clean-code-javascript

Table of Contents

1. Introduction
2. Variables
3. Functions
4. Objects and Data Structures
5. Classes
6. SOLID
7. Testing
8. Concurrency
9. Error Handling
10. Formatting
11. Comments
12. Translation

Introduction

!Humorous image of software quality estimation as a count of how many expletives
you shout when reading code

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 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

$3

Bad:

``javascript
const yyyymmdstr = moment().format("YYYY/MM/DD");
`

Good:

`javascript
const currentDate = moment().format("YYYY/MM/DD");
`

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$3

Bad:

`javascript
getUserInfo();
getClientData();
getCustomerRecord();
`

Good:

`javascript
getUser();
`

⬆ back to top

$3

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 and
ESLint
can help identify unnamed constants.

Bad:

`javascript
// What the heck is 86400000 for?
setTimeout(blastOff, 86400000);
`

Good:

`javascript
// Declare them as capitalized named constants.
const MILLISECONDS_PER_DAY = 60 60 24 * 1000; //86400000;

setTimeout(blastOff, MILLISECONDS_PER_DAY);
`

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$3

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);
`

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$3

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);
});
`

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$3

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, color) {
car.carColor = color;
}
`

Good:

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

function paintCar(car, color) {
car.color = color;
}
`

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$3

Default parameters 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(name = "Hipster Brew Co.") {
// ...
}
`

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Functions

$3

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 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:

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

createMenu("Foo", "Bar", "Baz", true);

`

Good:

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

createMenu({
title: "Foo",
body: "Bar",
buttonText: "Baz",
cancellable: true
});
`

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$3

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:

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

Good:

`javascript
function emailActiveClients(clients) {
clients.filter(isActiveClient).forEach(email);
}

function isActiveClient(client) {
const clientRecord = database.lookup(client);
return clientRecord.isActive();
}
`

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$3

Bad:

`javascript
function addToDate(date, month) {
// ...
}

const date = new Date();

// It's hard 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);
`

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$3

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 parseBetterJSAlternative(code) {
const tokens = tokenize(code);
const syntaxTree = parse(tokens);
syntaxTree.forEach(node => {
// parse...
});
}

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

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

return tokens;
}

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

return syntaxTree;
}
`

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$3

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();

const data = {
expectedSalary,
experience
};

switch (employee.type) {
case "manager":
data.portfolio = employee.getMBAProjects();
break;
case "developer":
data.githubLink = employee.getGithubLink();
break;
}

render(data);
});
}
`

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$3

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) {
let finalConfig = Object.assign(
{
title: "Foo",
body: "Bar",
buttonText: "Baz",
cancellable: true
},
config
);
return finalConfig
// config now equals: {title: "Order", body: "Bar", buttonText: "Send", cancellable: true}
// ...
}

createMenu(menuConfig);
`

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$3

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});
}
`

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$3

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'];
`

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$3

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 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() }];
};
`

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$3

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));
}
}
`

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$3

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:

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

let totalOutput = 0;

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

Good:

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

const totalOutput = programmerOutput.reduce(
(totalLines, output) => totalLines + output.linesOfCode,
0
);
`

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$3

Bad:

`javascript
if (fsm.state === "fetching" && isEmpty(listNode)) {
// ...
}
`

Good:

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

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

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$3

Bad:

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

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

Good:

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

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

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$3

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();
}
}
`

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$3

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"));
}
`

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$3

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:

`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

$3

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:

`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

$3

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");
`

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Objects and Data Structures

$3

Using getters and setters to access data on objects could be 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.
- You can lazy load your object's properties, let's say getting it from a
server.

Bad:

`javascript
function makeBankAccount() {
// ...

return {
balance: 0
// ...
};
}

const account = makeBankAccount();
account.balance = 100;
`

Good:

`javascript
function makeBankAccount() {
// this one is private
let balance = 0;

// a "getter", made public via the returned object below
function getBalance() {
return balance;
}

// a "setter", made public via the returned object below
function setBalance(amount) {
// ... validate before updating the balance
balance = amount;
}

return {
// ...
getBalance,
setBalance
};
}

const account = makeBankAccount();
account.setBalance(100);
`

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$3

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

Bad:

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

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

const employee = new Employee("John Doe");
console.log(Employee name: ${employee.getName()}); // Employee name: John Doe
delete employee.name;
console.log(Employee name: ${employee.getName()}); // Employee name: undefined
`

Good:

`javascript
function makeEmployee(name) {
return {
getName() {
return name;
}
};
}

const employee = makeEmployee("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

$3

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 ES2015/ES6 classes. However, prefer small functions over
classes until you find yourself needing larger and more complex objects.

