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# clean-code-typescript [![Tweet](https://img.shields.io/twitter/url/http/shields.io.svg?style=social)](https://twitter.com/intent/tweet?text=Clean%20Code%20Typescript&url=https://github.com/labs42io/clean-code-typescript)
Концепции чистого кода адаптированные для TypeScript, вдохновленные [clean-code-javascript](https://github.com/ryanmcdermott/clean-code-javascript).
Оригинал на английском [clean-code-typescript](https://github.com/labs42io/clean-code-typescript)
*У переводчика не идеальные знания английского, указывайте пожалуйста на ошибки!*
## Содержание
1. [Введение](#введение)
2. [Переменные](#переменные)
3. [Функции](#functions)
4. [Объекты и структуры данных](#objects-and-data-structures)
5. [Классы](#classes)
6. [SOLID](#solid)
7. [Тестирование](#testing)
8. [Асинхронность](#concurrency)
9. [Обработка ошибок](#error-handling)
10. [Форматирование](#formatting)
11. [Комментарии](#comments)
12. [Переводы](#translations)
## Введение
![Humorous image of software quality estimation as a count of how many expletives
you shout when reading code](https://www.osnews.com/images/comics/wtfm.jpg)
Инженерные принципы ПО, из книги Robert C. Martin'
[*Clean Code*](https://www.amazon.com/Clean-Code-Handbook-Software-Craftsmanship/dp/0132350882),
адаптированные для TypeScript. Это не руководство по стилю. Это руководство по написанию
[читаемого, переиспользуемого и пригодного для рефакторинга](https://github.com/ryanmcdermott/3rs-of-software-architecture) кода на TypeScript.
Не каждый принцип описанный здесь должен строго соблюдаться и еще меньше получать
всеобщего признаний. Это принципы и ни чего более, оно они накапливались в течение
многих лет с опытом коллектива авторов *Чистого Кода*
Ремеслу по написанию программного обеспечения чуть более 50 лет, но мы все еще многому учимся.
Когда программная архитектура постареет до возрастра самой архитектуры, быть может тогда у нас
появятся жесткие правила которым необходимо следовать. А сейчас пусть это служит критериями,
с помощью которого вы будете оценивать качество вашего TypeScript кода и вашей команды.
И еще одна вещь: знание этих принципов не делает вас сразу лучшим разработчиком ПО, а их использование в
течение многих лет не гарантирует, что вы не будете совершать ошибки. Каждый кусок кода начинается как
черновик, как мокрый кусок глины который только постеменно приобретает свою форму. Не упрекайте себя при
первых набросках кода, которые нуждаются в улучшении. Улучшайте код вместо этого!
**[⬆ Вернуться в начало](#содержание)**
## Переменные
### Используйте выразительные имена переменных
Различайте имена таким образом, чтобы читатель знал что они означают.
**Плохо:**
```ts
function between<T>(a1: T, a2: T, a3: T): boolean {
return a2 <= a1 && a1 <= a3;
}
```
**Хорошо:**
```ts
function between<T>(value: T, left: T, right: T): boolean {
return left <= value && value <= right;
}
```
**[⬆ Вернуться в начало](#содержание)**
### Используйте произносительные имена переменных
Если вы не можете произносить их, вы не можете обсуждать их не выглядя как идиот.
**Плохо:**
```ts
type DtaRcrd102 = {
genymdhms: Date;
modymdhms: Date;
pszqint: number;
}
```
**Хорошо:**
```ts
type Customer = {
generationTimestamp: Date;
modificationTimestamp: Date;
recordId: number;
}
```
**[⬆ Вернуться в начало](#содержание)**
### Используйте один и тот же словарь для одних и тех же типов переменных
**Плохо:**
```ts
function getUserInfo(): User;
function getUserDetails(): User;
function getUserData(): User;
```
**Хорошо:**
```ts
function getUser(): User;
```
**[⬆ Вернуться в начало](#содержание)**
### Используйте имена, доступные для поиска
Мы читаем больще кода, чем пишем. Это важно чтобы код, который мы пишем, был читаемым и достумным для поиска.
Не называйте переменные, которые в конечном итое имеют смысл только для наших программ мы вредим нашим читателям.
Делайте ваши имена доступными для поиска.
Такие инструменты, как [TSLint](https://palantir.github.io/tslint/rules/no-magic-numbers/) и [ESLint](https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-magic-numbers.md)
могут помочь идентифицировать не названные константы.
**Плохо:**
```ts
// What the heck is 86400000 for?
setTimeout(restart, 86400000);
```
**Хорошо:**
```ts
// Declare them as capitalized named constants.
const MILLISECONDS_IN_A_DAY = 24 * 60 * 60 * 1000;
setTimeout(restart, MILLISECONDS_IN_A_DAY);
```
**[⬆ Вернуться в начало](#содержание)**
### Используйте объясняющие переменные
**Плохо:**
```ts
declare const users: Map<string, User>;
for (const keyValue of users) {
// iterate through users map
}
```
**Хорошо:**
```ts
declare const users: Map<string, User>;
for (const [id, user] of users) {
// iterate through users map
}
```
**[⬆ Вернуться в начало](#содержание)**
### Избегайте ментальных связей
Явное лучше, чем неявное.
*Ясность - это король.*
**Плохо:**
```ts
const u = getUser();
const s = getSubscription();
const t = charge(u, s);
```
**Хорошо:**
```ts
const user = getUser();
const subscription = getSubscription();
const transaction = charge(user, subscription);
```
**[⬆ Вернуться в начало](#содержание)**
### Не добавляйте не нужный контекст
Если имя вашего класса/типа/объекта говорит само за себя, не повторяйте его в вашем именни переменной.
**Плохо:**
```ts
type Car = {
carMake: string;
carModel: string;
carColor: string;
}
function print(car: Car): void {
console.log(`${car.carMake} ${car.carModel} (${car.carColor})`);
}
```
**Хорошо:**
```ts
type Car = {
make: string;
model: string;
color: string;
}
function print(car: Car): void {
console.log(`${car.make} ${car.model} (${car.color})`);
}
```
**[⬆ Вернуться в начало](#содержание)**
### Используйте аргументы по умолчанию вместо замыканий или вычислений
Аргументы по умолчанию часто чище, чем короткое вычисление.
**Плохо:**
```ts
function loadPages(count?: number) {
const loadCount = count !== undefined ? count : 10;
// ...
}
```
**Хорошо:**
```ts
function loadPages(count: number = 10) {
// ...
}
```
**[⬆ Вернуться в начало](#содержание)**
### Используйте enum дл документирования
Enam'ы могут помочь документированию вашего кода. Например когда мы обеспокоены, что наши переменные
отличаются от значений.
**Плохо:**
```ts
const GENRE = {
ROMANTIC: 'romantic',
DRAMA: 'drama',
COMEDY: 'comedy',
DOCUMENTARY: 'documentary',
}
projector.configureFilm(GENRE.COMEDY);
class Projector {
// delactation of Projector
configureFilm(genre) {
switch (genre) {
case GENRE.ROMANTIC:
// some logic to be executed
}
}
}
```
**Хорошо:**
```ts
enum GENRE {
ROMANTIC,
DRAMA,
COMEDY,
DOCUMENTARY,
}
projector.configureFilm(GENRE.COMEDY);
class Projector {
// delactation of Projector
configureFilm(genre) {
switch (genre) {
case GENRE.ROMANTIC:
// some logic to be executed
}
}
}
```
**[⬆ Вернуться в начало](#содержание)**
## Функции
### Аргументы функции (идеально два или меньше)
Ограничение колличества парамметров функции невероятно важно, потому что это делает тестирование ваших
функций проще. Наличие более 3-х аргументов приводит к комбинаторному взрыву, где вы должны протестировать
множество вариантов с каждым отдельным аргументом
Один или два аргумента это идеальный случай, а три и более следует избегать, если это возможно.
