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. Enjoy!
Bad:
var yyyymmdstr = moment().format('YYYY/MM/DD');Good:
var yearMonthDay = moment().format('YYYY/MM/DD');Bad:
getUserInfo();
getClientData();
getCustomerRecord();Good:
getUser();We will read more code than we will ever write. It's important that the code we do write is readable and searchable. By not naming variables that end up being meaningful for understanding our program, we hurt our readers. Make your names searchable.
Bad:
// What the heck is 525600 for?
for (var i = 0; i < 525600; i++) {
runCronJob();
}Good:
// Declare them as capitalized `var` globals.
var MINUTES_IN_A_YEAR = 525600;
for (var i = 0; i < MINUTES_IN_A_YEAR; i++) {
runCronJob();
}Explicit is better than implicit.
Bad:
var locations = ['Austin', 'New York', 'San Francisco'];
locations.forEach((l) => {
doStuff();
doSomeOtherStuff();
...
...
...
// Wait, what is `l` for again?
dispatch(l);
});Good:
var locations = ['Austin', 'New York', 'San Francisco'];
locations.forEach((___location) => {
doStuff();
doSomeOtherStuff();
...
...
...
dispatch(___location);
});If your class/object name tells you something, don't repeat that in your variable name.
Bad:
var Car = {
carMake: 'Honda',
carModel: 'Accord',
carColor: 'Blue'
};
function paintCar(car) {
car.carColor = 'Red';
}Good:
var Car = {
make: 'Honda',
model: 'Accord',
color: 'Blue'
};
function paintCar(car) {
car.color = 'Red';
}Use an object if you are finding yourself needing a lot of parameters.
Bad:
function createMenu(title, body, buttonText, cancellable) {
...
}Good:
var menuConfig = {
title: 'Foo',
body: 'Bar',
buttonText: 'Baz'
cancellable: true
}
function createMenu(config) {
...
}This is by far the most important rule in software engineering. When functions do more than one thing, they are harder to compose, test, and reason about. When you can isolate a function to just one action, they can be refactored easily and your code will read much cleaner. If you take nothing else away from this guide other than this, you'll be ahead of many developers.
Bad:
function emailClients(clients) {
clients.forEach(client => {
let clientRecord = database.lookup(client);
if (clientRecord.isActive()) {
email(client);
}
});
}Good:
function emailClients(clients) {
clients.forEach(client => {
emailClientIfNeeded();
});
}
function emailClientIfNeeded(client) {
if (isClientActive(client)) {
email(client);
}
}
function isClientActive(client) {
let clientRecord = database.lookup(client);
return clientRecord.isActive();
}Never ever, ever, under any circumstance, have duplicate code. There's no reason for it and it's quite possibly the worst sin you can commit as a professional developer. Duplicate code means there's more than one place to alter something if you need to change some logic. JavaScript is untyped, so it makes having generic functions quite easy. Take advantage of that!
Bad:
function showDeveloperList(developers) {
developers.forEach(developers => {
var expectedSalary = developer.calculateExpectedSalary();
var experience = developer.getExperience();
var githubLink = developer.getGithubLink();
var data = {
expectedSalary: expectedSalary,
experience: experience,
githubLink: githubLink
};
render(data);
});
}
function showManagerList(managers) {
managers.forEach(manager => {
var expectedSalary = manager.calculateExpectedSalary();
var experience = manager.getExperience();
var portfolio = manager.getMBAProjects();
var data = {
expectedSalary: expectedSalary,
experience: experience,
portfolio: portfolio
};
render(data);
});
}Good:
function showList(employees) {
employees.forEach(employee => {
var expectedSalary = employee.calculateExpectedSalary();
var experience = employee.getExperience();
var portfolio;
if (employee.type === 'manager') {
portfolio = employee.getMBAProjects();
} else {
portfolio = employee.getGithubLink();
}
var favoriteManagerBooks = getMBAList()
var data = {
expectedSalary: expectedSalary,
experience: experience,
portfolio: portfolio
};
render(data);
});
}Bad:
function writeForumComment(subject, body) {
subject = subject || 'No Subject';
body = body || 'No text';
}Good:
function writeForumComment(subject='No subject', body='No text') {
...
