@spscommerce/eslint-config-typescript/README.md

SPS official linter configuration for TypeScript.

# SPS Commerce TypeScript Style Guide() {

## Table of Contents

  1. [Types](#types)
  1. [References](#references)
  1. [Objects](#objects)
  1. [Arrays](#arrays)
  1. [Destructuring](#destructuring)
  1. [Strings](#strings)
  1. [Promises](#promises)
  1. [Functions](#functions)
  1. [Arrow Functions](#arrow-functions)
  1. [Classes & Constructors](#classes--constructors)
  1. [Modules](#modules)
  1. [Iterators and Generators](#iterators-and-generators)
  1. [Properties](#properties)
  1. [Variables](#variables)
  1. [Comparison Operators & Equality](#comparison-operators--equality)
  1. [Blocks](#blocks)
  1. [Control Statements](#control-statements)
  1. [Comments](#comments)
  1. [Whitespace](#whitespace)
  1. [Commas](#commas)
  1. [Semicolons](#semicolons)
  1. [Type Casting & Coercion](#type-casting--coercion)
  1. [Naming Conventions](#naming-conventions)
  1. [Accessors](#accessors)
  1. [Events](#events)
  1. [Standard Library](#standard-library)
  1. [Language Proposals](#language-proposals)
  1. [Testing](#testing)
  1. [Resources](#resources)

## Types

<a name="types--primitives"></a>
💡 [**1.1**](#types--primitives) ‣ Primitives: When you access a primitive type you work directly on its value.

- `string`
- `number`
- `boolean`
- `null`
- `undefined`
- `symbol`
- `bigint`


```typescript
const foo = 1;
let bar = foo;

bar = 9;

console.log(foo, bar); // => 1, 9
```

Symbols and BigInts cannot be faithfully polyfilled, so they should not be used when targeting browsers/environments that don’t support them natively.

---

<a name="types--complex"></a>
💡 [**1.2**](#types--complex) ‣ Complex: When you access a complex type you work on a reference to its value.

- `object`
- `array`
- `function`


```typescript
const foo = [1, 2];
const bar = foo;

bar[0] = 9;

console.log(foo[0], bar[0]); // => 9, 9
```

---

<a name="types--assertions"></a>
[**1.3**](#types--assertions) ‣ Type assertions should be written in `as` style, rather than prefix style.

<img src="../eslint.svg" height="18" align="center"/> [`@typescript-eslint/consistent-type-assertions`](https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/consistent-type-assertions.md)

> Why? Because it uses angle brackets, the prefix style can be confused when generics are also in play, as well as potentially tripping up IDEs and tooling in `.tsx` files.

```typescript
function squareIt(num: number): number {
  return num ** 2;
}

const foo: string | number = 5;

// bad
console.log(squareIt(<number>foo));

// good
console.log(squareIt(foo as number));
```

---

<a name="types--enums"></a>
[**1.4**](#types--enums) ‣ Explicitly initialize the values of enum members.

<img src="../eslint.svg" height="18" align="center"/> [`@typescript-eslint/prefer-enum-initializers`](https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/prefer-enum-initializers.md)

> Why? If you allow member values to be inferred, then adding a new member can result in the values of pre-existing members changing. This can potentially introduce bugs.

```typescript
// bad
enum Status {
  Pending,
  Complete,
}

// good
enum Status {
  Pending = "PENDING",
  Complete = "COMPLETE",
}
```

**[⬆  back to top](#table-of-contents)**


## References

<a name="references--prefer-const"></a>
[**2.1**](#references--prefer-const) ‣ Use `const` for all of your references; avoid using `var`.

<img src="../eslint.svg" height="18" align="center"/> [`prefer-const`](https://eslint.org/docs/rules/prefer-const.html), [`no-const-assign`](https://eslint.org/docs/rules/no-const-assign.html)

> Why? This ensures that you can’t reassign your references, which can lead to bugs and difficult to comprehend code.

```typescript
// bad
var a = 1;
var b = 2;

// good
const a = 1;
const b = 2;
```

---

<a name="references--disallow-var"></a>
[**2.2**](#references--disallow-var) ‣ If you must reassign references, use `let` instead of `var`.

<img src="../eslint.svg" height="18" align="center"/> [`no-var`](https://eslint.org/docs/rules/no-var.html)

> Why? `let` is block-scoped rather than function-scoped like `var`.

```typescript
// bad
var count = 1;
if (true) {
  count += 1;
}

// good, use the let.
let count = 1;
if (true) {
  count += 1;
}
```

---

<a name="references--block-scope"></a>
💡 [**2.3**](#references--block-scope) ‣ Note that both `let` and `const` are block-scoped, whereas `var` is function-scoped.

```typescript
// const and let only exist in the blocks they are defined in.
{
  let a = 1;
  const b = 1;
  var c = 1;
}
console.log(a); // ReferenceError
console.log(b); // ReferenceError
console.log(c); // Prints 1
```

In the above code, you can see that referencing `a` and `b` will produce a ReferenceError, while `c` contains the number. This is because `a` and `b` are block scoped, while `c` is scoped to the containing function.