Большое колличество аргументов лучше объеденять. Обычно если вы используете более двух аргументов, то ваша функция пытается
делать слишком много. В случаях когда это не так, то лучше использовать объект верхнего уровня.
Подумайте о том чтобы использовать объектные литералы, если вам необходимо много аргументов.
Для того чтобы вы знали какие параметры ожидает функция, вы можете использовать
[синтаксис деструктуризации](https://basarat.gitbooks.io/typescript/docs/destructuring.html).
Он имеет несколько преимуществ:
1. Когда кто-то смотрит на синатуру функции, то сразу становится понятка какие свойства она использует.
2. Деструктуризация также клонирует примитивные значения аргумента-объекта переданного в функцию.
Это помогает избежать сайд эффекта. Заметка: объекты и массивы которые деструктурированы из аргумента-объекта не клонируются.
3. TypeScript предупреждает о неиспользуемых свойствах, это было бы не возможно без деструктуризации.
**Плохо:**
```ts
function createMenu(title: string, body: string, buttonText: string, cancellable: boolean) {
// ...
}
createMenu('Foo', 'Bar', 'Baz', true);
```
**Хорошо:**
```ts
function createMenu(options: { title: string, body: string, buttonText: string, cancellable: boolean }) {
// ...
}
createMenu({
title: 'Foo',
body: 'Bar',
buttonText: 'Baz',
cancellable: true
});
```
Вы можете еще больше повысить читаемость, если будете использовать [type aliases](https://www.typescriptlang.org/docs/handbook/advanced-types.html#type-aliases):
```ts
type MenuOptions = { title: string, body: string, buttonText: string, cancellable: boolean };
function createMenu(options: MenuOptions) {
// ...
}
createMenu({
title: 'Foo',
body: 'Bar',
buttonText: 'Baz',
cancellable: true
});
```
**[⬆ back to top](#содержание)**
### Функции должны выполнять одну задачу
Это одно из самых важных правил в разработке ПО. Когда функции решают больше одной задачи,
их труднее объеденять, тестировать. Если вы сможете изолировать функцию так чтобы она выполняла только одну задачу,
в дальнейшем она может быть легко переработана, а ваш код будет чище. Если вы запомните только это правило из этого
руководства, то вы уже будете лучще многих разработчиков.
**Плохо:**
```ts
function emailClients(clients: Client[]) {
clients.forEach((client) => {
const clientRecord = database.lookup(client);
if (clientRecord.isActive()) {
email(client);
}
});
}
```
**Хорошо:**
```ts
function emailClients(clients: Client[]) {
clients.filter(isActiveClient).forEach(email);
}
function isActiveClient(client: Client) {
const clientRecord = database.lookup(client);
return clientRecord.isActive();
}
```
**[⬆ back to top](#содержание)**
### Название функций должны описывать что они делают
**Плохо:**
```ts
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);
```
**Хорошо:**
```ts
function addMonthToDate(date: Date, month: number): Date {
// ...
}
const date = new Date();
addMonthToDate(date, 1);
```
**[⬆ back to top](#содержание)**
### Функции должны иметь один уровень абстракции
Если у вас больще одного уровня абстракции, то обычно эта функция делает слишком мноое. Разделение функций дает возможность
переиспользования и простого тестирования.
**Плохо:**
```ts
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...
});
}
```
**Хорошо:**
```ts
const REGEXES = [ /* ... */ ];
function parseCode(code: string) {
const tokens = tokenize(code);
const syntaxTree = parse(tokens);
syntaxTree.forEach((node) => {
// parse...
});
}
function tokenize(code: string): Token[] {
const statements = code.split(' ');
const tokens: Token[] = [];
REGEXES.forEach((regex) => {
statements.forEach((statement) => {
tokens.push( /* ... */ );
});
});
return tokens;
}
function parse(tokens: Token[]): SyntaxTree {
const syntaxTree: SyntaxTree[] = [];
tokens.forEach((token) => {
syntaxTree.push( /* ... */ );
});
return syntaxTree;
}
```
**[⬆ back to top](#содержание)**
### Удаляйте дублированный код
Делайте все возможное для избежания дублирования кода.
Дублирование кода плохо, тем что если вам придется править логику, её придется править в нескольких местах.
Представьте если вы открыли ресторан и ведете учет ваших продуктов: всех ваших томатов, лука, чеснока, специй и д.р..
Если у вас закажут блюда из томатов то вам придется вносить изменения во все ваши списки. Если список будет только один
то и править нужно будет только его.
Часто вы дублируете код из-за того что когда вам требуется реализовать два и более незначительно различающихся действий,
которые очень похожи, но их различия заставляют вас завести несколько функций, делающий практически одно и тоже. Удаление
дублирующихся частей кода, означает создание абстракции, обрабатывающий разную логику с помощью всего одной функции/модуля/класса.
Получение абстракции имеет важное значение, поэтому вы должны следовать принципам [SOLID](#solid). Плохие абстракции могут
оказаться хуже дублирующего кода, будьте осторожны! Если вы можете сделать хорошую абстракцию делайте. Не повторяйте себя
в противном случае вы можете обнаружить себя вносящим изменения в разные места, для одной единственной логики.
**Плохо:**
```ts
function showDeveloperList(developers: Developer[]) {
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: Manager[]) {
managers.forEach((manager) => {
const expectedSalary = manager.calculateExpectedSalary();
const experience = manager.getExperience();
const portfolio = manager.getMBAProjects();
const data = {
expectedSalary,
experience,
portfolio
};
render(data);
});
}
```
**Хорощо:**
```ts
class Developer {
// ...
getExtraDetails() {
return {
githubLink: this.githubLink,
}
}
}
class Manager {
// ...
getExtraDetails() {
return {
portfolio: this.portfolio,
}
}
}
function showEmployeeList(employee: Developer | Manager) {
employee.forEach((employee) => {
const expectedSalary = employee.calculateExpectedSalary();
const experience = employee.getExperience();
const extra = employee.getExtraDetails();
const data = {
expectedSalary,
experience,
extra,
};
render(data);
});
}
```
Вы должны критически относиться к дублированию кода. Иногда существует компромисс между дублированием кода и увеличением
сложности, вводя новую абстракцию. Когда две реализации из двух разных модулей выглядят одинаково, но существуют в разных
доменах, дублирование может быть приемлемым и предпочтительным вариантом, нежели объеденений в общий код. Перенос логики
в общий код, вводит косвенную зависимость между двумя модулями.
**[⬆ back to top](#содержание)**
### Устанавливайте объекты по умолчанию с помощью Object.assign или деструктуризации
**Плохо:**
```ts
type MenuConfig = { title?: string, body?: string, buttonText?: string, cancellable?: boolean };
function createMenu(config: MenuConfig) {
config.title = config.title || 'Foo';
config.body = config.body || 'Bar';
config.buttonText = config.buttonText || 'Baz';
config.cancellable = config.cancellable !== undefined ? config.cancellable : true;
// ...
}
createMenu({ body: 'Bar' });
```
**Хорошо:**
```ts
type MenuConfig = { title?: string, body?: string, buttonText?: string, cancellable?: boolean };
function createMenu(config: MenuConfig) {
const menuConfig = Object.assign({
title: 'Foo',
body: 'Bar',
buttonText: 'Baz',
cancellable: true
}, config);
// ...
}
createMenu({ body: 'Bar' });
```
Кроме того можно использовать деструктуризацию со значениями по умолчанию:
```ts
type MenuConfig = { title?: string, body?: string, buttonText?: string, cancellable?: boolean };
function createMenu({ title = 'Foo', body = 'Bar', buttonText = 'Baz', cancellable = true }: MenuConfig) {
// ...