}Bad:
var menuConfig = {
title: null,
body: 'Bar',
buttonText: null
cancellable: true
}
function createMenu(config) {
config.title = config.title || 'Foo'
config.body = config.body || 'Bar'
config.buttonText = config.title || 'Baz'
config.cancellable = config.cancellable === undefined ? config.cancellable : true;
}
createMenu(menuConfig);Good:
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);Flags tell your user that this function does more than one thing. Functions should do one thing. Split out your functions if they are following different code paths based on a boolean.
Bad:
function createFile(name, temp) {
if (temp) {
fs.create('./temp/' + name);
} else {
fs.create(name);
}
}Good:
function createTempFile(name) {
fs.create('./temp/' + name);
}
function createFile(name) {
fs.create(name);
}A function produces a side effect if it does anything other than take a value in and return another value or values. A side effect could be writing to a file, modifying some global variable, or accidentally wiring all your money to a Nigerian prince.
Now, you do need to have side effects in a program on occasion. Like the previous example, you might need to write to a file. What you want to do is to centralize where you are doing this. Don't have several functions and classes that write to a particular file. Have one service that does it. One and only one.
The main point is to avoid common pitfalls like sharing state between objects without any structure, using mutable data types that can be written to by anything, and not centralizing where your side effects occur. If you can do this, you will be happier than the vast majority of other programmers.
Bad:
// Global variable referenced by following function.
// If we had another function that used this name, now it'd be an array and it could break it.
var name = 'Ryan McDermott';
function splitIntoFirstAndLastName() {
name = name.split(' ');
}
console.log(name); // ['Ryan', 'McDermott'];Good:
function splitIntoFirstAndLastName(name) {
return name.split(' ');
}
var name = 'Ryan McDermott'
var newName = splitIntoFirstAndLastName(name);
console.log(name); // 'Ryan McDermott';
console.log(newName); // ['Ryan', 'McDermott'];Polluting globals is a bad practice in JavaScript because you could clash with another
library and the user of your API would be none-the-wiser until they get an
exception in production. Let's think about an example: what if you wanted to
extend JavaScript's native Array method to have a diff method that could
show the difference between two arrays? You could write your new function
to the Array.prototype, but it could clash with another library that tried
to do the same thing. What if that other library was just using diff to find
the difference between the first and last elements of an array? This is why it
would be much better to just use ES6 classes and simply extend the Array global.
Bad:
Array.prototype.diff = function(comparisonArray) {
var values = [];
var hash = {};
for (var i of comparisonArray) {
hash[i] = true;
}
for (var i of this) {
if (!hash[i]) {
values.push(i);
}
}
return values;
}Good:
class SuperArray extends Array {
constructor(...args) {
super(...args);
}
diff(comparisonArray) {
var values = [];
var hash = {};
for (var i of comparisonArray) {
hash[i] = true;
}
for (var i of this) {
if (!hash[i]) {
values.push(i);
}
}
return values;
}
}If Haskell were an IPA then JavaScript would be an O'Douls. That is to say, JavaScript isn't a functional language in the way that Haskell is, but it has a functional flavor to it. Functional languages are cleaner and easier to test. Favor this style of programming when you can.