**[⬆  back to top](#table-of-contents)**

## Objects

<a name="objects--no-new"></a>
[**3.1**](#objects--no-new) ‣ Use the literal syntax for object creation.

<img src="../eslint.svg" height="18" align="center"/> [`no-new-object`](https://eslint.org/docs/rules/no-new-object.html)

```typescript
// bad
const item = new Object();

// good
const item = {};
```

---

<a name="es6-computed-properties"></a>
[**3.2**](#es6-computed-properties) ‣ Use computed property names when creating objects with dynamic property names.

> Why? They allow you to define all the properties of an object in one place.

```typescript
function getKey(k: string) {
  return `a key named ${k}`;
}

// bad
const obj = {
  id: 5,
  name: 'San Francisco',
};
obj[getKey('enabled')] = true;

// good
const obj = {
  id: 5,
  name: 'San Francisco',
  [getKey('enabled')]: true,
};
```

---

<a name="es6-object-shorthand"></a>
[**3.3**](#es6-object-shorthand) ‣ Use object method shorthand.

<img src="../eslint.svg" height="18" align="center"/> [`object-shorthand`](https://eslint.org/docs/rules/object-shorthand.html)

```typescript
// bad
const atom = {
  value: 1,

  addValue: function (value: number) {
    return atom.value + value;
  },
};

// good
const atom = {
  value: 1,

  addValue(value: number) {
    return atom.value + value;
  },
};
```

---

<a name="es6-object-concise"></a>
[**3.4**](#es6-object-concise) ‣ Use property value shorthand.

<img src="../eslint.svg" height="18" align="center"/> [`object-shorthand`](https://eslint.org/docs/rules/object-shorthand.html)

> Why? It is shorter and descriptive.

```typescript
const lukeSkywalker = 'Luke Skywalker';

// bad
const obj = {
  lukeSkywalker: lukeSkywalker,
};

// good
const obj = {
  lukeSkywalker,
};
```

---

<a name="objects--quoted-props"></a>
[**3.5**](#objects--quoted-props) ‣ Only quote properties that are invalid identifiers.

<img src="../prettier.svg" height="18" align="center"/> **Enforced by Prettier**

<img src="../eslint.svg" height="18" align="center"/> [`quote-props`](https://eslint.org/docs/rules/quote-props.html)

> Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.

```typescript
// bad
const bad = {
  'foo': 3,
  'bar': 4,
  'data-blah': 5,
};

// good
const good = {
  foo: 3,
  bar: 4,
  'data-blah': 5,
};
```

---

<a name="objects--rest-spread"></a>
[**3.6**](#objects--rest-spread) ‣ Prefer the object spread syntax over [`Object.assign`](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/assign) to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted.

<img src="../eslint.svg" height="18" align="center"/> [`prefer-object-spread`](https://eslint.org/docs/rules/prefer-object-spread)

```typescript
// very bad
const original = { a: 1, b: 2 };
const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ
delete copy.a; // so does this

// bad
const original = { a: 1, b: 2 };
const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 }

// good
const original = { a: 1, b: 2 };
const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }

const { a, ...noA } = copy; // noA => { b: 2, c: 3 }
```

**[⬆  back to top](#table-of-contents)**

## Arrays

<a name="arrays--literals"></a>
[**4.1**](#arrays--literals) ‣ Use the literal syntax for array creation, except in the case of initializing a sparse array with a specific size.

<img src="../eslint.svg" height="18" align="center"/> [`no-array-constructor`](https://eslint.org/docs/rules/no-array-constructor.html)

```typescript
// bad
const items = new Array();

// good
const items = [];

// good
const items = new Array(8);
```

---

<a name="arrays--push"></a>
[**4.2**](#arrays--push) ‣ Use [Array#push](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/push) instead of direct assignment to add items to an array.

```typescript
const someStack = [];

// bad
someStack[someStack.length] = 'abracadabra';

// good
someStack.push('abracadabra');
```

---

<a name="es6-array-spreads"></a>
[**4.3**](#es6-array-spreads) ‣ Use array spreads `...` to copy arrays.

```typescript
// bad
const len = items.length;
const itemsCopy = [];
let i;

for (i = 0; i < len; i += 1) {
  itemsCopy[i] = items[i];
}

// good
const itemsCopy = [...items];
```