}
createMenu({ body: 'Bar' });
```
Чтобы избежать каких-либо побочных эффектов и неожиданноо поведения передавая явно `undefined` или `null` вы можете сказать
компилятору TypeScript чтобы он не разрешал этого.
Смотрите [`--strictNullChecks`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-0.html#--strictnullchecks) опция для TypeScript.
**[⬆ back to top](#содержание)**
### Не используйте флаги в качестве параметров функции
Флаги говорят пользователю, что функция совершает более одного действия. Функция должна решать одну задачу.
Разделяйте функции, если они исполняют различные варианты кода на основе логического значения.
**Плохо:**
```ts
function createFile(name: string, temp: boolean) {
if (temp) {
fs.create(`./temp/${name}`);
} else {
fs.create(name);
}
}
```
**Хорошо:**
```ts
function createTempFile(name: string) {
createFile(`./temp/${name}`);
}
function createFile(name: string) {
fs.create(name);
}
```
**[⬆ back to top](#содержание)**
### Избегайте побочных эффектов (часть 1)
Функция производит побочный эффект, если она совершает какое-либо действие помимо получения значения и
возврата другого значения или значений. Побочный эффект может быть записью в файл, изменением каких-то
глобальных переменных или случайным переводом всех ваших денег неизвестным лицам.
Впрочем, побочные эффекты в программе необходимы. Пусть, как и в предыдущем примере, вам требуется запись в файл.
Опишите то, что вы хотите сделать, строго в одном месте. Не создавайте несколько функций и классов, которые пишут что-то
в конкретный файл. Создайте один сервис, который всем этим занимается. Один и только один.
Суть в том, чтобы избегать распространенных ошибок, таких как, например, передача состояния между объектами
без какой-либо структуры, с помощью изменяемых данных, которые может перезаписывать кто угодно, в обход
централизованного места применения побочных эффектов. Если научитесь так делать, вы станете счастливее, чем подавляющее
большинство других программистов.
**Плохо:**
```ts
// Global variable referenced by following function.
let name = 'Robert C. Martin';
function toBase64() {
name = btoa(name);
}
toBase64();
// If we had another function that used this name, now it'd be a Base64 value
console.log(name); // expected to print 'Robert C. Martin' but instead 'Um9iZXJ0IEMuIE1hcnRpbg=='
```
**Хорошо:**
```ts
const name = 'Robert C. Martin';
function toBase64(text: string): string {
return btoa(text);
}
const encodedName = toBase64(name);
console.log(name);
```
**[⬆ back to top](#содержание)**
### Избегайте побочных эффектов (Часть 2)
В JavaScript примитивы передаются по значению, а объекты и массивы передаются по ссылке. В случае объектов или массивов,
если ваша функция вносит изменения в корзину покупок(массив), например при добавлении элемента в массив, то любая другая
функция использующаяя `корзину` массив будет зависеть от этого добавления. Это может быть как хорошо, так и плохо. Давайте
представим плохую ситуации:
Пользователь нажимает кнопку "Купить" вызывующюю функцию `purchase` которая делает сетевой запрос и отправляет `корзину`
массив на сервер. Если происходит плохое подключение к сети функция должна отправить повторный запрос. Теперь, если пользователь
случайно нажимает на кнопку "Добавить в корзину", но пока не хочет покупать товар? Если это произойдет и в этот момент начнется
запрос на сервер, то функция purchase отправит случайно добавленный элемент, так как он имеет ссылку на корзину покупок,
котора была изменена функцией `addItemToCart`. Путем добавления нежелательного элемента.
Хорошим бы рещением было бы что бы функция `addItemToCart` всегда клонировала бы массив `cart` редактировала его и
возвращала клон. Это бы гарантировало, что никакие другие функции, использующие ссылку на массив корзины покупок,
не будут затронуты какими-либо изменениями.
Два предостережения по-поводу такого подхода:
1. Возможны случаи, когда вы на самом деле хотите изменить объект по ссылке, но такие случаи крайне редки.
Большинство функций могут быть объявлены без сайд эффектов! (Смотрите [pure function](https://en.wikipedia.org/wiki/Pure_function))
2.Клонирование больших объектов может быть очень нагрузочным и влиять на производительность. К счастью, это не является
большой проблемой на практике, потому что есть отличные библиотеки, которые позволяют клонировать объекты с меньшей
нагрузкой на память в отличии от клонирования вручную.
**Плохо:**
```ts
function addItemToCart(cart: CartItem[], item: Item): void {
cart.push({ item, date: Date.now() });
};
```
**Хорошо:**
```ts
function addItemToCart(cart: CartItem[], item: Item): CartItem[] {
return [...cart, { item, date: Date.now() }];
};
```
**[⬆ back to top](#table-of-contents)**
### 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 classes and simply extend the `Array` global.
**Bad:**
```ts
declare global {
interface Array<T> {
diff(other: T[]): Array<T>;
}
}
if (!Array.prototype.diff) {
Array.prototype.diff = function <T>(other: T[]): T[] {
const hash = new Set(other);
return this.filter(elem => !hash.has(elem));
};
}
```
**Good:**
```ts
class MyArray<T> extends Array<T> {
diff(other: T[]): T[] {
const hash = new Set(other);
return this.filter(elem => !hash.has(elem));
};
}
```
**[⬆ back to top](#table-of-contents)**
### Favor functional programming over imperative programming
Favor this style of programming when you can.
**Bad:**
```ts
const contributions = [
{
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 < contributions.length; i++) {
totalOutput += contributions[i].linesOfCode;
}
```
**Good:**
```ts
const contributions = [
{
name: 'Uncle Bobby',
linesOfCode: 500
}, {
name: 'Suzie Q',
linesOfCode: 1500
}, {
name: 'Jimmy Gosling',
linesOfCode: 150
}, {
name: 'Gracie Hopper',
linesOfCode: 1000
}
];
const totalOutput = contributions
.reduce((totalLines, output) => totalLines + output.linesOfCode, 0);
```
**[⬆ back to top](#table-of-contents)**
### Encapsulate conditionals
**Bad:**
```ts
if (subscription.isTrial || account.balance > 0) {
// ...
}
```
**Good:**
```ts
function canActivateService(subscription: Subscription, account: Account) {
return subscription.isTrial || account.balance > 0
}
if (canActivateService(subscription, account)) {
// ...
}
```
**[⬆ back to top](#table-of-contents)**
### Avoid negative conditionals
**Bad:**
```ts
function isEmailNotUsed(email: string): boolean {
// ...
}
if (isEmailNotUsed(email)) {
// ...
}
```
**Good:**
```ts
function isEmailUsed(email): boolean {
// ...
}
if (!isEmailUsed(node)) {
// ...
}
```
**[⬆ back to top](#table-of-contents)**
### 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:**
```ts
class Airplane {
private type: string;
// ...
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();
default:
throw new Error('Unknown airplane type.');
}
}
private getMaxAltitude(): number {
// ...
}
}
```
**Good:**
```ts
abstract class Airplane {
protected getMaxAltitude(): number {
// shared logic with subclasses ...
}
// ...
}
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();
}
}
```
**[⬆ back to top](#table-of-contents)**
### Avoid type checking
TypeScript is a strict syntactical superset of JavaScript and adds optional static type checking to the language.
Always prefer to specify types of variables, parameters and return values to leverage the full power of TypeScript features.
It makes refactoring more easier.
