actions = function() {
  // ...
}

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Avoid legal comments in source files

That's what your LICENSE file at the top of your source tree is for.

Bad:

/*
The MIT License (MIT)

Copyright (c) 2016 Ryan McDermott

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE
*/

function calculateBill() {
  // ...
}

Good:

function calculateBill() {
  // ...
}

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