clean-code-typescript/README.md

clean-code-typescript

Clean Code concepts adapted for TypeScript.
Inspired from clean-code-javascript

Table of Contents

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

Introduction

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 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!

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Variables

Use meaningful variable names

Distinguish names in such a way that the reader knows what the differences offer.

Bad:

function between<T>(a1: T, a2: T, a3: T) {
  return a2 <= a1 && a1 <= a3;
}

Good:

function between<T>(value: T, left: T, right: T) {
  return left <= value && value <= right;
}

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Use pronounceable variable names

If you can’t pronounce it, you can’t discuss it without sounding like an idiot.

Bad:

class DtaRcrd102 {
  private genymdhms: Date;
  private modymdhms: Date;
  private pszqint = '102';
}

Good:

class Customer {
  private generationTimestamp: Date;
  private modificationTimestamp: Date;
  private recordId = '102';
}

Use the same vocabulary for the same type of variable

Bad:

function getUserInfo(): User;
function getUserDetails(): User;
function getUserData(): User;

Good:

function getUser(): User;

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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. Tools like TSLint can help identify unnamed constants.

Bad:

// What the heck is 86400000 for?
setTimeout(restart, 86400000);

Good:

// Declare them as capitalized named constants.
const MILLISECONDS_IN_A_DAY = 24 * 60 * 60 * 1000;

setTimeout(restart, MILLISECONDS_IN_A_DAY);

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Use explanatory variables

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
}

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Avoid Mental Mapping

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);

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Don't add unneeded context

If your class/object name tells you something, don't repeat that in your variable name.

Bad:

class Car {
  carMake: string;
  carModel: string;
  carColor: string;

  name(): string{
    return `${this.carMake} ${this.carModel} (${this.carColor})`;
  }
}

Good:

class Car {
  make: string;
  model: string;
  color: string;

  name(): string{
    return `${this.make} ${this.model} (${this.color})`;
  }
}

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Use default arguments instead of short circuiting or conditionals

Default arguments are often cleaner than short circuiting.

Bad:

function loadPages(count: number) {
  const loadCount = count || 10;
}

Good:

function loadPages(count: number = 10) {
}

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Functions

Function arguments (2 or fewer 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.

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:

1. When someone looks at the function signature, it's immediately clear what properties are being used.

2. 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.

3. 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 TypeScript's 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
});

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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: Client) {
  clients.forEach((client) => {
    const clientRecord = database.lookup(client);
    if (clientRecord.isActive()) {
      email(client);
    }
  });
}

Good:

function emailClients(clients: Client) {
  clients.filter(isActiveClient).forEach(email);
}

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

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Function names should say what they do

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);

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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 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:

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;
}

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Remove duplicate code

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:

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);
  });
}

Good:

class Developer {
  // ...
  getExtraDetails() {
    return {
      githubLink: this.githubLink,
    }
  }
}

class Manager {
  // ...
  getExtraDetails() {
    return {
      portfolio: this.portfolio,
    }
  }
}

function showEmployeeList(employee: Developer | Manager) {
  employee.forEach((employee) => {
    const expectedSalary = developer.calculateExpectedSalary();
    const experience = developer.getExperience();
    const extra = employee.getExtraDetails();

    const data = {
      expectedSalary,
      experience,
      extra,
    };

    render(data);
  });
}

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Set default objects with Object.assign or destructuring

Bad:

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;
}

const menuConfig = {
  title: null,
  body: 'Bar',
  buttonText: null,
  cancellable: true
};

createMenu(menuConfig);

Good:

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' });

Alternatively, you can use destructuring with default values:

type MenuConfig = {title?: string, body?: string, buttonText?: string, cancellable?: boolean};

function createMenu({title = 'Foo', body = 'Bar', buttonText = 'Baz', cancellable = true}: MenuConfig) {
  // ...
}

createMenu({ body: 'Bar' });

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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:string, temp:boolean) {
  if (temp) {
    fs.create(`./temp/${name}`);
  } else {
    fs.create(name);
  }
}

Good:

function createFile(name:string) {
  fs.create(name);
}

function createTempFile(name:string) {
  fs.create(`./temp/${name}`);
}

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Avoid Side Effects (part 1)

A function produces a side effect if it does anything other than take a value in and return another value or values. A side effect could be writing to a file, modifying some global variable, or accidentally wiring all your money to a stranger.