**Bad:**
```ts
function travelToTexas(vehicle: Bicycle | Car) {
if (vehicle instanceof Bicycle) {
vehicle.pedal(currentLocation, new Location('texas'));
} else if (vehicle instanceof Car) {
vehicle.drive(currentLocation, new Location('texas'));
}
}
```
**Good:**
```ts
type Vehicle = Bicycle | Car;
function travelToTexas(vehicle: Vehicle) {
vehicle.move(currentLocation, new Location('texas'));
}
```
**[⬆ back to top](#table-of-contents)**
### Don't over-optimize
Modern browsers do a lot of optimization under-the-hood at runtime. A lot of times, if you are optimizing then you are just wasting your time. There are good [resources](https://github.com/petkaantonov/bluebird/wiki/Optimization-killers) for seeing where optimization is lacking. Target those in the meantime, until they are fixed if they can be.
**Bad:**
```ts
// 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:**
```ts
for (let i = 0; i < list.length; i++) {
// ...
}
```
**[⬆ back to top](#table-of-contents)**
### 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:**
```ts
function oldRequestModule(url: string) {
// ...
}
function requestModule(url: string) {
// ...
}
const req = requestModule;
inventoryTracker('apples', req, 'www.inventory-awesome.io');
```
**Good:**
```ts
function requestModule(url: string) {
// ...
}
const req = requestModule;
inventoryTracker('apples', req, 'www.inventory-awesome.io');
```
**[⬆ back to top](#table-of-contents)**
### Use iterators and generators
Use generators and iterables when working with collections of data used like a stream.
There are some good reasons:
- decouples the callee from the generator implementation in a sense that callee decides how many
items to access
- lazy execution, items are streamed on demand
- built-in support for iterating items using the `for-of` syntax
- iterables allow to implement optimized iterator patterns
**Bad:**
```ts
function fibonacci(n: number): number[] {
if (n === 1) return [0];
if (n === 2) return [0, 1];
const items: number[] = [0, 1];
while (items.length < n) {
items.push(items[items.length - 2] + items[items.length - 1]);
}
return items;
}
function print(n: number) {
fibonacci(n).forEach(fib => console.log(fib));
}
// Print first 10 Fibonacci numbers.
print(10);
```
**Good:**
```ts
// Generates an infinite stream of Fibonacci numbers.
// The generator doesn't keep the array of all numbers.
function* fibonacci(): IterableIterator<number> {
let [a, b] = [0, 1];
while (true) {
yield a;
[a, b] = [b, a + b];
}
}
function print(n: number) {
let i = 0;
for (const fib of fibonacci()) {
if (i++ === n) break;
console.log(fib);
}
}
// Print first 10 Fibonacci numbers.
print(10);
```
There are libraries that allow working with iterables in a similar way as with native arrays, by
chaining methods like `map`, `slice`, `forEach` etc. See [itiriri](https://www.npmjs.com/package/itiriri) for
an example of advanced manipulation with iterables (or [itiriri-async](https://www.npmjs.com/package/itiriri-async) for manipulation of async iterables).
```ts
import itiriri from 'itiriri';
function* fibonacci(): IterableIterator<number> {
let [a, b] = [0, 1];
while (true) {
yield a;
[a, b] = [b, a + b];
}
}
itiriri(fibonacci())
.take(10)
.forEach(fib => console.log(fib));
```
**[⬆ back to top](#table-of-contents)**
## Objects and Data Structures
### Use getters and setters
TypeScript supports getter/setter syntax.
Using getters and setters to access data from objects that encapsulate behavior could be better than simply looking for a property on an object.
"Why?" you might ask. Well, here's a list of reasons:
- 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:**
```ts
type BankAccount = {
balance: number;
// ...
}
const value = 100;
const account: BankAccount = {
balance: 0,
// ...
};
if (value < 0) {
throw new Error('Cannot set negative balance.');
}
account.balance = value;
```
**Good:**
```ts
class BankAccount {
private accountBalance: number = 0;
get balance(): number {
return this.accountBalance;
}
set balance(value: number) {
if (value < 0) {
throw new Error('Cannot set negative balance.');
}
this.accountBalance = value;
}
// ...
}
// Now `BankAccount` encapsulates the validation logic.
// If one day the specifications change, and we need extra validation rule,
// we would have to alter only the `setter` implementation,
// leaving all dependent code unchanged.
const account = new BankAccount();
account.balance = 100;
```
**[⬆ back to top](#table-of-contents)**
### Make objects have private/protected members
TypeScript supports `public` *(default)*, `protected` and `private` accessors on class members.
**Bad:**
```ts
class Circle {
radius: number;
constructor(radius: number) {
this.radius = radius;
}
perimeter() {
return 2 * Math.PI * this.radius;
}
surface() {
return Math.PI * this.radius * this.radius;
}
}
```
**Good:**
```ts
class Circle {
constructor(private readonly radius: number) {
}
perimeter() {
return 2 * Math.PI * this.radius;
}
surface() {
return Math.PI * this.radius * this.radius;
}
}
```
**[⬆ back to top](#table-of-contents)**
### Prefer immutability
TypeScript's type system allows you to mark individual properties on an interface / class as *readonly*. This allows you to work in a functional way (unexpected mutation is bad).
For more advanced scenarios there is a built-in type `Readonly` that takes a type `T` and marks all of its properties as readonly using mapped types (see [mapped types](https://www.typescriptlang.org/docs/handbook/advanced-types.html#mapped-types)).
**Bad:**
```ts
interface Config {
host: string;
port: string;
db: string;
}
```
**Good:**
```ts
interface Config {
readonly host: string;
readonly port: string;
readonly db: string;
}
```
Case of Array, you can create a read-only array by using `ReadonlyArray<T>`.
do not allow changes such as `push()` and `fill()`, but can use features such as `concat()` and `slice()` that do not change the value.
**Bad:**
```ts
const array: number[] = [ 1, 3, 5 ];
array = []; // error
array.push(100); // array will updated
```
**Good:**
```ts
const array: ReadonlyArray<number> = [ 1, 3, 5 ];
array = []; // error
array.push(100); // error
```
Declaring read-only arguments in [TypeScript 3.4 is a bit easier](https://github.com/microsoft/TypeScript/wiki/What's-new-in-TypeScript#improvements-for-readonlyarray-and-readonly-tuples).
```ts
function hoge(args: readonly string[]) {
args.push(1); // error
}
```
Prefer [const assertions](https://github.com/microsoft/TypeScript/wiki/What's-new-in-TypeScript#const-assertions) for literal values.
**Bad:**
```ts
const config = {
hello: 'world'
};
config.hello = 'world'; // value is changed
const array = [ 1, 3, 5 ];
array[0] = 10; // value is changed
// writable objects is returned
function readonlyData(value: number) {
return { value };
}
const result = readonlyData(100);
result.value = 200; // value is changed
```
**Good:**
```ts
// read-only object
const config = {
hello: 'world'
} as const;
config.hello = 'world'; // error
// read-only array
const array = [ 1, 3, 5 ] as const;
array[0] = 10; // error
// You can return read-only objects
function readonlyData(value: number) {
return { value } as const;
}
const result = readonlyData(100);
result.value = 200; // error
```
**[⬆ back to top](#table-of-contents)**
### type vs. interface
Use type when you might need a union or intersection. Use interface when you want `extends` or `implements`. There is no strict rule however, use the one that works for you.
For a more detailed explanation refer to this [answer](https://stackoverflow.com/questions/37233735/typescript-interfaces-vs-types/54101543#54101543) about the differences between `type` and `interface` in TypeScript.
**Bad:**
```ts
interface EmailConfig {
// ...
}
interface DbConfig {
// ...
}
interface Config {
// ...
}
//...
type Shape = {
// ...
}
```
**Good:**
```ts
type EmailConfig = {
// ...
}
type DbConfig = {
// ...
}
type Config = EmailConfig | DbConfig;
// ...
interface Shape {
// ...
}
class Circle implements Shape {
// ...
}
class Square implements Shape {
// ...
}
```
**[⬆ back to top](#table-of-contents)**
## Classes
### Classes should be small
The class' size is measured by its responsibility. Following the *Single Responsibility principle* a class should be small.