---

<a name="arrays--from-iterable"></a>
[**4.4**](#arrays--from-iterable) ‣ To convert an iterable object to an array, use spreads `...` instead of [`Array.from`](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/from).

```typescript
const foo = document.querySelectorAll('.foo');

// good
const nodes = Array.from(foo);

// best
const nodes = [...foo];
```

---

<a name="arrays--from-array-like"></a>
[**4.5**](#arrays--from-array-like) ‣ Use [`Array.from`](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/from) for converting an array-like object to an array.

```typescript
const arrLike = { 0: 'foo', 1: 'bar', 2: 'baz', length: 3 };

// bad
const arr = Array.prototype.slice.call(arrLike);

// good
const arr = Array.from(arrLike);
```

---

<a name="arrays--mapping"></a>
[**4.6**](#arrays--mapping) ‣ Use [`Array.from`](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/from) instead of spread `...` for mapping over iterables, because it avoids creating an intermediate array.

```typescript
// bad
const baz = [...foo].map(bar);

// good
const baz = Array.from(foo, bar);
```

---

<a name="arrays--callback-return"></a>
[**4.7**](#arrays--callback-return) ‣ Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, following [**9.2**](#arrows--implicit-return).

<img src="../eslint.svg" height="18" align="center"/> [`array-callback-return`](https://eslint.org/docs/rules/array-callback-return)

```typescript
// good
[1, 2, 3].map((x) => {
  const y = x + 1;
  return x * y;
});

// good
[1, 2, 3].map((x) => x + 1);

// bad - no returned value means `acc` becomes undefined after the first iteration
[[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
  const flatten = acc.concat(item);
});

// good
[[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
  const flatten = acc.concat(item);
  return flatten;
});

// bad
inbox.filter((msg) => {
  const { subject, author } = msg;
  if (subject === 'Mockingbird') {
    return author === 'Harper Lee';
  }
});

// good
inbox.filter((msg) => {
  const { subject, author } = msg;
  if (subject === 'Mockingbird') {
    return author === 'Harper Lee';
  }

  return false;
});
```

---

<a name="arrays--bracket-newline"></a>
[**4.8**](#arrays--bracket-newline) ‣ Use line breaks after open and before close array brackets if an array has multiple lines.

<img src="../prettier.svg" height="18" align="center"/> **Enforced by Prettier**

```typescript
// bad
const arr = [
  [0, 1],
];

const objectInArray = [{
  id: 1,
}, {
  id: 2,
}];

const numberInArray = [
  1, 2,
];

// good
const arr = [[0, 1]];

const objectInArray = [
  {
    id: 1,
  },
  {
    id: 2,
  },
];

const stringInArray = [
  'foofoofoofoofoofoofoofoo',
  'foofoofoofoofoofoofoofoo',
  'foofoofoofoofoofoofoofoo',
];
```

---

<a name="arrays--sort-compare"></a>
[**4.9**](#arrays--sort-compare) ‣ Always provide a comparison function to `Array#sort`.

<img src="../eslint.svg" height="18" align="center"/> [`@typescript-eslint/require-array-sort-compare`](https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/require-array-sort-compare.md)

> Why? `Array#sort` is one of those old-school Javascript things that does not behave the way you would expect. If you don't pass in your own comparison function, it converts all the contents to strings and sorts alphabetically.

```typescript
// bad
const thanksBrendan = [30, 10, 3, 2, 20, 1].sort();
// -> [1, 10, 2, 20, 3, 30]

// good
const ahThatsBetter = [30, 10, 3, 2, 20, 1].sort((a, b) => a - b);
// -> [1, 2, 3, 10, 20, 30]
```

**[⬆  back to top](#table-of-contents)**

## Destructuring

<a name="destructuring--object"></a>
[**5.1**](#destructuring--object) ‣ Use object destructuring when accessing and using multiple properties of an object.

<img src="../eslint.svg" height="18" align="center"/> [`prefer-destructuring`](https://eslint.org/docs/rules/prefer-destructuring)

> Why? Destructuring saves you from creating temporary references for those properties, and from repetitive access of the object. Repeating object access creates more repetitive code, requires more reading, and creates more opportunities for mistakes. Destructuring objects also provides a single site of definition of the object structure that is used in the block, rather than requiring reading the entire block to determine what is used.

```typescript
// bad
function getFullName(user) {
  const firstName = user.firstName;
  const lastName = user.lastName;

  return `${firstName} ${lastName}`;
}

// good
function getFullName(user) {
  const { firstName, lastName } = user;
  return `${firstName} ${lastName}`;
}

// best
function getFullName({ firstName, lastName }) {
  return `${firstName} ${lastName}`;
}
```

---

<a name="destructuring--array"></a>
[**5.2**](#destructuring--array) ‣ Use array destructuring.

