https://github.com/ryanmcdermott/clean-code-javascript
clean-code-javascript
Table of Contents
- Introduction
- Variables
- Functions
- Objects and Data Structures
- Classes
- Testing
- Concurrency
- Formatting
- Comments
Introduction
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 less will be universally agreed upon. These are guidelines and nothing more, but they are ones codified over many years of collective experience by the authors of Clean Code.
Our craft of software engineering is just a bit over 50 years old, and we are still learning a lot. When software architecture is as old as architecture itself, maybe then we will have harder rules to follow. For now, let these guidelines serve as a touchstone by which to assess the quality of the JavaScript code that you and your team produce.
One more thing: knowing these won’t immediately make you a better software developer, and working with them for many years doesn’t mean you won’t make mistakes. Every piece of code starts as a first draft, like wet clay getting shaped into its final form. Finally, we chisel away the imperfections when we review it with our peers. Don’t beat yourself up for first drafts that need improvement. Beat up the code instead!
Variables
Use meaningful and pronounceable variable names
Bad:
var yyyymmdstr = moment().format('YYYY/MM/DD');
Good:
var yearMonthDay = moment().format('YYYY/MM/DD');
Use the same vocabulary for the same type of variable
Bad:
getUserInfo();
getClientData();
getCustomerRecord();
Good:
getUser();
Use searchable names
We will read more code than we will ever write. It’s important that the code we do write is readable and searchable. By not naming variables that end up being meaningful for understanding our program, we hurt our readers. Make your names searchable.
Bad:
// 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();
}
Use explanatory variables
Bad:
let cityStateRegex = /^(.+)[,\\s]+(.+?)\s*(\d{5})?$/;
saveCityState(cityStateRegex.match(cityStateRegex)[1], cityStateRegex.match(cityStateRegex)[2]);
Good:
let cityStateRegex = /^(.+)[,\\s]+(.+?)\s*(\d{5})?$/;
let match = cityStateRegex.match(cityStateRegex)
let city = match[1];
let state = match[2];
saveCityState(city, state);
Avoid Mental Mapping
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);
});
Don’t add unneeded context
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';
}
Short-circuiting is cleaner than conditionals
Bad:
function createMicrobrewery(name) {
var breweryName;
if (name) {
breweryName = name;
} else {
breweryName = 'Hipster Brew Co.';
}
}
Good:
function createMicrobrewery(name) {
var breweryName = name || 'Hipster Brew Co.'
}
Functions
Function arguments (2 or less ideally)
Limiting the amount of function parameters is incredibly important because it makes testing your function easier. Having more than three leads to a combinatorial explosion where you have to test tons of different cases with each separate argument.
Zero arguments is the ideal case. One or two arguments is ok, and three should be avoided. Anything more than that should be consolidated. Usually, if you have more than two arguments then your function is trying to do too much. In cases where it’s not, most of the time a higher-level object will suffice as an argument.
Since JavaScript allows us to make objects on the fly, without a lot of class boilerplate, you can use an object if you are finding yourself needing a lot of arguments.
Bad:
function createMenu(title, body, buttonText, cancellable) {
...
}
Good:
var menuConfig = {
title: 'Foo',
body: 'Bar',
buttonText: 'Baz',
cancellable: true
}
function createMenu(menuConfig) {
...
}
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:
function emailClients(clients) {
clients.forEach(client => {
let clientRecord = database.lookup(client);
if (clientRecord.isActive()) {
email(client);
}
});
}
Good:
function emailClients(clients) {
clients.forEach(client => {
emailClientIfNeeded(client);
});
}
function emailClientIfNeeded(client) {
if (isClientActive(client)) {
email(client);
}
}
function isClientActive(client) {
let clientRecord = database.lookup(client);
return clientRecord.isActive();
}
Function names should say what they do
Bad:
function dateAdd(date, month) {
// ...
}
let date = new Date();
// It's hard to to tell from the function name what is added
dateAdd(date, 1);
Good:
function dateAddMonth(date, month) {
// ...
}
let date = new Date();
dateAddMonth(date, 1);
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:
function parseBetterJSAlternative(code) {
let REGEXES = [
// ...
];
let statements = code.split(' ');
let tokens;
REGEXES.forEach((REGEX) => {
statements.forEach((statement) => {
// ...
})
});
let ast;
tokens.forEach((token) => {
// lex...
});
ast.forEach((node) => {
// parse...
})
}
Good:
function tokenize(code) {
let REGEXES = [
// ...