Bad:
const programmerOutput = [
{
name: 'Uncle Bobby',
linesOfCode: 500
}, {
name: 'Suzie Q',
linesOfCode: 1500
}, {
name: 'Jimmy Gosling',
linesOfCode: 150
}, {
name: 'Gracie Hopper',
linesOfCode: 1000
}
];
var totalOutput = 0;
for (var i = 0; i < programmerOutput.length; i++) {
totalOutput += programmerOutput[i].linesOfCode;
}Good:
const programmerOutput = [
{
name: 'Uncle Bobby',
linesOfCode: 500
}, {
name: 'Suzie Q',
linesOfCode: 1500
}, {
name: 'Jimmy Gosling',
linesOfCode: 150
}, {
name: 'Gracie Hopper',
linesOfCode: 1000
}
];
var totalOutput = programmerOutput
.map((programmer) => programmer.linesOfCode)
.reduce((acc, linesOfCode) => acc + linesOfCode, 0);This seems like an impossible task. Upon first hearing this, most people say,
"how am I supposed to do anything without an if statement?" The answer is that
you can use polymorphism to achieve the same task in many cases. The second
question is usually, "well that's great but why would I want to do that?" The
answer is a previous clean code concept we learned: a function should only do
one thing. When you have classes and functions that have if statements, you
are telling your user that your function does more than one thing. Remember,
just do one thing.
Bad:
class Airplane {
//...
getCruisingAltitude() {
switch (this.type) {
case '777':
return getMaxAltitude() - getPassengerCount();
case 'Air Force One':
return getMaxAltitude();
case 'Cesna':
return getMaxAltitude() - getFuelExpenditure();
}
}
}Good:
class Airplane {
//...
}
class Boeing777 extends Airplane {
//...
getCruisingAltitude() {
return getMaxAltitude() - getPassengerCount();
}
}
class AirForceOne extends Airplane {
//...
getCruisingAltitude() {
return getMaxAltitude();
}
}
class Cesna extends Airplane {
//...
getCruisingAltitude() {
return getMaxAltitude() - getFuelExpenditure();
}
}JavaScript is untyped, which means your functions can take any type of argument. Sometimes you are bitten by this freedom and it becomes tempting to do type-checking in your functions. There are many ways to avoid having to do this. The first thing to consider is consistent APIs.
Bad:
function travelToTexas(vehicle) {
if (obj instanceof Bicylce) {
vehicle.peddle(this.currentLocation, new Location('texas'));
} else if (obj instanceof Car) {
vehicle.drive(this.currentLocation, new Location('texas'));
}
}Good:
function travelToTexas(vehicle) {
vehicle.move(this.currentLocation, new Location('texas'));
}If you are working with basic primitive values like strings, integers, and arrays, and you can't use polymorphism but you still feel the need to type-check, you should consider using TypeScript. It is an excellent alternative to normal JavaScript, as it provides you with static typing on top of standard JavaScript syntax. The problem with manually type-checking normal JavaScript is that doing it well requires so much extra verbiage that the faux "type-safety" you get doesn't make up for the lost readability. Keep your JavaScript, clean, write good tests, and have good code reviews. Otherwise, do all of that but with TypeScript (which, like I said, is a great alternative!).
Bad:
function combine(val1, val2) {
if (typeof val1 == "number" && typeof val2 == "number" ||
typeof val1 == "string" && typeof val2 == "string") {
return val1 + val2;
} else {
throw new Error('Must be of type String or Number');
}
}Good:
function combine(val1, val2) {
return val1 + val2;
}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 if 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:
class BankAccount {
constructor() {
this.balance = 1000;
}
}
let bankAccount = new BankAccount();
// Buy shoes...
bankAccount.balance = bankAccount.balance - 100;Good:
class BankAccount {
constructor() {
this.balance = 1000;
}
// It doesn't have to be prefixed with `get` or `set` to be a getter/setter
withdraw(amount) {
if (verifyAmountCanBeDeducted(amount)) {
this.balance -= amount;
}
}
}
let bankAccount = new BankAccount();
// Buy shoes...
bankAccount.withdraw(100);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.
Bad:
class UserSettings {
constructor(user) {
this.user = user;
}
changeSettings(settings) {
if (this.verifyCredentials(user)) {
// ...
}
}
verifyCredentials(user) {
// ...
}
}Good:
class UserAuth {
constructor(user) {
this.user = user;
}
verifyCredentials() {
// ...
}
}
class UserSettings {
constructor(user) {
this.user = user;
this.auth = new UserAuth(user)
}
changeSettings(settings) {
if (this.auth.verifyCredentials()) {
// ...