Bad:

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

this.age = age;
};

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

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

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

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

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

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

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

Good:

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

move() {
/ ... /
}
}

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

liveBirth() {
/ ... /
}
}

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

speak() {
/ ... /
}
}
`

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$3

This pattern is very useful in JavaScript and you see it in many libraries such
as jQuery and Lodash. It allows your code to be expressive, and less verbose.
For that reason, I say, use method chaining and take a look at how clean your code
will be. In your class functions, simply return this at the end of every function,
and you can chain further class methods onto it.

Bad:

`javascript
class Car {
constructor(make, model, color) {
this.make = make;
this.model = model;
this.color = color;
}

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

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

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

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

const car = new Car("Ford", "F-150", "red");
car.setColor("pink");
car.save();
`

Good:

`javascript
class Car {
constructor(make, model, color) {
this.make = make;
this.model = model;
this.color = color;
}

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

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

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

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

const car = new Car("Ford", "F-150", "red").setColor("pink").save();
`

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$3

As stated famously in _Design Patterns_ by the Gang of Four,
you should prefer composition over inheritance where you can. There are lots of
good reasons to use inheritance and lots of good reasons to use composition.
The main point for this maxim is that if your mind instinctively goes for
inheritance, try to think if composition could model your problem better. In some
cases it can.

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

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

Bad:

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

// ...
}

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

// ...
}
`

Good:

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

// ...
}

class Employee {
constructor(name, email) {
this.name = name;
this.email = email;
}

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

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SOLID

$3

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

Bad:

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

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

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

Good:

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

verifyCredentials() {
// ...
}
}

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

changeSettings(settings) {
if (this.auth.verifyCredentials()) {
// ...
}
}
}
`

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$3

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
add new functionalities without changing existing code.

Bad:

`javascript
class AjaxAdapter extends Adapter {
constructor() {
super();
this.name = "ajaxAdapter";
}
}

class NodeAdapter extends Adapter {
constructor() {
super();
this.name = "nodeAdapter";
}
}

class HttpRequester {
constructor(adapter) {
this.adapter = adapter;
}

fetch(url) {
if (this.adapter.name === "ajaxAdapter") {
return makeAjaxCall(url).then(response => {
// transform response and return
});
} else if (this.adapter.name === "nodeAdapter") {
return makeHttpCall(url).then(response => {
// transform response and return
});
}
}
}

function makeAjaxCall(url) {
// request and return promise
}

function makeHttpCall(url) {
// request and return promise
}
`

Good:

`javascript
class AjaxAdapter extends Adapter {
constructor() {
super();
this.name = "ajaxAdapter";
}

request(url) {
// request and return promise
}
}

class NodeAdapter extends Adapter {
constructor() {
super();
this.name = "nodeAdapter";
}

request(url) {
// request and return promise
}
}

class HttpRequester {
constructor(adapter) {
this.adapter = adapter;
}

fetch(url) {
return this.adapter.request(url).then(response => {
// transform response and return
});
}
}
`

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$3

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

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

Bad:

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

setColor(color) {
// ...
}

render(area) {
// ...
}

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

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

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

class Square extends Rectangle {
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);
const area = rectangle.getArea(); // BAD: Returns 25 for Square. Should be 20.
rectangle.render(area);
});
}

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

Good:

`javascript
class Shape {
setColor(color) {
// ...
}

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

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

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

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

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

function renderLargeShapes(shapes) {
shapes.forEach(shape => {
const area = shape.getArea();
shape.render(area);
});
}

const shapes = [new Rectangle(4, 5), new Rectangle(4, 5), new Square(5)];
renderLargeShapes(shapes);
`

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$3

JavaScript doesn't have interfaces so this principle doesn't apply as strictly
as others. However, it's important and relevant even with JavaScript's lack of
type system.

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

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

Bad:

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

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

traverse() {
// ...
}
}

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

Good:

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

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

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

traverse() {
// ...
}
}

const $ = new DOMTraverser({
rootNode: document.getElementsByTagName("body"),
options: {
animationModule() {}
}
});
`

⬆ back to top

$3

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

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

Bad:

`javascript
class InventoryRequester {
constructor() {
this.REQ_METHODS = ["HTTP"];
}

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

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);
});
}
}

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

Good:

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

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

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

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

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

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

// By constructing our dependencies externally and injecting them, we can easily
// substitute our request module for a fancy new one that uses WebSockets.
const inventoryTracker = new InventoryTracker(
["apples", "bananas"],
new InventoryRequesterV2()
);
inventoryTracker.requestItems();
`

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Testing

Testing is more important than shipping. If you 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 are plenty of good JS test frameworks, 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.