**Bad:**
```ts
class Dashboard {
getLanguage(): string { /* ... */ }
setLanguage(language: string): void { /* ... */ }
showProgress(): void { /* ... */ }
hideProgress(): void { /* ... */ }
isDirty(): boolean { /* ... */ }
disable(): void { /* ... */ }
enable(): void { /* ... */ }
addSubscription(subscription: Subscription): void { /* ... */ }
removeSubscription(subscription: Subscription): void { /* ... */ }
addUser(user: User): void { /* ... */ }
removeUser(user: User): void { /* ... */ }
goToHomePage(): void { /* ... */ }
updateProfile(details: UserDetails): void { /* ... */ }
getVersion(): string { /* ... */ }
// ...
}
```
**Good:**
```ts
class Dashboard {
disable(): void { /* ... */ }
enable(): void { /* ... */ }
getVersion(): string { /* ... */ }
}
// split the responsibilities by moving the remaining methods to other classes
// ...
```
**[⬆ back to top](#table-of-contents)**
### High cohesion and low coupling
Cohesion defines the degree to which class members are related to each other. Ideally, all fields within a class should be used by each method.
We then say that the class is *maximally cohesive*. In practice, this however is not always possible, nor even advisable. You should however prefer cohesion to be high.
Coupling refers to how related or dependent are two classes toward each other. Classes are said to be low coupled if changes in one of them doesn't affect the other one.
Good software design has **high cohesion** and **low coupling**.
**Bad:**
```ts
class UserManager {
// Bad: each private variable is used by one or another group of methods.
// It makes clear evidence that the class is holding more than a single responsibility.
// If I need only to create the service to get the transactions for a user,
// I'm still forced to pass and instance of `emailSender`.
constructor(
private readonly db: Database,
private readonly emailSender: EmailSender) {
}
async getUser(id: number): Promise<User> {
return await db.users.findOne({ id });
}
async getTransactions(userId: number): Promise<Transaction[]> {
return await db.transactions.find({ userId });
}
async sendGreeting(): Promise<void> {
await emailSender.send('Welcome!');
}
async sendNotification(text: string): Promise<void> {
await emailSender.send(text);
}
async sendNewsletter(): Promise<void> {
// ...
}
}
```
**Good:**
```ts
class UserService {
constructor(private readonly db: Database) {
}
async getUser(id: number): Promise<User> {
return await this.db.users.findOne({ id });
}
async getTransactions(userId: number): Promise<Transaction[]> {
return await this.db.transactions.find({ userId });
}
}
class UserNotifier {
constructor(private readonly emailSender: EmailSender) {
}
async sendGreeting(): Promise<void> {
await this.emailSender.send('Welcome!');
}
async sendNotification(text: string): Promise<void> {
await this.emailSender.send(text);
}
async sendNewsletter(): Promise<void> {
// ...
}
}
```
**[⬆ back to top](#table-of-contents)**
### Prefer composition over inheritance
As stated famously in [Design Patterns](https://en.wikipedia.org/wiki/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:**
```ts
class Employee {
constructor(
private readonly name: string,
private readonly email: string) {
}
// ...
}
// Bad because Employees "have" tax data. EmployeeTaxData is not a type of Employee
class EmployeeTaxData extends Employee {
constructor(
name: string,
email: string,
private readonly ssn: string,
private readonly salary: number) {
super(name, email);
}
// ...
}
```
**Good:**
```ts
class Employee {
private taxData: EmployeeTaxData;
constructor(
private readonly name: string,
private readonly email: string) {
}
setTaxData(ssn: string, salary: number): Employee {
this.taxData = new EmployeeTaxData(ssn, salary);
return this;
}
// ...
}
class EmployeeTaxData {
constructor(
public readonly ssn: string,
public readonly salary: number) {
}
// ...
}
```
**[⬆ back to top](#table-of-contents)**
### Use method chaining
This pattern is very useful and commonly used in many libraries. It allows your code to be expressive, and less verbose. For that reason, use method chaining and take a look at how clean your code will be.
**Bad:**
```ts
class QueryBuilder {
private collection: string;
private pageNumber: number = 1;
private itemsPerPage: number = 100;
private orderByFields: string[] = [];
from(collection: string): void {
this.collection = collection;
}
page(number: number, itemsPerPage: number = 100): void {
this.pageNumber = number;
this.itemsPerPage = itemsPerPage;
}
orderBy(...fields: string[]): void {
this.orderByFields = fields;
}
build(): Query {
// ...
}
}
// ...
const queryBuilder = new QueryBuilder();
queryBuilder.from('users');
queryBuilder.page(1, 100);
queryBuilder.orderBy('firstName', 'lastName');
const query = queryBuilder.build();
```
**Good:**
```ts
class QueryBuilder {
private collection: string;
private pageNumber: number = 1;
private itemsPerPage: number = 100;
private orderByFields: string[] = [];
from(collection: string): this {
this.collection = collection;
return this;
}
page(number: number, itemsPerPage: number = 100): this {
this.pageNumber = number;
this.itemsPerPage = itemsPerPage;
return this;
}
orderBy(...fields: string[]): this {
this.orderByFields = fields;
return this;
}
build(): Query {
// ...
}
}
// ...
const query = new QueryBuilder()
.from('users')
.page(1, 100)
.orderBy('firstName', 'lastName')
.build();
```
**[⬆ back to top](#table-of-contents)**
## SOLID
### Single Responsibility Principle (SRP)
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:**
```ts
class UserSettings {
constructor(private readonly user: User) {
}
changeSettings(settings: UserSettings) {
if (this.verifyCredentials()) {
// ...
}
}
verifyCredentials() {
// ...
}
}
```
**Good:**
```ts
class UserAuth {
constructor(private readonly user: User) {
}
verifyCredentials() {
// ...
}
}
class UserSettings {
private readonly auth: UserAuth;
constructor(private readonly user: User) {
this.auth = new UserAuth(user);
}
changeSettings(settings: UserSettings) {
if (this.auth.verifyCredentials()) {
// ...
}
}
}
```
**[⬆ back to top](#table-of-contents)**
### Open/Closed Principle (OCP)
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:**
```ts
class AjaxAdapter extends Adapter {
constructor() {
super();
}
// ...
}
class NodeAdapter extends Adapter {
constructor() {
super();
}
// ...
}
class HttpRequester {
constructor(private readonly adapter: Adapter) {
}
async fetch<T>(url: string): Promise<T> {
if (this.adapter instanceof AjaxAdapter) {
const response = await makeAjaxCall<T>(url);
// transform response and return
} else if (this.adapter instanceof NodeAdapter) {
const response = await makeHttpCall<T>(url);
// transform response and return
}
}
}
function makeAjaxCall<T>(url: string): Promise<T> {
// request and return promise
}
function makeHttpCall<T>(url: string): Promise<T> {
// request and return promise
}
```
**Good:**
```ts
abstract class Adapter {
abstract async request<T>(url: string): Promise<T>;
// code shared to subclasses ...
}
class AjaxAdapter extends Adapter {
constructor() {
super();
}
async request<T>(url: string): Promise<T>{
// request and return promise
}
// ...
}
class NodeAdapter extends Adapter {
constructor() {
super();
}
async request<T>(url: string): Promise<T>{
// request and return promise
}
// ...
}
class HttpRequester {
constructor(private readonly adapter: Adapter) {
}
async fetch<T>(url: string): Promise<T> {
const response = await this.adapter.request<T>(url);
// transform response and return
}
}
```
**[⬆ back to top](#table-of-contents)**
### Liskov Substitution Principle (LSP)
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 parent 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:**
```ts
class Rectangle {
constructor(
protected width: number = 0,
protected height: number = 0) {
}
setColor(color: string): this {
// ...