}
}
}It's very difficult to get readable class inheritance, construction, and method definitions for classical ES5 classes. If you need inheritance (and be aware that you might not), then prefer classes. However, prefer small functions over classes until you find yourself needing larger and more complex objects.
Bad:
var Animal = function(age) {
if (!(this instanceof Animal)) {
throw new Error("Instantiate Animal with `new`");
}
this.age = age;
};
Animal.prototype.move = function() {};
var Mammal = function(age, furColor) {
if (!(this instanceof Mammal)) {
throw new Error("Instantiate Mammal with `new`");
}
Animal.call(this, age);
this.furColor = furColor;
};
Mammal.prototype = Object.create(Animal.prototype);
Mammal.prototype.constructor = Mammal;
Mammal.prototype.liveBirth = function() {};
var Human = function(age, furColor, languageSpoken) {
if (!(this instanceof Human)) {
throw new Error("Instantiate Human with `new`");
}
Mammal.call(this, age, furColor);
this.languageSpoken = languageSpoken;
};
Human.prototype = Object.create(Mammal.prototype);
Human.prototype.constructor = Human;
Human.prototype.speak = function() {};Good:
class Animal {
constructor(age) {
this.age = age;
}
move() {}
}
class Mammal extends Animal {
constructor(age, furColor) {
super(age);
this.furColor = furColor;
}
liveBirth() {}
}
class Human extends Mammal {
constructor(age, furColor, languageSpoken) {
super(age, furColor);
this.languageSpoken = languageSpoken;
}
speak() {}
}Against the advice of Clean Code, this is one place where we will have to differ.
It has been argued that method chaining is unclean and violates the Law of Demeter.
Maybe it's true, but this pattern is very useful in JavaScript and you see it in
many libraries such as jQuery and Lodash. It allows your code to be expressive,
and less verbose. For that reason, I say, use method chaining and take a look at
how clean your code will be. In your class functions, simply return this at
the end of every function, and you can chain further class methods onto it.
Bad:
class Car {
constructor() {
this.make = 'Honda';
this.model = 'Accord';
this.color = 'white';
}
setMake(make) {
this.name = name;
}
setModel(model) {
this.model = model;
}
setColor(color) {
this.color = color;
}
save() {
console.log(this.make, this.model, this.color);
}
}
let car = new Car();
car.setColor('pink');
car.setMake('Ford');
car.setModel('F-150')
car.save();Good:
class Car {
constructor() {
this.make = 'Honda';
this.model = 'Accord';
this.color = 'white';
}
setMake(make) {
this.name = name;
// NOTE: Returning this for chaining
return this;
}
setModel(model) {
this.model = model;
// NOTE: Returning this for chaining
return this;
}
setColor(color) {
this.color = color;
// NOTE: Returning this for chaining
return this;
}
save() {
console.log(this.make, this.model, this.color);
}
}
let car = new Car()
.setColor('pink')
.setMake('Ford')
.setModel('F-150')
.save();Callbacks aren't clean, and they cause excessive amounts of nesting. With ES6, Promises are a built-in global type. Use them!
Bad:
require('request').get('https://en.wikipedia.org/wiki/Robert_Cecil_Martin', function(err, response) {
if (err) {
console.error(err);
}
else {
require('fs').writeFile('article.html', response.body, function(err) {
if (err) {
console.error(err);
} else {
console.log('File written');
}
})
}
})Good:
require('request-promise').get('https://en.wikipedia.org/wiki/Robert_Cecil_Martin')
.then(function(response) {
return require('fs-promise').writeFile('article.html', response);
})
.then(function() {
console.log('File written');
})
.catch(function(err) {
console.log(err);
})Promises are a very clean alternative to callbacks, but ES7 brings async and await
which offer an even cleaner solution. All you need is a function that is prefixed
in an async keyword, and then you can write your logic imperatively without
a then chain of functions. Use this if you can take advantage of ES7 features
today!