$3

Bad:

`javascript
import assert from "assert";

describe("MomentJS", () => {
it("handles date boundaries", () => {
let date;

date = new MomentJS("1/1/2015");
date.addDays(30);
assert.equal("1/31/2015", date);

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

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

Good:

`javascript
import assert from "assert";

describe("MomentJS", () => {
it("handles 30-day months", () => {
const date = new MomentJS("1/1/2015");
date.addDays(30);
assert.equal("1/31/2015", date);
});

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

it("handles non-leap year", () => {
const date = new MomentJS("2/1/2015");
date.addDays(28);
assert.equal("03/01/2015", date);
});
});
`

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Concurrency

$3

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

Bad:

`javascript
import { get } from "request";
import { writeFile } from "fs";

get(
"https://en.wikipedia.org/wiki/Robert_Cecil_Martin",
(requestErr, response, body) => {
if (requestErr) {
console.error(requestErr);
} else {
writeFile("article.html", body, writeErr => {
if (writeErr) {
console.error(writeErr);
} else {
console.log("File written");
}
});
}
}
);
`

Good:

`javascript
import { get } from "request-promise";
import { writeFile } from "fs-extra";

get("https://en.wikipedia.org/wiki/Robert_Cecil_Martin")
.then(body => {
return writeFile("article.html", body);
})
.then(() => {
console.log("File written");
})
.catch(err => {
console.error(err);
});
`

⬆ back to top

$3

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

Bad:

`javascript
import { get } from "request-promise";
import { writeFile } from "fs-extra";

get("https://en.wikipedia.org/wiki/Robert_Cecil_Martin")
.then(body => {
return writeFile("article.html", body);
})
.then(() => {
console.log("File written");
})
.catch(err => {
console.error(err);
});
`

Good:

`javascript
import { get } from "request-promise";
import { writeFile } from "fs-extra";

async function getCleanCodeArticle() {
try {
const body = await get(
"https://en.wikipedia.org/wiki/Robert_Cecil_Martin"
);
await writeFile("article.html", body);
console.log("File written");
} catch (err) {
console.error(err);
}
}

getCleanCodeArticle()
`

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Error Handling

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

$3

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

Bad:

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

Good:

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

$3

For the same reason you shouldn't ignore caught errors
from try/catch.

Bad:

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

Good:

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

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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.

$3

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

Bad:

`javascript
const DAYS_IN_WEEK = 7;
const daysInMonth = 30;

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

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

class animal {}
class Alpaca {}
`

Good:

`javascript
const DAYS_IN_WEEK = 7;
const DAYS_IN_MONTH = 30;

const SONGS = ["Back In Black", "Stairway to Heaven", "Hey Jude"];
const ARTISTS = ["ACDC", "Led Zeppelin", "The Beatles"];

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

class Animal {}
class Alpaca {}
`

⬆ back to top

$3

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

Bad:

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

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

lookupManager() {
return db.lookup(this.employee, "manager");
}

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

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

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

getSelfReview() {
// ...
}
}

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

Good:

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

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

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

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

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

lookupManager() {
return db.lookup(this.employee, "manager");
}

getSelfReview() {
// ...
}
}

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

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Comments

$3

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

Bad:

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

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

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

Good:

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

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

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

⬆ back to top

$3

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

Bad:

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

Good:

`javascript
doStuff();
`

⬆ back to top

$3

Remember, use version control! There's no need for dead code, commented code,
and especially journal comments. Use git log to get history!

Bad:

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

Good:

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

⬆ back to top

$3

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

Bad:

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

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

Good:

`javascript
$scope.model = {
menu: "foo",
nav: "bar"
};

const actions = function() {
// ...
};
``

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Translation

This is also available in other languages:

- !am Armenian: hanumanum/clean-code-javascript/
- !bd Bangla(বাংলা): InsomniacSabbir/clean-code-javascript/
- !br Brazilian Portuguese: fesnt/clean-code-javascript
- !cn Simplified Chinese:
- alivebao/clean-code-js
- beginor/clean-code-javascript
- !tw Traditional Chinese: AllJointTW/clean-code-javascript
- !fr French: eugene-augier/clean-code-javascript-fr
- !de German: marcbruederlin/clean-code-javascript
- !id Indonesia: andirkh/clean-code-javascript/
- !it Italian: frappacchio/clean-code-javascript/
- !ja Japanese: mitsuruog/clean-code-javascript/
- !kr Korean: qkraudghgh/clean-code-javascript-ko
- !pl Polish: greg-dev/clean-code-javascript-pl
- !ru Russian:
- BoryaMogila/clean-code-javascript-ru/
- maksugr/clean-code-javascript
- !es Spanish: tureey/clean-code-javascript
- !es Spanish: andersontr15/clean-code-javascript
- !rs Serbian: doskovicmilos/clean-code-javascript/
- !tr Turkish: bsonmez/clean-code-javascript
- !ua Ukrainian: mindfr1k/clean-code-javascript-ua
- !vi Vietnamese: hienvd/clean-code-javascript/
- !ir Persian: hamettio/clean-code-javascript

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