Now, you do need to have side effects in a program on occasion. Like the previous example, you might need to write to a file. What you want to do is to centralize where you are doing this. Don't have several functions and classes that write to a particular file. Have one service that does it. One and only one.

The main point is to avoid common pitfalls like sharing state between objects without any structure, using mutable data types that can be written to by anything, and not centralizing where your side effects occur. If you can do this, you will be happier than the vast majority of other programmers.

Bad:

// Global variable referenced by following function.
// If we had another function that used this name, now it'd be an array and it could break it.
let name = 'Robert C. Martin';

function toBase64() {
  name = btoa(name);
}

toBase64(); // produces side effects to `name` variable

console.log(name); // expected to print 'Robert C. Martin' but instead 'Um9iZXJ0IEMuIE1hcnRpbg=='

Good:

// 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.
const name = 'Robert C. Martin';

function toBase64(text:string):string {
  return btoa(text);
}

const encodedName = toBase64();

console.log(name);

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Avoid Side Effects (part 2)

In JavaScript, primitives are passed by value and objects/arrays are passed by reference. In the case of objects and arrays, if your function makes a change in a shopping cart array, for example, by adding an item to purchase, then any other function that uses that cart array will be affected by this addition. That may be great, however it can be bad too. Let's imagine a bad situation:

The user clicks the "Purchase", button which calls a purchase function that spawns a network request and sends the cart array to the server. Because of a bad network connection, the purchase function has to keep retrying the request. Now, what if in the meantime the user accidentally clicks "Add to Cart" button on an item they don't actually want before the network request begins? If that happens and the network request begins, then that purchase function will send the accidentally added item because it has a reference to a shopping cart array that the addItemToCart function modified by adding an unwanted item.

A great solution would be for the addItemToCart to always clone the cart, edit it, and return the clone. This ensures that no other functions that are holding onto a reference of the shopping cart will be affected by any changes.

Two caveats to mention to this approach:

1. There might be cases where you actually want to modify the input object, but when you adopt this programming practice you will find that those cases are pretty rare. Most things can be refactored to have no side effects! (see pure function)

2. Cloning big objects can be very expensive in terms of performance. Luckily, this isn't a big issue in practice because there are great libraries that allow this kind of programming approach to be fast and not as memory intensive as it would be for you to manually clone objects and arrays.

Bad:

function addItemToCart(cart: CartItem[], item:Item):void {
  cart.push({ item, date: Date.now() });
};

Good:

function addItemToCart(cart: CartItem[], item:Item):CartItem[] {
  return {...cart, { item, date: Date.now() }};
};

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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:

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:

class MyArray<T> extends Array<T> {
  diff(other: T[]): T[] {
    const hash = new Set(other);
    return this.filter(elem => !hash.has(elem));
  };
}

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Favor functional programming over imperative programming

Favor this style of programming when you can.

Bad:

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:

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)

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Encapsulate conditionals

Bad:

if (subscription.isTrial || account.balance > 0) {
  // ...
}

Good:

function canActivateService(subscription: Subscription, account: Account) {
  return subscription.isTrial || account.balance > 0
}

if (canActivateService(subscription, account)) {
  // ...
}

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Avoid negative conditionals

Bad:

function isEmailNotUsed(email: string) {
  // ...
}

if (isEmailNotUsed(email)) {
  // ...
}

Good:

function isEmailUsed(email) {
  // ...
}

if (!isEmailUsed(node)) {
  // ...
}

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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 {
  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.');
    }
  }
}

Good:

class Airplane {
  // ...
}

class Boeing777 extends Airplane {
  // ...
  getCruisingAltitude() {
    return this.getMaxAltitude() - this.getPassengerCount();
  }
}

class AirForceOne extends Airplane {
  // ...
  getCruisingAltitude() {
    return this.getMaxAltitude();
  }
}

class Cessna extends Airplane {
  // ...
  getCruisingAltitude() {
    return this.getMaxAltitude() - this.getFuelExpenditure();
  }
}

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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:

function travelToTexas(vehicle: Bicycle | Car) {
  if (vehicle instanceof Bicycle) {
    vehicle.pedal(this.currentLocation, new Location('texas'));
  } else if (vehicle instanceof Car) {
    vehicle.drive(this.currentLocation, new Location('texas'));
  }
}

Good:

type Vehicle = Bicycle | Car;

function travelToTexas(vehicle: Vehicle) {
  vehicle.move(this.currentLocation, new Location('texas'));
}