<img src="../eslint.svg" height="18" align="center"/> [`prefer-destructuring`](https://eslint.org/docs/rules/prefer-destructuring)

```typescript
const arr = [1, 2, 3, 4];

// bad
const first = arr[0];
const second = arr[1];

// good
const [first, second] = arr;
```

---

<a name="destructuring--object-over-array"></a>
[**5.3**](#destructuring--object-over-array) ‣ Use object destructuring for multiple return values, not array destructuring.

> Why? You can add new properties over time or change the order of things without breaking call sites.

```typescript
// bad
function processInput(input) {
  // then a miracle occurs
  return [left, right, top, bottom];
}

// the caller needs to think about the order of return data
const [left, __, top] = processInput(input);

// good
function processInput(input) {
  // then a miracle occurs
  return { left, right, top, bottom };
}

// the caller selects only the data they need
const { left, top } = processInput(input);
```

**[⬆  back to top](#table-of-contents)**

## Strings

<a name="strings--quotes"></a>
[**6.1**](#strings--quotes) ‣ Use double quotes `""` for strings.

<img src="../prettier.svg" height="18" align="center"/> **Enforced by Prettier**

<img src="../eslint.svg" height="18" align="center"/> [`quotes`](https://eslint.org/docs/rules/quotes.html)

> Why? In a JSX world, double quotes result in your code needing fewer escapes than single quotes.

```typescript
// bad
const name = 'Let\'s all go to the movies';

// bad - template literals should contain interpolation or newlines
const name = `Let's all go to the movies`;

// good
const name = "Let's all go to the movies";
```

---

<a name="strings--line-length"></a>
[**6.2**](#strings--line-length) ‣ Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation.

> Why? Broken strings are painful to work with and make code less searchable.

```typescript
// bad
const errorMessage = 'This is a super long error that was thrown because \
of Batman. When you stop to think about how Batman had anything to do \
with this, you would get nowhere \
fast.';

// bad
const errorMessage = 'This is a super long error that was thrown because ' +
  'of Batman. When you stop to think about how Batman had anything to do ' +
  'with this, you would get nowhere fast.';

// good
const errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';
```

---

<a name="es6-template-literals"></a>
[**6.3**](#es6-template-literals) ‣ When programmatically building up strings, use template strings instead of concatenation.

<img src="../eslint.svg" height="18" align="center"/> [`prefer-template`](https://eslint.org/docs/rules/prefer-template.html), [`template-curly-spacing`](https://eslint.org/docs/rules/template-curly-spacing)

> Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

```typescript
// bad
function sayHi(name) {
  return 'How are you, ' + name + '?';
}

// bad
function sayHi(name) {
  return ['How are you, ', name, '?'].join();
}

// bad
function sayHi(name) {
  return `How are you, ${ name }?`;
}

// good
function sayHi(name) {
  return `How are you, ${name}?`;
}
```

---

<a name="strings--eval"></a>
[**6.4**](#strings--eval) ‣ Never use `eval()` on a string, it opens too many vulnerabilities.

<img src="../eslint.svg" height="18" align="center"/> [`no-eval`](https://eslint.org/docs/rules/no-eval)

---

<a name="strings--escaping"></a>
[**6.5**](#strings--escaping) ‣ Do not unnecessarily escape characters in strings.

<img src="../prettier.svg" height="18" align="center"/> **Enforced by Prettier**

<img src="../eslint.svg" height="18" align="center"/> [`no-useless-escape`](https://eslint.org/docs/rules/no-useless-escape)

> Why? Backslashes harm readability, thus they should only be present when necessary.

```typescript
// bad
const foo = '\'this\' \i\s \"quoted\"';

// good
const foo = '\'this\' is "quoted"';
const foo = `my name is '${name}'`;
```

**[⬆  back to top](#table-of-contents)**

## Promises

<a name="promises--handle-errors"></a>
[**7.1**](#promises--handle-errors) ‣ When awaiting a promise, the potential for it to be rejected must be handled.

<img src="../eslint.svg" height="18" align="center"/> [`@typescript-eslint/no-floating-promises`](https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-floating-promises.md)

> Why? We've all seen "Unhandled Promise rejection" before and not once has that error ever been useful in debugging the issue. Do yourself a favor and catch Promise errors so you can at least log out a useful error object.

```typescript
// bad
someBackendCall().then((result) => {
  // ...
});

// good
someBackendCall()
  .then((result) => {
    // ...
  })
  .catch((err) => {
    console.error(err);
    // other error handling maybe
  });

// bad
const result = await someBackendCall();

// good
try {
  const result = await someBackendCall();
} catch (err) {
  console.error(err);
  // other error handling maybe
}
```

<a name="promises--void"></a>
[**7.2**](#promises--void) ‣ You can have a "fire-and-forget" Promise that is not awaited if you explicitly mark it as such with the `void` operator.

```typescript
// bad, will produce an eslint error for the above rule about handling errors
someBackendCall(); // (returns a Promise)

// good, now it is explicit that we mean to just fire this off and move on
void someBackendCall();
```

**[⬆  back to top](#table-of-contents)**

## Functions

<a name="functions--in-blocks"></a>
[**8.1**](#functions--in-blocks) ‣ Never declare a function in a non-function block (`if`, `while`, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears.

