## 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. [SOLID](#solid)

7. [Testing](#testing)

8. [Concurrency](#concurrency)

9. [Error Handling](#error-handling)

10. [Formatting](#formatting)

11. [Comments](#comments)

12. [Translation](#translation)

## Introduction

![Humorous image of software quality estimation as a count of how many expletives

you shout when reading code](https://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](https://github.com/ryanmcdermott/3rs-of-software-architecture) 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:**

**Good:**

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

**Bad:**

**Good:**

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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. 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:**

**Good:**

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### Use explanatory variables

**Bad:**

**Good:**

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### Avoid Mental Mapping

Explicit is better than implicit.

**Bad:**

**Good:**

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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:**

**Good:**

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### 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:**

**Good:**

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## **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

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 ES2015/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. It can be used to simulate named parameters.

3. 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.

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

without destructuring.

**Bad:**

**Good:**

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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, it 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:**

**Good:**

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### Function names should say what they do

**Bad:**

**Good:**

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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

**Bad:**

**Good:**

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### 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:**

**Good:**

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### Set default objects with Object.assign

**Bad:**

**Good:**

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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:**

fs.create(`./temp/${name}`);

**Good:**

createFile(`./temp/${name}`);

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### 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

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:**

**Good:**

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### Avoid Side Effects (part 2)

In JavaScript, some values are unchangeable (immutable) and some are changeable

(mutable). Objects and arrays are two kinds of mutable values so it's important

to handle them carefully when they're passed as parameters to a function. A

JavaScript function can change an object's properties or alter the contents of

an array which could easily cause bugs elsewhere.

Suppose there's a function that accepts an array parameter representing a

shopping cart. If the function makes a change in that shopping cart array -

by adding an item to purchase, for example - then any other function that

uses that same `cart` array will be affected by this addition. That may be

great, however it could also be bad. 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 an "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 the `cart` array was modified.

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

`cart`, edit it, and return the clone. This would ensure that functions that are still

using the old shopping cart wouldn't be affected by the 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 cases

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:**

**Good:**

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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 ES2015/ES6 classes and simply extend the `Array` global.

**Bad:**

**Good:**

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### 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 can be cleaner and easier to test.

Favor this style of programming when you can.

**Bad:**

**Good:**

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### Encapsulate conditionals

**Bad:**

**Good:**

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### Avoid negative conditionals

**Bad:**

**Good:**

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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:**

**Good:**

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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:**

**Good:**

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### Avoid type-checking (part 2)

If you are working with basic primitive values like strings and integers,

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:**

**Good:**

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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](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:**

**Good:**