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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 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:

for (let i = 0; i < list.length; i++) {
  // ...
}

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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: string) {
  // ...
}

function requestModule(url: string) {
  // ...
}

const req = requestModule;
inventoryTracker('apples', req, 'www.inventory-awesome.io');

Good:

for (let i = 0; i < list.length; i++) {
  // ...
}

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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 that 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:

class BankAccount {
  balance: number = 0;
  // ...
}

const value = 100;
const account = new BankAccount();

if (value < 0) {
  throw new Error('Cannot set negative balance.');
}

account.balance = value;

Good:

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;
  }

  // ...
}

const account = new BankAccount();
account.balance = 100;

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Make objects have private/protected members

TypeScript supports public (default), protected and private accessors on class members.

Bad:

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:

class Circle {
  constructor(private readonly radius: number) {
  }

  perimeter(){
    return 2 * Math.PI * this.radius;
  }

  surface(){
    return Math.PI * this.radius * this.radius;
  }
}

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Prefer readonly properties

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).

Bad:

interface Config {
  host: string;
  port: string;
  db: string;
}

Good:

interface Config {
  readonly host: string;
  readonly port: string;
  readonly db: string;
}

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, for general purpose see pify, es6-promisify)

Bad:

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:

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 patterns that could be useful in some cases:

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.

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Async/Await are even cleaner than Promises

With async/await syntax you can write code that is far cleaner and more understandable that 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:

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:

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);
}

Formatting

Formatting is subjective. Like many rules herein, there is no hard and fast rule that you must follow. The main point is DO NOT ARGUE over formatting. There are tons of tools to automate this. Use one! It's a waste of time and money for engineers to argue over formatting.

For TypeScript consider using the 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 should consider:

• TSLint Config Airbnb - great for general Typescript coding guidelines

• TSLint Config Standard - can be used as a baseline to extend with additional rules

• TSLint react - lint rules related to React & JSX

• TSLint + Prettier - lint rules for Prettier code formatter

• ESLint rules for TSLint - ESLint rules for TypeScript

Refer also to this great TypeScript StyleGuide and Coding Conventions 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:

const DAYS_IN_WEEK = 7;
const daysInMonth = 30;

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

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

class animal {}
class Container {}

Good:

const DAYS_IN_WEEK = 7;
const DAYS_IN_MONTH = 30;

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

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

class Animal {}
class Container {}

Prefer using PascalCase for class, interface, type and namespace names.
Prefer using camelCase for variables, functions and class members.

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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:

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:

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();

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Don't prefix interfaces with I

Bad:

interface IService {
  // ...
}

Good:

interface Service {
  // ...
}

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Prefer single quotes '

When you can't use double quotes, try using back ticks `

Bad:

const text = "Hello world.";
const question = "How're you?";

Good:

const text = 'Hello world.';
const question = `How're you?`;

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Use 2 spaces, no tabs

Bad:

function sum(a: number, b: number) {
	return a + b;
}

Good:

function sum(a: number, b: number) {
  return a + b;
}

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type vs. interface

Use type when you might need a union or intersection.
Use interface when you want extends or implements.

Bad:

interface EmailConfig {
  // ...
}

interface DbConfig {
  // ...
}

interface Config {
  // ...
}

//...

type Shape {
  // ...
}

Good:

type EmailConfig {
  // ...
}

type DbConfig {
  // ...
}

type Config  = EmailConfig | DbConfig;

// ...

interface Shape {

}

class Circle implements Shape {
  // ...
}

class Square implements Shape {
  // ...
}

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:

// Check if subscription is active.
if (subscription.endDate > Date.now) {  }

Good:

const isSubscriptionActive = subscription.endDate > Date.now;
if (isSubscriptionActive) { /* ... */ }

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Don't leave commented out code in your codebase

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

Bad:

class User {
  name: string;
  email: string;
  // age: number;
  // jobPosition: string;
}

Good:

class User {
  name: string;
  email: string;
}

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:

/**
 * 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:

function combine(a:number, b:number): number {
  return a + b;
}

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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.
Optionally you can use IDE support for code folding (see Visual Studio Code folding regions).

Bad:

////////////////////////////////////////////////////////////////////////////////
// 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:

class Client {
  id: number;
  name: string;
  address: Address;
  contact: Contact;

  public describe(): string {
    // ...
  }

  private describeAddress(): string {
    // ...
  }

  private describeContact(): string {
    // ...
  }
};

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