<img src="../eslint.svg" height="18" align="center"/> [`no-loop-func`](https://eslint.org/docs/rules/no-loop-func.html)

---

<a name="functions--note-on-blocks"></a>
💡 [**8.2**](#functions--note-on-blocks) ‣ **Note:** ECMA-262 defines a `block` as a list of statements. A function declaration is not a statement.

```typescript
// bad
if (currentUser) {
  function test() {
    console.log('Nope.');
  }
}

// good
let test;
if (currentUser) {
  test = () => {
    console.log('Yup.');
  };
}
```

---

<a name="es6-rest"></a>
[**8.3**](#es6-rest) ‣ Never use `arguments`, opt to use rest syntax `...` instead.

<img src="../eslint.svg" height="18" align="center"/> [`prefer-rest-params`](https://eslint.org/docs/rules/prefer-rest-params)

> Why? `...` is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like like `arguments`.

```typescript
// bad
function concatenateAll() {
  const args = Array.prototype.slice.call(arguments);
  return args.join('');
}

// good
function concatenateAll(...args) {
  return args.join('');
}
```

---

<a name="es6-default-parameters"></a>
[**8.4**](#es6-default-parameters) ‣ Use default parameter syntax rather than mutating function arguments.

```typescript
// really bad
function handleThings(opts) {
  // No! We shouldn’t mutate function arguments.
  // Double bad: if opts is falsy it'll be set to an object which may
  // be what you want but it can introduce subtle bugs.
  opts = opts || {};
  // ...
}

// still bad
function handleThings(opts) {
  if (opts === void 0) {
    opts = {};
  }
  // ...
}

// good
function handleThings(opts = {}) {
  // ...
}
```

---

<a name="functions--default-side-effects"></a>
[**8.5**](#functions--default-side-effects) ‣ Avoid side effects with default parameters.

> Why? They are confusing to reason about.


```typescript
let b = 1;

// bad
function count(a = b++) {
  console.log(a);
}
count();  // 1
count();  // 2
count(3); // 3
count();  // 3
```

---

<a name="functions--defaults-last"></a>
[**8.6**](#functions--defaults-last) ‣ Always put default parameters last.

<img src="../eslint.svg" height="18" align="center"/> [`default-param-last`](https://eslint.org/docs/rules/default-param-last)

```typescript
// bad
function handleThings(opts: IHandleThingsOpts = {}, name: string) {
  // ...
}

// good
function handleThings(name: string, opts: IHandleThingsOpts = {}) {
  // ...
}
```

---

<a name="functions--constructor"></a>
[**8.7**](#functions--constructor) ‣ Never use the Function constructor to create a new function.

<img src="../eslint.svg" height="18" align="center"/> [`no-new-func`](https://eslint.org/docs/rules/no-new-func)

> Why? Creating a function in this way evaluates a string similarly to `eval()`, which opens vulnerabilities.

```typescript
// bad
const add = new Function('a', 'b', 'return a + b');

// still bad
const subtract = Function('a', 'b', 'return a - b');
```

---

<a name="functions--signature-spacing"></a>
[**8.8**](#functions--signature-spacing) ‣ Spacing in a function signature.

<img src="../prettier.svg" height="18" align="center"/> **Enforced by Prettier**

<img src="../eslint.svg" height="18" align="center"/> [`space-before-function-paren`](https://eslint.org/docs/rules/space-before-function-paren), [`space-before-blocks`](https://eslint.org/docs/rules/space-before-blocks)

> Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.

```typescript
// bad
const f = function(){};
const g = function (){};
const h = function() {};

// good
const x = function () {};
const y = function a() {};
```

---

<a name="functions--mutate-params"></a>
[**8.10**](#functions--mutate-params) ‣ Never mutate parameters.

<img src="../eslint.svg" height="18" align="center"/> [`no-param-reassign`](https://eslint.org/docs/rules/no-param-reassign.html)

> Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.

```typescript
// bad
function f1(obj: ObjType) {
  obj.key = 1;
  // ...
}

// good
function f2(obj: ObjType) {
  const objCopy = {
    ...obj,
    key: 1,
  };
  // ...
}
```

---

<a name="functions--reassign-params"></a>
[**8.11**](#functions--reassign-params) ‣ Never reassign parameters.