Bad:
require('request-promise').get('https://en.wikipedia.org/wiki/Robert_Cecil_Martin')
.then(function(response) {
return require('fs-promise').writeFile('article.html', response);
})
.then(function() {
console.log('File written');
})
.catch(function(err) {
console.log(err);
})Good:
async function getCleanCodeArticle() {
try {
var request = await require('request-promise')
var response = await request.get('https://en.wikipedia.org/wiki/Robert_Cecil_Martin');
var fileHandle = await require('fs-promise');
await fileHandle.writeFile('article.html', response);
console.log('File written');
} catch(err) {
console.log(err);
}
}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! 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.
JavaScript is untyped, so capitalization tells you a lot about your variables, functions, etc. These rules are subjective, so your team can choose whatever they want. The point is, no matter what you all choose, just be consistent.
Bad:
var DAYS_IN_WEEK = 7;
var daysInMonth = 30;
var songs = ['Back In Black', 'Stairway to Heaven', 'Hey Jude'];
var Artists = ['ACDC', 'Led Zeppelin', 'The Beatles'];
function eraseDatabase() {}
function restore_database() {}
class animal {}
class Alpaca {}Good:
var DAYS_IN_WEEK = 7;
var DAYS_IN_MONTH = 30;
var songs = ['Back In Black', 'Stairway to Heaven', 'Hey Jude'];
var artists = ['ACDC', 'Led Zeppelin', 'The Beatles'];
function eraseDatabase() {}
function restoreDatabase() {}
class Animal {}
class Alpaca {}If a function calls another, keep those functions vertically close in the source file. Ideally, keep the caller right above the callee. We tend to read code from top-to-bottom, like a newspaper. Because of this, make your code read that way.
Bad:
class PerformanceReview {
constructor(employee) {
this.employee = employee;
}
lookupPeers() {
return db.lookup(this.employee, 'peers');
}
lookupMananger() {
return db.lookup(this.employee, 'manager');
}
getPeerReviews() {
let peers = this.lookupPeers();
// ...
}
perfReview() {
getPeerReviews();
getManagerReview();
getSelfReview();
}
getManagerReview() {
let manager = this.lookupManager();
}
getSelfReview() {
// ...
}
}
let review = new PerformanceReview(user);
review.perfReview();Good:
class PerformanceReview {
constructor(employee) {
this.employee = employee;
}
perfReview() {
getPeerReviews();
getManagerReview();
getSelfReview();
}
getPeerReviews() {
let peers = this.lookupPeers();
// ...
}
lookupPeers() {
return db.lookup(this.employee, 'peers');
}
getManagerReview() {
let manager = this.lookupManager();
}
lookupMananger() {
return db.lookup(this.employee, 'manager');
}
getSelfReview() {
// ...
}
}
let review = new PerformanceReview(employee);
review.perfReview();Comments are an apology, not a requirement. Good code mostly documents itself.
Bad:
function hashIt(data) {
// The hash
var hash = 0;
// Length of string
var length = data.length;
// Loop through every character in data
for (var i = 0; i < length; i++) {
// Get character code.
var char = i.charCodeAt(i);
// Make the hash
hash = ((hash << 5) - hash) + char;
// Convert to 32-bit integer
hash = hash & hash;
}
}Good:
function hashIt(data) {
var hash = 0;
var length = data.length;
for (var i = 0; i < length; i++) {
var char = i.charCodeAt(i);
hash = ((hash << 5) - hash) + char;
// Convert to 32-bit integer
hash = hash & hash;
}
}Version control exists for a reason. Leave old code in your history.
Bad:
doStuff();
// doOtherStuff();
// doSomeMoreStuff();
// doSoMuchStuff();Good:
doStuff();Remember, use version control! There's no need for dead code, commented code,
and especially journal comments. Use git log to get history!
Bad:
/**
* 2016-12-20: Removed monads, didn't understand them (RM)
* 2016-10-01: Improved using special monads (JP)
* 2016-02-03: Removed type-checking (LI)
* 2015-03-14: Added combine with type-checking (JR)
*/
function combine(a, b) {
return a + b;
}Good:
function combine(a, b) {
return a + b;
}