];
let statements = code.split(' ');
let tokens;
REGEXES.forEach((REGEX) => {
statements.forEach((statement) => {
// ...
})
});
return tokens;
}
function lexer(tokens) {
let ast;
tokens.forEach((token) => {
// lex...
});
return ast;
}
function parseBetterJSAlternative(code) {
let tokens = tokenize(code);
let ast = lexer(tokens);
ast.forEach((node) => {
// parse...
})
}
Remove duplicate code
Never ever, ever, under any circumstance, have duplicate code. There’s no reason for it and it’s quite possibly the worst sin you can commit as a professional developer. Duplicate code means there’s more than one place to alter something if you need to change some logic. JavaScript is untyped, so it makes having generic functions quite easy. Take advantage of that!
Bad:
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 data = {
expectedSalary: expectedSalary,
experience: experience,
portfolio: portfolio
};
render(data);
});
}
Use default arguments instead of short circuiting
Bad:
function writeForumComment(subject, body) {
subject = subject || 'No Subject';
body = body || 'No text';
}
Good:
function writeForumComment(subject = 'No subject', body = 'No text') {
...
}
Set default objects with Object.assign
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);
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:
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);
}
Avoid Side Effects
A function produces a side effect if it does anything other than take a value in and return another value or values. A side effect could be writing to a file, modifying some global variable, or accidentally wiring all your money to a Nigerian prince.
Now, you do need to have side effects in a program on occasion. Like the previous example, you might need to write to a file. What you want to do is to centralize where you are doing this. Don’t have several functions and classes that write to a particular file. Have one service that does it. One and only one.
The main point is to avoid common pitfalls like sharing state between objects without any structure, using mutable data types that can be written to by anything, and not centralizing where your side effects occur. If you can do this, you will be happier than the vast majority of other programmers.
Bad:
// 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'];
Don’t write to global functions
Polluting globals is a bad practice in JavaScript because you could clash with another
library and the user of your API would be none-the-wiser until they get an
exception in production. Let’s think about an example: what if you wanted to
extend JavaScript’s native Array method to have a diff method that could
show the difference between two arrays? You could write your new function
to the Array.prototype, but it could clash with another library that tried
to do the same thing. What if that other library was just using diff to find
the difference between the first and last elements of an array? This is why it
would be much better to just use ES6 classes and simply extend the Array global.
Bad:
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;
}
}
Favor functional programming over imperative programming
If Haskell were an IPA then JavaScript would be an O’Douls. That is to say, JavaScript isn’t a functional language in the way that Haskell is, but it has a functional flavor to it. Functional languages are cleaner and easier to test. Favor this style of programming when you can.
Bad:
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);
Encapsulate conditionals
Bad:
if (fsm.state === 'fetching' && isEmpty(listNode)) {
/// ...
}
Good:
function shouldShowSpinner(fsm, listNode) {
return fsm.state === 'fetching' && isEmpty(listNode);
}
if (shouldShowSpinner(fsmInstance, listNodeInstance)) {
// ...
}
Avoid negative conditionals
Bad:
function isDOMNodeNotPresent(node) {
// ...
}
if (!isDOMNodeNotPresent(node)) {
// ...
}
Good:
function isDOMNodePresent(node) {
// ...
}
if (isDOMNodePresent(node)) {
// ...
}
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:
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();
}
}
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:
function travelToTexas(vehicle) {
if (obj instanceof Bicycle) {
vehicle.peddle(this.currentLocation, new Location('texas'));
} else if (obj instanceof Car) {
vehicle.drive(this.currentLocation, new Location('texas'));
}
}
Good:
function travelToTexas(vehicle) {
vehicle.move(this.currentLocation, new Location('texas'));
}
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:
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;
}
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 for seeing where optimization is lacking. Target those in the meantime, until they are fixed if they can be.
Bad:
// On old browsers, each iteration would be costly because `len` would be
// recomputed. In modern browsers, this is optimized.
for (var i = 0, len = list.length; i < len; i++) {
// ...
}
Good:
for (var i = 0; i < list.length; i++) {
// ...
}
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:
function oldRequestModule(url) {
// ...
}
function newRequestModule(url) {
// ...
}
var req = newRequestModule;
inventoryTracker('apples', req, 'www.inventory-awesome.io');
Good:
function newRequestModule(url) {
// ...
}
var req = newRequestModule;
inventoryTracker('apples', req, 'www.inventory-awesome.io');
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:
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 -=