<img src="../eslint.svg" height="18" align="center"/> [`no-param-reassign`](https://eslint.org/docs/rules/no-param-reassign.html)

> Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the `arguments` object. It can also cause optimization issues, especially in V8.

```typescript
// bad
function f1(a: number) {
  a = 1;
  // ...
}

function f2(a: number) {
  if (!a) { a = 1; }
  // ...
}

// good
function f3(a: number) {
  const b = a || 1;
  // ...
}

function f4(a: number = 1) {
  // ...
}
```

---

<a name="functions--spread-vs-apply"></a>
[**8.12**](#functions--spread-vs-apply) ‣ Prefer the use of the spread syntax `...` to call variadic functions.

<img src="../eslint.svg" height="18" align="center"/> [`prefer-spread`](https://eslint.org/docs/rules/prefer-spread)

> Why? It’s cleaner, you don’t need to supply a context, and you can not easily compose `new` with `apply`.

```typescript
// bad
const x = [1, 2, 3, 4, 5];
console.log.apply(console, x);

// good
const x = [1, 2, 3, 4, 5];
console.log(...x);

// bad
new (Function.prototype.bind.apply(Date, [null, 2016, 8, 5]));

// good
new Date(...[2016, 8, 5]);
```

---

<a name="functions--signature-invocation-indentation"></a>
[**8.13**](#functions--signature-invocation-indentation) ‣ Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item.

<img src="../prettier.svg" height="18" align="center"/> **Enforced by Prettier**

```typescript
// bad
function foo(bar: string,
              baz: number,
              quux: boolean) {
  // ...
}

// good
function foo(
  bar: string,
  baz: number,
  quux: boolean,
) {
  // ...
}

// bad
console.log(foo,
  bar,
  baz);

// good
console.log(
  foo,
  bar,
  baz,
);
```

---

<a name="functions--return-type"></a>
[**8.14**](#functions--return-type) ‣ Functions and methods should include an explicit return type.

<img src="../eslint.svg" height="18" align="center"/> [`@typescript-eslint/explicit-function-return-type`](https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/explicit-function-return-type.md)

> Why? If you explicitly specify what the function is intended to return, then a bug where it returns something unintended will be caught immediately in your IDE. If you allow TypeScript to infer the return type, it will assume anything the function ends up returning is fine. (We acknowledge the examples throughout this document often don't follow this. Time permitting we will possibly correct that.)

```typescript
// bad
function foo(isTwo: boolean) {
  if (isTwo) {
    return 2;
  }
}

/** good - this will show a TS error because the
 * code as written could return `undefined`, prompting
 * you to think about what you want: should it actually
 * always return a number, or should it be the way it is?
 * If so, then you can explicitly change the return type
 * to `number | undefined`. */
function foo(isTwo: boolean): number {
  if (isTwo) {
    return 2;
  }
}
```

**[⬆  back to top](#table-of-contents)**

## Arrow Functions

<a name="arrows--use-them"></a>
[**9.1**](#arrows--use-them) ‣ When you must use an anonymous function (as when passing an inline callback), use arrow function notation.

<img src="../eslint.svg" height="18" align="center"/> [`prefer-arrow-callback`](https://eslint.org/docs/rules/prefer-arrow-callback.html), [`arrow-spacing`](https://eslint.org/docs/rules/arrow-spacing.html)

> Why? It creates a version of the function that executes in the context of `this`, which is usually what you want, and is a more concise syntax.

> Why not? If you have a fairly complicated function, you might move that logic out into its own named function expression.

```typescript
// bad
[1, 2, 3].map(function (x) {
  const y = x + 1;
  return x * y;
});

// good
[1, 2, 3].map((x) => {
  const y = x + 1;
  return x * y;
});
```

---

<a name="arrows--implicit-return"></a>
[**9.2**](#arrows--implicit-return) ‣ If the function body consists of a single statement returning an [expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators#Expressions) without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a `return` statement.

<img src="../eslint.svg" height="18" align="center"/> [`arrow-parens`](https://eslint.org/docs/rules/arrow-parens.html), [`arrow-body-style`](https://eslint.org/docs/rules/arrow-body-style.html)

> Why? Syntactic sugar. It reads well when multiple functions are chained together.

```typescript
// bad
[1, 2, 3].map((number) => {
  const nextNumber = number + 1;
  `A string containing the ${nextNumber}.`;
});

// good
[1, 2, 3].map((number) => `A string containing the ${number + 1}.`);

// good
[1, 2, 3].map((number) => {
  const nextNumber = number + 1;
  return `A string containing the ${nextNumber}.`;
});

// good
[1, 2, 3].map((number, index) => ({
  [index]: number,
}));

// No implicit return with side effects
function foo(callback: () => boolean) {
  const val = callback();
  if (val === true) {
    // Do something if callback returns true
  }
}

let bool = false;

// bad
foo(() => bool = true);

// good
foo(() => {
  bool = true;
});
```

---

<a name="arrows--paren-wrap"></a>
[**9.3**](#arrows--paren-wrap) ‣ In case the expression spans over multiple lines, wrap it in parentheses for better readability.

