### Use the same vocabulary for the same type of variable
**Bad:**
```ts
function getUserInfo(): User;
function getUserDetails(): User;
function getUserData(): User;
```
**Good:**
```ts
function getUser(): User;
```
**[⬆ back to top](#table-of-contents)**
### 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](https://palantir.github.io/tslint/rules/no-magic-numbers/) can help identify unnamed constants.
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](https://basarat.gitbooks.io/typescript/docs/destructuring.html) 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:**
```ts
function createMenu(title: string, body: string, buttonText: string, cancellable: boolean) {
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:**
```ts
function emailClients(clients: Client) {
clients.forEach((client) => {
const clientRecord = database.lookup(client);
if (clientRecord.isActive()) {
email(client);
}
});
}
```
**Good:**
```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](#table-of-contents)**
### Function names should say what they do
**Bad:**
```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);
```
**Good:**
```ts
function addMonthToDate(date: Date, month: number): Date {
// ...
}
const date = new Date();
addMonthToDate(date, 1);
```
**[⬆ back to top](#table-of-contents)**
### 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.
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](#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:**
```ts
function showDeveloperList(developers: Developer[]) {
You should be critical about code duplication. Sometimes there is a tradeoff between duplicated code and increased complexity by introducing unnecessary abstraction. When two implementations from two different modules look similar but live in different domains, duplication might be acceptable and preferred over extracting the common code. The extracted common code in this case introduces an indirect dependency between the two modules.
To avoid any side effects and unexpected behavior by passing in explicitly the `undefined` or `null` value, you can tell the TypeScript compiler to not allow it.
See [`--strictNullChecks`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-0.html#--strictnullchecks) option in TypeScript.
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:**
```ts
function createFile(name:string, temp:boolean) {
if (temp) {
fs.create(`./temp/${name}`);
} else {
fs.create(name);
}
}
```
**Good:**
```ts
function createFile(name:string) {
fs.create(name);
}
function createTempFile(name:string) {
fs.create(`./temp/${name}`);
}
```
**[⬆ back to top](#table-of-contents)**
### 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:**
```ts
// 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:**
```ts
// 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.
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](https://en.wikipedia.org/wiki/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.
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
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.
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.
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)).
// 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.
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).
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.
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> {
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 base 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.
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 ISmartPrinter {
print();
fax();
scan();
}
class AllInOnePrinter implements ISmartPrinter {
print() {
// ...
}
fax() {
// ...
}
scan() {
// ...
}
}
class EconomicPrinter implements ISmartPrinter {
print() {
// ...
}
fax() {
throw new Error('Fax not supported.');
}
scan() {
throw new Error('Scan not supported.');
}
}
```
**Good:**
```ts
interface IPrinter {
print();
}
interface IFax {
fax();
}
interface IScanner {
scan();
}
class AllInOnePrinter implements IPrinter, IFax, IScanner {
print() {
// ...
}
fax() {
// ...
}
scan() {
// ...
}
}
class EconomicPrinter implements IPrinter {
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`
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.
There's no excuse to not write tests. There are plenty of good JS test frameworks 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.
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');
date.addDays(30);
assert.equal('1/31/2015', date);
date = new AwesomeDate('2/1/2016');
date.addDays(28);
assert.equal('02/29/2016', date);
date = new AwesomeDate('2/1/2015');
date.addDays(28);
assert.equal('03/01/2015', date);
});
});
```
**Good:**
```ts
import { assert } from 'chai';
describe('AwesomeDate', () => {
it('handles 30-day months', () => {
const date = new AwesomeDate('1/1/2015');
date.addDays(30);
assert.equal('1/31/2015', date);
});
it('handles leap year', () => {
const date = new AwesomeDate('2/1/2016');
date.addDays(28);
assert.equal('02/29/2016', date);
});
it('handles non-leap year', () => {
const date = new AwesomeDate('2/1/2015');
date.addDays(28);
assert.equal('03/01/2015', date);
});
});
```
**[⬆ back to top](#table-of-contents)**
### The name of the test should reveal it's intention
When a test fail, it's name is the first indication of what may have gone wrong.
(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))
| `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.
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:**
```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> {
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.
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).
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`.
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 Clean Code](https://www.npmjs.com/package/tslint-clean-code) - TSLint rules inspired be the [Clean Code: A Handbook of Agile Software Craftsmanship](https://www.amazon.ca/Clean-Code-Handbook-Software-Craftsmanship/dp/0132350882)
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'];
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.
Refer to this [explanation](https://stackoverflow.com/questions/37233735/typescript-interfaces-vs-types/54101543#54101543) about the differences between `type` and `interface` in TypeScript.
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.
Optionally you can use IDE support for code folding (see Visual Studio Code [folding regions](https://code.visualstudio.com/updates/v1_17#_folding-regions)).
