# 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)

11. [Translation](#translation)

## 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 fewer 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 currentDate = 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. Tools like

[buddy.js](https://github.com/danielstjules/buddy.js) and

[ESLint](https://github.com/eslint/eslint/blob/660e0918933e6e7fede26bc675a0763a6b357c94/docs/rules/no-magic-numbers.md)

can help identify unnamed constants.

**Bad:**

```javascript

// What the heck is 86400000 for?

setTimeout(blastOff, 86400000);

```

**Good:**

```javascript

// Declare them as capitalized `const` globals.

const MILLISECONDS_IN_A_DAY = 86400000;

setTimeout(blastOff, MILLISECONDS_IN_A_DAY);

```

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

### Use explanatory variables

**Bad:**

```javascript

const address = 'One Infinite Loop, Cupertino 95014';

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

saveCityZipCode(address.match(cityZipCodeRegex)[1], address.match(cityZipCodeRegex)[2]);

```

**Good:**

```javascript

const address = 'One Infinite Loop, Cupertino 95014';

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

const [, city, zipCode] = address.match(cityZipCodeRegex) || [];

saveCityZipCode(city, zipCode);

```

**[⬆ 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)**

### Use default arguments instead of short circuiting or conditionals

Default arguments are often cleaner than short circuiting. Be aware that if you

use them, your function will only provide default values for `undefined`

arguments. Other "falsy" values such as `''`, `""`, `false`, `null`, `0`, and

`NaN`, will not be replaced by a default value.

**Bad:**

```javascript

function createMicrobrewery(name) {

const breweryName = name || 'Hipster Brew Co.';

// ...

}

```

**Good:**

```javascript

function createMicrobrewery(breweryName = 'Hipster Brew Co.') {

// ...

}

```

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

## **Functions**

### Function arguments (2 or fewer 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.

One or two arguments is the ideal case, and three should be avoided if possible.

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 you 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.

To make it obvious what properties the function expects, you can use the es6

destructuring syntax. This has a few advantages:

1. When someone looks at the function signature, it's immediately clear what

properties are being used.

2. Destructuring also clones the specified primitive values of the argument

object passed into the function. This can help prevent side effects. Note:

objects and arrays that are destructured from the argument object are NOT

cloned.

3. Linters can warn you about unused properties, which would be impossible

without destructuring.

**Bad:**

```javascript

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

// ...

}

```

**Good:**

```javascript

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

// ...

}

createMenu({

title: 'Foo',

body: 'Bar',

buttonText: 'Baz',

cancellable: true

});

```

**[⬆ 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 addToDate(date, month) {

// ...

}

const date = new Date();

// It's hard to to tell from the function name what is added

addToDate(date, 1);

```

**Good:**

```javascript

function addMonthToDate(month, date) {

// ...

}

const date = new Date();

addMonthToDate(1, date);

```

**[⬆ 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

Do your absolute best to avoid duplicate code. Duplicate code is bad because it

means that there's more than one place to alter something if you need to change

some logic.

Imagine if you run a restaurant and you keep track of your inventory: all your

tomatoes, onions, garlic, spices, etc. If you have multiple lists that

you keep this on, then all have to be updated when you serve a dish with

tomatoes in them. If you only have one list, there's only one place to update!

Oftentimes you have duplicate code because you have two or more slightly

different things, that share a lot in common, but their differences force you

to have two or more separate functions that do much of the same things. Removing

duplicate code means creating an abstraction that can handle this set of

different things with just one function/module/class.

Getting the abstraction right is critical, that's why you should follow the

SOLID principles laid out in the *Classes* section. Bad abstractions can be

worse than duplicate code, so be careful! Having said this, if you can make

a good abstraction, do it! Don't repeat yourself, otherwise you'll find yourself

updating multiple places anytime you want to change one thing.

**Bad:**

```javascript

function showDeveloperList(developers) {

developers.forEach((developer) => {

const expectedSalary = developer.calculateExpectedSalary();

const experience = developer.getExperience();

const githubLink = developer.getGithubLink();

const data = {

expectedSalary,

experience,

githubLink

};

render(data);

});