}
render(area: number) {
// ...
}
setWidth(width: number): this {
this.width = width;
return this;
}
setHeight(height: number): this {
this.height = height;
return this;
}
getArea(): number {
return this.width * this.height;
}
}
class Square extends Rectangle {
setWidth(width: number): this {
this.width = width;
this.height = width;
return this;
}
setHeight(height: number): this {
this.width = height;
this.height = height;
return this;
}
}
function renderLargeRectangles(rectangles: Rectangle[]) {
rectangles.forEach((rectangle) => {
const area = rectangle
.setWidth(4)
.setHeight(5)
.getArea(); // BAD: Returns 25 for Square. Should be 20.
rectangle.render(area);
});
}
const rectangles = [new Rectangle(), new Rectangle(), new Square()];
renderLargeRectangles(rectangles);
```
**Good:**
```ts
abstract class Shape {
setColor(color: string): this {
// ...
}
render(area: number) {
// ...
}
abstract getArea(): number;
}
class Rectangle extends Shape {
constructor(
private readonly width = 0,
private readonly height = 0) {
super();
}
getArea(): number {
return this.width * this.height;
}
}
class Square extends Shape {
constructor(private readonly length: number) {
super();
}
getArea(): number {
return this.length * this.length;
}
}
function renderLargeShapes(shapes: Shape[]) {
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);
```
**[⬆ back to top](#table-of-contents)**
### Interface Segregation Principle (ISP)
ISP states that "Clients should not be forced to depend upon interfaces that they do not use.". This principle is very much related to the Single Responsibility Principle.
What it really means is that you should always design your abstractions in a way that the clients that are using the exposed methods do not get the whole pie instead. That also include imposing the clients with the burden of implementing methods that they don’t actually need.
**Bad:**
```ts
interface SmartPrinter {
print();
fax();
scan();
}
class AllInOnePrinter implements SmartPrinter {
print() {
// ...
}
fax() {
// ...
}
scan() {
// ...
}
}
class EconomicPrinter implements SmartPrinter {
print() {
// ...
}
fax() {
throw new Error('Fax not supported.');
}
scan() {
throw new Error('Scan not supported.');
}
}
```
**Good:**
```ts
interface Printer {
print();
}
interface Fax {
fax();
}
interface Scanner {
scan();
}
class AllInOnePrinter implements Printer, Fax, Scanner {
print() {
// ...
}
fax() {
// ...
}
scan() {
// ...
}
}
class EconomicPrinter implements Printer {
print() {
// ...
}
}
```
**[⬆ back to top](#table-of-contents)**
### Dependency Inversion Principle (DIP)
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 Angular, 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.
DIP is usually achieved by a using an inversion of control (IoC) container. An example of a powerful IoC container for TypeScript is [InversifyJs](https://www.npmjs.com/package/inversify)
**Bad:**
```ts
import { readFile as readFileCb } from 'fs';
import { promisify } from 'util';
const readFile = promisify(readFileCb);
type ReportData = {
// ..
}
class XmlFormatter {
parse<T>(content: string): T {
// Converts an XML string to an object T
}
}
class ReportReader {
// BAD: We have created a dependency on a specific request implementation.
// We should just have ReportReader depend on a parse method: `parse`
private readonly formatter = new XmlFormatter();
async read(path: string): Promise<ReportData> {
const text = await readFile(path, 'UTF8');
return this.formatter.parse<ReportData>(text);
}
}
// ...
const reader = new ReportReader();
await report = await reader.read('report.xml');
```
**Good:**
```ts
import { readFile as readFileCb } from 'fs';
import { promisify } from 'util';
const readFile = promisify(readFileCb);
type ReportData = {
// ..
}
interface Formatter {
parse<T>(content: string): T;
}
class XmlFormatter implements Formatter {
parse<T>(content: string): T {
// Converts an XML string to an object T
}
}
class JsonFormatter implements Formatter {
parse<T>(content: string): T {
// Converts a JSON string to an object T
}
}
class ReportReader {
constructor(private readonly formatter: Formatter) {
}
async read(path: string): Promise<ReportData> {
const text = await readFile(path, 'UTF8');
return this.formatter.parse<ReportData>(text);
}
}
// ...
const reader = new ReportReader(new XmlFormatter());
await report = await reader.read('report.xml');
// or if we had to read a json report
const reader = new ReportReader(new JsonFormatter());
await report = await reader.read('report.json');
```
**[⬆ back to top](#table-of-contents)**
## 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](https://github.com/gotwarlost/istanbul).
There's no excuse to not write tests. There are [plenty of good JS test frameworks](http://jstherightway.org/#testing-tools) with typings support for TypeScript, 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.
### The three laws of TDD
1. You are not allowed to write any production code unless it is to make a failing unit test pass.
2. You are not allowed to write any more of a unit test than is sufficient to fail; and compilation failures are failures.
3. You are not allowed to write any more production code than is sufficient to pass the one failing unit test.
**[⬆ back to top](#table-of-contents)**
### F.I.R.S.T. rules
Clean tests should follow the rules:
- **Fast** tests should be fast because we want to run them frequently.
- **Independent** tests should not depend on each other. They should provide same output whether run independently or all together in any order.
- **Repeatable** tests should be repeatable in any environment and there should be no excuse for why they fail.
- **Self-Validating** a test should answer with either *Passed* or *Failed*. You don't need to compare log files to answer if a test passed.
- **Timely** unit tests should be written before the production code. If you write tests after the production code, you might find writing tests too hard.
**[⬆ back to top](#table-of-contents)**
### Single concept per test
Tests should also follow the *Single Responsibility Principle*. Make only one assert per unit test.
**Bad:**
```ts
import { assert } from 'chai';
describe('AwesomeDate', () => {
it('handles date boundaries', () => {
let date: AwesomeDate;
date = new AwesomeDate('1/1/2015');
assert.equal('1/31/2015', date.addDays(30));
date = new AwesomeDate('2/1/2016');
assert.equal('2/29/2016', date.addDays(28));
date = new AwesomeDate('2/1/2015');
assert.equal('3/1/2015', date.addDays(28));
});
});
```
**Good:**
```ts
import { assert } from 'chai';
describe('AwesomeDate', () => {
it('handles 30-day months', () => {
const date = new AwesomeDate('1/1/2015');
assert.equal('1/31/2015', date.addDays(30));
});
it('handles leap year', () => {
const date = new AwesomeDate('2/1/2016');
assert.equal('2/29/2016', date.addDays(28));
});
it('handles non-leap year', () => {
const date = new AwesomeDate('2/1/2015');
assert.equal('3/1/2015', date.addDays(28));
});
});
```
**[⬆ back to top](#table-of-contents)**
### The name of the test should reveal its intention
When a test fail, its name is the first indication of what may have gone wrong.
**Bad:**
```ts
describe('Calendar', () => {
it('2/29/2020', () => {
// ...
});
it('throws', () => {
// ...
});
});
```
**Good:**
```ts
describe('Calendar', () => {
it('should handle leap year', () => {
// ...
});
it('should throw when format is invalid', () => {
// ...
});
});
```
**[⬆ back to top](#table-of-contents)**
## Concurrency
### Prefer promises vs callbacks
Callbacks aren't clean, and they cause excessive amounts of nesting *(the callback hell)*.
