Clean Code concepts adapted for TypeScript
Clean Code concepts adapted for TypeScript
Clean Code concepts adapted for TypeScript.
Inspired from clean-code-javascript.
Software engineering principles, from Robert C. Martin's book Clean Code, adapted for TypeScript. This is not a style guide. It's a guide to producing readable, reusable, and refactorable software in TypeScript.
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 TypeScript 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!
Distinguish names in such a way that the reader knows what the differences offer.
Bad:
function between<T>(a1: T, a2: T, a3: T): boolean {
return a2 <= a1 && a1 <= a3;
}
Good:
function between<T>(value: T, left: T, right: T): boolean {
return left <= value && value <= right;
}
If you can’t pronounce it, you can’t discuss it without sounding like an idiot.
Bad:
type DtaRcrd102 = {
genymdhms: Date;
modymdhms: Date;
pszqint: number;
}
Good:
type Customer = {
generationTimestamp: Date;
modificationTimestamp: Date;
recordId: number;
}
Bad:
function getUserInfo(): User;
function getUserDetails(): User;
function getUserData(): User;
Good:
function getUser(): User;
We will read more code than we will ever write. It's important that the code we do write must be 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 ESLint can help identify unnamed constants (also known as magic strings and magic numbers).
Bad:
// What the heck is 86400000 for?
setTimeout(restart, 86400000);
Good:
// Declare them as capitalized named constants.
const MILLISECONDS_PER_DAY = 24 * 60 * 60 * 1000; // 86400000
setTimeout(restart, MILLISECONDS_PER_DAY);
Bad:
declare const users: Map<string, User>;
for (const keyValue of users) {
// iterate through users map
}
Good:
declare const users: Map<string, User>;
for (const [id, user] of users) {
// iterate through users map
}
Explicit is better than implicit.
Clarity is king.
Bad:
const u = getUser();
const s = getSubscription();
const t = charge(u, s);
Good:
const user = getUser();
const subscription = getSubscription();
const transaction = charge(user, subscription);
If your class/type/object name tells you something, don't repeat that in your variable name.
Bad:
type Car = {
carMake: string;
carModel: string;
carColor: string;
}
function print(car: Car): void {
console.log(`${car.carMake} ${car.carModel} (${car.carColor})`);
}
Good:
type Car = {
make: string;
model: string;
color: string;
}
function print(car: Car): void {
console.log(`${car.make} ${car.model} (${car.color})`);
}
Default arguments are often cleaner than short circuiting.
Bad:
function loadPages(count?: number) {
const loadCount = count !== undefined ? count : 10;
// ...
}
Good:
function loadPages(count: number = 10) {
// ...
}
Enums can help you document the intent of the code. For example when we are concerned about values being different rather than the exact value of those.
Bad:
const GENRE = {
ROMANTIC: 'romantic',
DRAMA: 'drama',
COMEDY: 'comedy',
DOCUMENTARY: 'documentary',
}
projector.configureFilm(GENRE.COMEDY);
class Projector {
// declaration of Projector
configureFilm(genre) {
switch (genre) {
case GENRE.ROMANTIC:
// some logic to be executed
}
}
}
Good:
enum GENRE {
ROMANTIC,
DRAMA,
COMEDY,
DOCUMENTARY,
}
projector.configureFilm(GENRE.COMEDY);
class Projector {
// declaration of Projector
configureFilm(genre) {
switch (genre) {
case GENRE.ROMANTIC:
// some logic to be executed
}
}
}
Limiting the number 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.
Consider using object literals if you are finding yourself needing a lot of arguments.
To make it obvious what properties the function expects, you can use the destructuring syntax. This has a few advantages:
When someone looks at the function signature, it's immediately clear what properties are being used.
It can be used to simulate named parameters.
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.
TypeScript warns you about unused properties, which would be impossible without destructuring.
Bad:
function createMenu(title: string, body: string, buttonText: string, cancellable: boolean) {
// ...
}
createMenu('Foo', 'Bar', 'Baz', true);
Good:
function createMenu(options: { title: string, body: string, buttonText: string, cancellable: boolean }) {
// ...
}
createMenu({
title: 'Foo',
body: 'Bar',
buttonText: 'Baz',
cancellable: true
});
You can further improve readability by using type aliases:
type MenuOptions = { title: string, body: string, buttonText: string, cancellable: boolean };
function createMenu(options: MenuOptions) {
// ...
}
createMenu({
title: 'Foo',
body: 'Bar',
buttonText: 'Baz',
cancellable: true
});
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:
function emailActiveClients(clients: Client[]) {
clients.forEach((client) => {
const clientRecord = database.lookup(client);
if (clientRecord.isActive()) {
email(client);
}
});
}
Good:
function emailActiveClients(clients: Client[]) {
clients.filter(isActiveClient).forEach(email);
}
function isActiveClient(client: Client) {
const clientRecord = database.lookup(client);
return clientRecord.isActive();
}
Bad:
function addToDate(date: Date, month: number): Date {
// ...
}
const date = new Date();
// It's hard to tell from the function name what is added
addToDate(date, 1);
Good:
function addMonthToDate(date: Date, month: number): Date {
// ...
}
const date = new Date();
addMonthToDate(date, 1);
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 parseCode(code: string) {
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:
…
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. 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:
class Developer {
// ...
getExtraDetails() {
return {
githubLink: this.githubLink,
}
}
}
class Manager {
// ...
getExtraDetails()
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