> Why? It shows clearly where the function starts and ends.

```typescript
// bad
['get', 'post', 'put'].map((httpMethod) => Object.prototype.hasOwnProperty.call(
    httpMagicObjectWithAVeryLongName,
    httpMethod,
  )
);

// good
['get', 'post', 'put'].map((httpMethod) => (
  Object.prototype.hasOwnProperty.call(
    httpMagicObjectWithAVeryLongName,
    httpMethod,
  )
));
```

---

<a name="arrows--one-arg-parens"></a>
[**9.4**](#arrows--one-arg-parens) ‣ Always include parentheses around arguments for clarity and consistency.

<img src="../prettier.svg" height="18" align="center"/> **Enforced by Prettier**

<img src="../eslint.svg" height="18" align="center"/> [`arrow-parens`](https://eslint.org/docs/rules/arrow-parens.html)

> Why? Minimizes diff churn when adding or removing arguments.

```typescript
// bad
[1, 2, 3].map(x => x * x);

// good
[1, 2, 3].map((x) => x * x);

// bad
[1, 2, 3].map(number => (
  `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
));

// good
[1, 2, 3].map((number) => (
  `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
));

// bad
[1, 2, 3].map(x => {
  const y = x + 1;
  return x * y;
});

// good
[1, 2, 3].map((x) => {
  const y = x + 1;
  return x * y;
});
```

---

<a name="arrows--confusing"></a>
[**9.5**](#arrows--confusing) ‣ Avoid confusing arrow function syntax (`=>`) with comparison operators (`<=`, `>=`).

<img src="../eslint.svg" height="18" align="center"/> [`no-confusing-arrow`](https://eslint.org/docs/rules/no-confusing-arrow)

```typescript
// bad
const itemHeight = (item: ItemType) => item.height <= 256 ? item.largeSize : item.smallSize;

// bad
const itemHeight = (item: ItemType) => item.height >= 256 ? item.largeSize : item.smallSize;

// good
const itemHeight = (item: ItemType) => (item.height <= 256 ? item.largeSize : item.smallSize);

// good
const itemHeight = (item: ItemType) => {
  const { height, largeSize, smallSize } = item;
  return height <= 256 ? largeSize : smallSize;
};
```

---

<a name="arrows--implicit-arrow-linebreak"></a>
[**9.6**](#arrows--implicit-arrow-linebreak) ‣ Enforce the location of arrow function bodies with implicit returns.

<img src="../prettier.svg" height="18" align="center"/> **Enforced by Prettier**

```typescript
// bad
(foo) =>
  bar;

(foo) =>
  (bar);

// good
(foo) => bar;
(foo) => (bar);
(foo) => (
    bar
)
```

**[⬆  back to top](#table-of-contents)**

## Classes & Constructors

<a name="constructors--use-class"></a>
[**10.1**](#constructors--use-class) ‣ Always use `class`. Avoid manipulating `prototype` directly.

> Why? `class` syntax is more concise and easier to reason about.

```typescript
// bad
function Queue(contents = []) {
  this.queue = [...contents];
}
Queue.prototype.pop = function () {
  const value = this.queue[0];
  this.queue.splice(0, 1);
  return value;
};

// good
class Queue<T> {
  queue: T[];

  constructor(contents: T[] = []) {
    this.queue = [...contents];
  }

  pop(): T {
    const value = this.queue[0];
    this.queue.splice(0, 1);
    return value;
  }
}
```

---

<a name="constructors--extends"></a>
[**10.2**](#constructors--extends) ‣ Use `extends` for inheritance.

> Why? It is a built-in way to inherit prototype functionality without breaking `instanceof`.

```typescript
// bad
const inherits = require('inherits');
function PeekableQueue(contents) {
  Queue.apply(this, contents);
}
inherits(PeekableQueue, Queue);
PeekableQueue.prototype.peek = function () {
  return this.queue[0];
};

// good
class PeekableQueue<T> extends Queue<T> {
  peek(): T {
    return this.queue[0];
  }
}
```

---

<a name="constructors--chaining"></a>
💡 [**10.3**](#constructors--chaining) ‣ Methods can return `this` to help with method chaining.

```typescript
// bad
Jedi.prototype.jump = function () {
  this.jumping = true;
  return true;
};

Jedi.prototype.setHeight = function (height) {
  this.height = height;
};

const luke = new Jedi();
luke.jump(); // => true
luke.setHeight(20); // => undefined

// good
class Jedi {
  jumping = false;
  height: number;

  jump(): this {
    this.jumping = true;
    return this;
  }

  setHeight(height: number): this {
    this.height = height;
    return this;
  }
}

const luke = new Jedi();

luke.jump()
  .setHeight(20);
```