There are utilities that transform existing functions using the callback style to a version that returns promises
(for Node.js see [`util.promisify`](https://nodejs.org/dist/latest-v8.x/docs/api/util.html#util_util_promisify_original), for general purpose see [pify](https://www.npmjs.com/package/pify), [es6-promisify](https://www.npmjs.com/package/es6-promisify))
**Bad:**
```ts
import { get } from 'request';
import { writeFile } from 'fs';
function downloadPage(url: string, saveTo: string, callback: (error: Error, content?: string) => void) {
get(url, (error, response) => {
if (error) {
callback(error);
} else {
writeFile(saveTo, response.body, (error) => {
if (error) {
callback(error);
} else {
callback(null, response.body);
}
});
}
});
}
downloadPage('https://en.wikipedia.org/wiki/Robert_Cecil_Martin', 'article.html', (error, content) => {
if (error) {
console.error(error);
} else {
console.log(content);
}
});
```
**Good:**
```ts
import { get } from 'request';
import { writeFile } from 'fs';
import { promisify } from 'util';
const write = promisify(writeFile);
function downloadPage(url: string, saveTo: string): Promise<string> {
return get(url)
.then(response => write(saveTo, response));
}
downloadPage('https://en.wikipedia.org/wiki/Robert_Cecil_Martin', 'article.html')
.then(content => console.log(content))
.catch(error => console.error(error));
```
Promises supports a few helper methods that help make code more conscise:
| Pattern | Description |
| ------------------------ | ----------------------------------------- |
| `Promise.resolve(value)` | Convert a value into a resolved promise. |
| `Promise.reject(error)` | Convert an error into a rejected promise. |
| `Promise.all(promises)` |Returns a new promise which is fulfilled with an array of fulfillment values for the passed promises or rejects with the reason of the first promise that rejects. |
| `Promise.race(promises)`|Returns a new promise which is fulfilled/rejected with the result/error of the first settled promise from the array of passed promises. |
`Promise.all` is especially useful when there is a need to run tasks in parallel. `Promise.race` makes it easier to implement things like timeouts for promises.
**[⬆ back to top](#table-of-contents)**
### Async/Await are even cleaner than Promises
With `async`/`await` syntax you can write code that is far cleaner and more understandable than chained promises. Within a function prefixed with `async` keyword you have a way to tell the JavaScript runtime to pause the execution of code on the `await` keyword (when used on a promise).
**Bad:**
```ts
import { get } from 'request';
import { writeFile } from 'fs';
import { promisify } from 'util';
const write = util.promisify(writeFile);
function downloadPage(url: string, saveTo: string): Promise<string> {
return get(url).then(response => write(saveTo, response));
}
downloadPage('https://en.wikipedia.org/wiki/Robert_Cecil_Martin', 'article.html')
.then(content => console.log(content))
.catch(error => console.error(error));
```
**Good:**
```ts
import { get } from 'request';
import { writeFile } from 'fs';
import { promisify } from 'util';
const write = promisify(writeFile);
async function downloadPage(url: string, saveTo: string): Promise<string> {
const response = await get(url);
await write(saveTo, response);
return response;
}
// somewhere in an async function
try {
const content = await downloadPage('https://en.wikipedia.org/wiki/Robert_Cecil_Martin', 'article.html');
console.log(content);
} catch (error) {
console.error(error);
}
```
**[⬆ back to top](#table-of-contents)**
## 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.
### Always use Error for throwing or rejecting
JavaScript as well as TypeScript allow you to `throw` any object. A Promise can also be rejected with any reason object.
It is advisable to use the `throw` syntax with an `Error` type. This is because your error might be caught in higher level code with a `catch` syntax.
It would be very confusing to catch a string message there and would make
[debugging more painful](https://basarat.gitbooks.io/typescript/docs/types/exceptions.html#always-use-error).
For the same reason you should reject promises with `Error` types.
**Bad:**
```ts
function calculateTotal(items: Item[]): number {
throw 'Not implemented.';
}
function get(): Promise<Item[]> {
return Promise.reject('Not implemented.');
}
```
**Good:**
```ts
function calculateTotal(items: Item[]): number {
throw new Error('Not implemented.');
}
function get(): Promise<Item[]> {
return Promise.reject(new Error('Not implemented.'));
}
// or equivalent to:
async function get(): Promise<Item[]> {
throw new Error('Not implemented.');
}
```
The benefit of using `Error` types is that it is supported by the syntax `try/catch/finally` and implicitly all errors have the `stack` property which
is very powerful for debugging.
There are also another alternatives, not to use the `throw` syntax and instead always return custom error objects. TypeScript makes this even easier.
Consider following example:
```ts
type Result<R> = { isError: false, value: R };
type Failure<E> = { isError: true, error: E };
type Failable<R, E> = Result<R> | Failure<E>;
function calculateTotal(items: Item[]): Failable<number, 'empty'> {
if (items.length === 0) {
return { isError: true, error: 'empty' };
}
// ...
return { isError: false, value: 42 };
}
```
For the detailed explanation of this idea refer to the [original post](https://medium.com/@dhruvrajvanshi/making-exceptions-type-safe-in-typescript-c4d200ee78e9).
**[⬆ back to top](#table-of-contents)**
### Don't ignore caught errors
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:**
```ts
try {
functionThatMightThrow();
} catch (error) {
console.log(error);
}
// or even worse
try {
functionThatMightThrow();
} catch (error) {
// ignore error
}
```
**Good:**
```ts
import { logger } from './logging'
try {
functionThatMightThrow();
} catch (error) {
logger.log(error);
}
```
**[⬆ back to top](#table-of-contents)**
### Don't ignore rejected promises
For the same reason you shouldn't ignore caught errors from `try/catch`.
**Bad:**
```ts
getUser()
.then((user: User) => {
return sendEmail(user.email, 'Welcome!');
})
.catch((error) => {
console.log(error);
});
```
**Good:**
```ts
import { logger } from './logging'
getUser()
.then((user: User) => {
return sendEmail(user.email, 'Welcome!');
})
.catch((error) => {
logger.log(error);
});
// or using the async/await syntax:
try {
const user = await getUser();
await sendEmail(user.email, 'Welcome!');
} catch (error) {
logger.log(error);
}
```
**[⬆ back to top](#table-of-contents)**
## 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. The general rule to follow is *keep consistent formatting rules*.
For TypeScript there is a powerful tool called [TSLint](https://palantir.github.io/tslint/). It's a static analysis tool that can help you improve dramatically the readability and maintainability of your code. There are ready to use TSLint configurations that you can reference in your projects:
- [TSLint Config Standard](https://www.npmjs.com/package/tslint-config-standard) - standard style rules
- [TSLint Config Airbnb](https://www.npmjs.com/package/tslint-config-airbnb) - Airbnb style guide
- [TSLint Clean Code](https://www.npmjs.com/package/tslint-clean-code) - TSLint rules inspired by the [Clean Code: A Handbook of Agile Software Craftsmanship](https://www.amazon.ca/Clean-Code-Handbook-Software-Craftsmanship/dp/0132350882)
- [TSLint react](https://www.npmjs.com/package/tslint-react) - lint rules related to React & JSX
- [TSLint + Prettier](https://www.npmjs.com/package/tslint-config-prettier) - lint rules for [Prettier](https://github.com/prettier/prettier) code formatter
- [ESLint rules for TSLint](https://www.npmjs.com/package/tslint-eslint-rules) - ESLint rules for TypeScript
- [Immutable](https://www.npmjs.com/package/tslint-immutable) - rules to disable mutation in TypeScript
Refer also to this great [TypeScript StyleGuide and Coding Conventions](https://basarat.gitbooks.io/typescript/docs/styleguide/styleguide.html) source.
### Use consistent capitalization
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:**
```ts
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() {}
type animal = { /* ... */ }
type Container = { /* ... */ }
```
**Good:**
```ts
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() {}
type Animal = { /* ... */ }
type Container = { /* ... */ }
```
Prefer using `PascalCase` for class, interface, type and namespace names.
Prefer using `camelCase` for variables, functions and class members.
**[⬆ back to top](#table-of-contents)**
### Function callers and callees should be close
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:**
```ts
class PerformanceReview {
constructor(private readonly employee: Employee) {
}
private lookupPeers() {
return db.lookup(this.employee.id, 'peers');
}
private lookupManager() {
return db.lookup(this.employee, 'manager');
}
private getPeerReviews() {
const peers = this.lookupPeers();
// ...
