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# 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. [Formatting](#formatting)

9. [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

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

```

**Good**:

```javascript

var 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 (var i = 0; i < 525600; i++) {

runCronJob();

}

```

**Good**:

```javascript

// Declare them as capitalized `var` globals.

var MINUTES_IN_A_YEAR = 525600;

for (var i = 0; i < MINUTES_IN_A_YEAR; i++) {

runCronJob();

}

```

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

### Use explanatory variables

**Bad:**

```javascript

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

saveCityState(cityStateRegex.match(cityStateRegex)[1], cityStateRegex.match(cityStateRegex)[2]);

```

**Good**:

```javascript

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

let match = cityStateRegex.match(cityStateRegex)

let city = match[1];

let state = match[2];

saveCityState(city, state);

```

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

### Avoid Mental Mapping

Explicit is better than implicit.

**Bad:**

```javascript

var locations = ['Austin', 'New York', 'San Francisco'];

locations.forEach((l) => {

doStuff();

doSomeOtherStuff();

...

// Wait, what is `l` for again?

dispatch(l);

});

```

**Good**:

```javascript

var 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

var Car = {

carMake: 'Honda',

carModel: 'Accord',

carColor: 'Blue'

};

function paintCar(car) {

car.carColor = 'Red';

}

```

**Good**:

```javascript

var 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) {

var breweryName;

if (name) {

breweryName = name;

} else {

breweryName = 'Hipster Brew Co.';

}

```

**Good**:

```javascript

function createMicrobrewery(name) {

var 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

var 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 => {

let clientRecord = database.lookup(client);

if (clientRecord.isActive()) {

email(client);

}

});

}

```

**Good**:

```javascript

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();

}

```

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

### Function names should say what they do

**Bad:**

```javascript

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**:

```javascript

function dateAddMonth(date, month) {

// ...

}

let 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) {

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**:

```javascript

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...

})

}

```

**[⬆ 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!

**Bad:**

```javascript

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**:

```javascript

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);

});

}

```

**[⬆ 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

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.title || 'Baz'

config.cancellable = config.cancellable === undefined ? config.cancellable : true;

}

createMenu(menuConfig);

```

**Good**:

```javascript

var menuConfig = {

title: null,

body: 'Bar',

buttonText: null,

cancellable: true

}

function createMenu(config) {

Object.assign(config, {

title: 'Foo',

body: 'Bar',

buttonText: 'Baz',

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 createTempFile(name) {

fs.create('./temp/' + name);

}

function createFile(name) {

fs.create(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

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:**

```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.

var name = 'Ryan McDermott';

function splitIntoFirstAndLastName() {

name = name.split(' ');

}

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

```

**Good**:

```javascript

function splitIntoFirstAndLastName(name) {

return name.split(' ');

}

var name = 'Ryan McDermott'

var 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 ES6 classes and simply extend the `Array` global.

**Bad:**

```javascript

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:**

```javascript

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;

}

```

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

### 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:**

```javascript

const programmerOutput = [

{

linesOfCode: 500

}, {

linesOfCode: 1500

}, {

linesOfCode: 150

}, {

linesOfCode: 1000

}

];

var totalOutput = 0;

for (var i = 0; i < programmerOutput.length; i++) {

totalOutput += programmerOutput[i].linesOfCode;

}

```

**Good**:

```javascript

const programmerOutput = [

{

linesOfCode: 500

}, {

linesOfCode: 1500

}, {

linesOfCode: 150

}, {

linesOfCode: 1000

}

];

var 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 'Cesna':

return getMaxAltitude() - getFuelExpenditure();

}

```

**Good**:

```javascript

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();

}

```

**[⬆ 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 (obj instanceof Bicycle) {

vehicle.peddle(this.currentLocation, new Location('texas'));

} else if (obj 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 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**:

```javascript

for (var 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) {

// ...

}

var req = newRequestModule;

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

```

**Good**:

```javascript

function newRequestModule(url) {

// ...

}

var 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;

}

let 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;

}

let 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

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**:

```javascript

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

```

**[⬆ 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 functioanlity 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.

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