}

function showManagerList(managers) {

managers.forEach((manager) => {

const expectedSalary = manager.calculateExpectedSalary();

const experience = manager.getExperience();

const portfolio = manager.getMBAProjects();

const data = {

expectedSalary,

experience,

portfolio

};

render(data);

});

}

```

**Good:**

```javascript

function showEmployeeList(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,

experience,

portfolio

};

render(data);

});

}

```

**[⬆ 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 (part 1)

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)**

### Avoid Side Effects (part 2)

In JavaScript, primitives are passed by value and objects/arrays are passed by

reference. In the case of objects and arrays, if our function makes a change

in a shopping cart array, for example, by adding an item to purchase,

then any other function that uses that `cart` array will be affected by this

addition. That may be great, however it can be bad too. Let's imagine a bad

situation:

The user clicks the "Purchase", button which calls a `purchase` function that

spawns a network request and sends the `cart` array to the server. Because

of a bad network connection, the `purchase` function has to keep retrying the

request. Now, what if in the meantime the user accidentally clicks "Add to Cart"

button on an item they don't actually want before the network request begins?

If that happens and the network request begins, then that purchase function

will send the accidentally added item because it has a reference to a shopping

cart array that the `addItemToCart` function modified by adding an unwanted

item.

A great solution would be for the `addItemToCart` to always clone the `cart`,

edit it, and return the clone. This ensures that no other functions that are

holding onto a reference of the shopping cart will be affected by any changes.

Two caveats to mention to this approach:

1. There might be cases where you actually want to modify the input object,

but when you adopt this programming practice you will find that those case

are pretty rare. Most things can be refactored to have no side effects!

2. Cloning big objects can be very expensive in terms of performance. Luckily,

this isn't a big issue in practice because there are

[great libraries](https://facebook.github.io/immutable-js/) that allow

this kind of programming approach to be fast and not as memory intensive as

it would be for you to manually clone objects and arrays.

**Bad:**

```javascript

const addItemToCart = (cart, item) => {

cart.push({ item, date: Date.now() });

};

```

**Good:**

```javascript

const addItemToCart = (cart, item) => {

return [...cart, { item, date : Date.now() }];

};

```

**[⬆ 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 diff(comparisonArray) {

const hash = new Set(comparisonArray);

return this.filter(elem => !hash.has(elem));

};

```

**Good:**

```javascript

class SuperArray extends Array {

diff(comparisonArray) {

const hash = new Set(comparisonArray);

return this.filter(elem => !hash.has(elem));

}

}

```

**[⬆ 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 INITIAL_VALUE = 0;

const totalOutput = programmerOutput

.map((programmer) => programmer.linesOfCode)

.reduce((acc, linesOfCode) => acc + linesOfCode, INITIAL_VALUE);

```

**[⬆ 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 this.getMaxAltitude() - this.getPassengerCount();

case 'Air Force One':

return this.getMaxAltitude();

case 'Cessna':

return this.getMaxAltitude() - this.getFuelExpenditure();

}

}

}

```

**Good:**

```javascript

class Airplane {

// ...

}

class Boeing777 extends Airplane {

// ...

getCruisingAltitude() {

return this.getMaxAltitude() - this.getPassengerCount();

}

}

class AirForceOne extends Airplane {

// ...

getCruisingAltitude() {

return this.getMaxAltitude();

}

}

class Cessna extends Airplane {

// ...

getCruisingAltitude() {

return this.getMaxAltitude() - this.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.pedal(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;

}

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:

* 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.

* Makes adding validation simple when doing a `set`.

* Encapsulates the internal representation.

* Easy to add logging and error handling when getting and setting.

* Inheriting this class, you can override default functionality.

* 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 -= 100;

```

**Good:**

```javascript

class BankAccount {

constructor(balance = 1000) {

this._balance = balance;

}

// It doesn't have to be prefixed with `get` or `set` to be a getter/setter

set balance(amount) {

if (this.verifyIfAmountCanBeSetted(amount)) {

this._balance = amount;

}

}

get balance() {

return this._balance;

}

verifyIfAmountCanBeSetted(val) {

// ...

}

}

const bankAccount = new BankAccount();

// Buy shoes...

bankAccount.balance -= shoesPrice;

// Get balance

let balance = bankAccount.balance;

```

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

### Make objects have private members

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

**Bad:**

```javascript