}
review() {
this.getPeerReviews();
this.getManagerReview();
this.getSelfReview();
// ...
}
private getManagerReview() {
const manager = this.lookupManager();
}
private getSelfReview() {
// ...
}
}
const review = new PerformanceReview(employee);
review.review();
```
**Good:**
```ts
class PerformanceReview {
constructor(private readonly employee: Employee) {
}
review() {
this.getPeerReviews();
this.getManagerReview();
this.getSelfReview();
// ...
}
private getPeerReviews() {
const peers = this.lookupPeers();
// ...
}
private lookupPeers() {
return db.lookup(this.employee.id, 'peers');
}
private getManagerReview() {
const manager = this.lookupManager();
}
private lookupManager() {
return db.lookup(this.employee, 'manager');
}
private getSelfReview() {
// ...
}
}
const review = new PerformanceReview(employee);
review.review();
```
**[⬆ back to top](#table-of-contents)**
### Organize imports
With clean and easy to read import statements you can quickly see the dependencies of current code. Make sure you apply following good practices for `import` statements:
- Import statements should be alphabetized and grouped.
- Unused imports should be removed.
- Named imports must be alphabetized (i.e. `import {A, B, C} from 'foo';`)
- Import sources must be alphabetized within groups, i.e.: `import * as foo from 'a'; import * as bar from 'b';`
- Groups of imports are delineated by blank lines.
- Groups must respect following order:
- Polyfills (i.e. `import 'reflect-metadata';`)
- Node builtin modules (i.e. `import fs from 'fs';`)
- external modules (i.e. `import { query } from 'itiriri';`)
- internal modules (i.e `import { UserService } from 'src/services/userService';`)
- modules from a parent directory (i.e. `import foo from '../foo'; import qux from '../../foo/qux';`)
- modules from the same or a sibling's directory (i.e. `import bar from './bar'; import baz from './bar/baz';`)
**Bad:**
```ts
import { TypeDefinition } from '../types/typeDefinition';
import { AttributeTypes } from '../model/attribute';
import { ApiCredentials, Adapters } from './common/api/authorization';
import fs from 'fs';
import { ConfigPlugin } from './plugins/config/configPlugin';
import { BindingScopeEnum, Container } from 'inversify';
import 'reflect-metadata';
```
**Good:**
```ts
import 'reflect-metadata';
import fs from 'fs';
import { BindingScopeEnum, Container } from 'inversify';
import { AttributeTypes } from '../model/attribute';
import { TypeDefinition } from '../types/typeDefinition';
import { ApiCredentials, Adapters } from './common/api/authorization';
import { ConfigPlugin } from './plugins/config/configPlugin';
```
**[⬆ back to top](#table-of-contents)**
### Use typescript aliases
Create prettier imports by defining the paths and baseUrl properties in the compilerOptions section in the `tsconfig.json`
This will avoid long relative paths when doing imports.
**Bad:**
```ts
import { UserService } from '../../../services/UserService';
```
**Good:**
```ts
import { UserService } from '@services/UserService';
```
```js
// tsconfig.json
...
"compilerOptions": {
...
"baseUrl": "src",
"paths": {
"@services": ["services/*"]
}
...
}
...
```
**[⬆ back to top](#table-of-contents)**
## Comments
The use of a comments is an indication of failure to express without them. Code should be the only source of truth.
> Don’t comment bad code—rewrite it.
> — *Brian W. Kernighan and P. J. Plaugher*
### Prefer self explanatory code instead of comments
Comments are an apology, not a requirement. Good code *mostly* documents itself.
**Bad:**
```ts
// Check if subscription is active.
if (subscription.endDate > Date.now) { }
```
**Good:**
```ts
const isSubscriptionActive = subscription.endDate > Date.now;
if (isSubscriptionActive) { /* ... */ }
```
**[⬆ back to top](#table-of-contents)**
### Don't leave commented out code in your codebase
Version control exists for a reason. Leave old code in your history.
**Bad:**
```ts
type User = {
name: string;
email: string;
// age: number;
// jobPosition: string;
}
```
**Good:**
```ts
type User = {
name: string;
email: string;
}
```
**[⬆ back to top](#table-of-contents)**
### Don't have journal comments
Remember, use version control! There's no need for dead code, commented code, and especially journal comments. Use `git log` to get history!
**Bad:**
```ts
/**
* 2016-12-20: Removed monads, didn't understand them (RM)
* 2016-10-01: Improved using special monads (JP)
* 2016-02-03: Added type-checking (LI)
* 2015-03-14: Implemented combine (JR)
*/
function combine(a: number, b: number): number {
return a + b;
}
```
**Good:**
```ts
function combine(a: number, b: number): number {
return a + b;
}
```
**[⬆ back to top](#table-of-contents)**
### Avoid positional markers
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.
Most IDE support code folding feature that allows you to collapse/expand blocks of code (see Visual Studio Code [folding regions](https://code.visualstudio.com/updates/v1_17#_folding-regions)).
**Bad:**
```ts
////////////////////////////////////////////////////////////////////////////////
// Client class
////////////////////////////////////////////////////////////////////////////////
class Client {
id: number;
name: string;
address: Address;
contact: Contact;
////////////////////////////////////////////////////////////////////////////////
// public methods
////////////////////////////////////////////////////////////////////////////////
public describe(): string {
// ...
}
////////////////////////////////////////////////////////////////////////////////
// private methods
////////////////////////////////////////////////////////////////////////////////
private describeAddress(): string {
// ...
}
private describeContact(): string {
// ...
}
};
```
**Good:**
```ts
class Client {
id: number;
name: string;
address: Address;
contact: Contact;
public describe(): string {
// ...
}
private describeAddress(): string {
// ...
}
private describeContact(): string {
// ...
}
};
```
**[⬆ back to top](#table-of-contents)**
### TODO comments
When you find yourself that you need to leave notes in the code for some later improvements,
do that using `// TODO` comments. Most IDE have special support for those kind of comments so that
you can quickly go over the entire list of todos.
Keep in mind however that a *TODO* comment is not an excuse for bad code.
**Bad:**
```ts
function getActiveSubscriptions(): Promise<Subscription[]> {
// ensure `dueDate` is indexed.
return db.subscriptions.find({ dueDate: { $lte: new Date() } });
}
```
**Good:**
```ts
function getActiveSubscriptions(): Promise<Subscription[]> {
// TODO: ensure `dueDate` is indexed.
return db.subscriptions.find({ dueDate: { $lte: new Date() } });
}
```
**[⬆ back to top](#table-of-contents)**
## Translations
This is also available in other languages:
- ![br](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Brazil.png) **Brazilian Portuguese**: [vitorfreitas/clean-code-typescript](https://github.com/vitorfreitas/clean-code-typescript)
- ![cn](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/China.png) **Chinese**:
- [beginor/clean-code-typescript](https://github.com/beginor/clean-code-typescript)
- [pipiliang/clean-code-typescript](https://github.com/pipiliang/clean-code-typescript)
- ![ja](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Japan.png) **Japanese**: [MSakamaki/clean-code-typescript](https://github.com/MSakamaki/clean-code-typescript)
- ![tr](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Turkey.png) **Turkish**: [ozanhonamlioglu/clean-code-typescript](https://github.com/ozanhonamlioglu/clean-code-typescript)
There is work in progress for translating this to other languages:
- ![kr](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/South-Korea.png) Korean
References will be added once translations are completed.
Check this [discussion](https://github.com/labs42io/clean-code-typescript/issues/15) for more details and progress.
You can make an indispensable contribution to *Clean Code* community by translating this to your language.
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