---

<a name="constructors--tostring"></a>
[**10.4**](#constructors--tostring) ‣ It’s okay to write a custom `toString()` method, just make sure it works successfully and causes no side effects.

```typescript
class Jedi {
  name: string;

  constructor(options: IJediOptions = {}) {
    this.name = options.name || 'no name';
  }

  getName(): string {
    return this.name;
  }

  toString(): string {
    return `Jedi - ${this.getName()}`;
  }
}
```

---

<a name="constructors--no-useless"></a>
[**10.5**](#constructors--no-useless) ‣ Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary.

<img src="../eslint.svg" height="18" align="center"/> [`no-useless-constructor`](https://eslint.org/docs/rules/no-useless-constructor)

```typescript
// bad
class Jedi {
  name: string;

  constructor() {}

  getName() {
    return this.name;
  }
}

// bad
class Rey extends Jedi {
  constructor(...args) {
    super(...args);
  }
}

// good
class Rey extends Jedi {
  constructor(...args) {
    super(...args);
    this.name = 'Rey';
  }
}
```

---

<a name="classes--no-duplicate-members"></a>
[**10.6**](#classes--no-duplicate-members) ‣ Avoid duplicate class members.

<img src="../eslint.svg" height="18" align="center"/> [`no-dupe-class-members`](https://eslint.org/docs/rules/no-dupe-class-members)

> Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug.

```typescript
// bad
class Foo {
  bar() { return 1; }
  bar() { return 2; }
}

// good
class Foo {
  bar() { return 1; }
}

// good
class Foo {
  bar() { return 2; }
}
```

---

<a name="classes--methods-use-this"></a>
[**10.7**](#classes--methods-use-this) ‣ Class methods should use `this` or be made into a static method unless an external library or framework requires using specific non-static methods. Being an instance method should indicate that it behaves differently based on properties of the receiver.

<img src="../eslint.svg" height="18" align="center"/> [`class-methods-use-this`](https://eslint.org/docs/rules/class-methods-use-this)

```typescript
// bad
class Foo {
  bar() {
    console.log('bar');
  }
}

// good - `this` is used
class Foo {
  bar() {
    console.log(this.bar);
  }
}

// good - constructor is exempt
class Foo {
  constructor() {
    // ...
  }
}

// good - static methods aren't expected to use this
class Foo {
  static bar() {
    console.log('bar');
  }
}
```

**[⬆  back to top](#table-of-contents)**

## Modules

<a name="modules--use-them"></a>
[**11.1**](#modules--use-them) ‣ Always use modules (`import`/`export`) over a non-standard module system. You can always transpile to your preferred module system.

> Why? Modules are the future, let’s start using the future now.

```typescript
// bad
const AirbnbStyleGuide = require('./AirbnbStyleGuide');
module.exports = AirbnbStyleGuide.es6;

// ok
import AirbnbStyleGuide from './AirbnbStyleGuide';
export default AirbnbStyleGuide.es6;

// best
import { es6 } from './AirbnbStyleGuide';
export default es6;
```

---

<a name="modules--no-duplicate-imports"></a>
[**11.2**](#modules--no-duplicate-imports) ‣ Only import from a path in one place.

<img src="../eslint.svg" height="18" align="center"/> [`no-duplicate-imports`](https://eslint.org/docs/rules/no-duplicate-imports)

> Why? Having multiple lines that import from the same path can make code harder to maintain.

```typescript
// bad
import foo from 'foo';
// … some other imports … //
import { named1, named2 } from 'foo';

// good
import foo, { named1, named2 } from 'foo';

// good
import foo, {
  named1,
  named2,
} from 'foo';
```

---

<a name="modules--no-mutable-exports"></a>
[**11.3**](#modules--no-mutable-exports) ‣ Do not export mutable bindings.

<img src="../eslint.svg" height="18" align="center"/> [`import/no-mutable-exports`](https://github.com/benmosher/eslint-plugin-import/blob/master/docs/rules/no-mutable-exports.md)

> Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.

```typescript
// bad
let foo = 3;
export { foo };

// good
const foo = 3;
export { foo };
```

---

<a name="modules--imports-first"></a>
[**11.4**](#modules--imports-first) ‣ Put all `import`s above non-import statements.

<img src="../eslint.svg" height="18" align="center"/> [`import/first`](https://github.com/benmosher/eslint-plugin-import/blob/master/docs/rules/first.md)

> Why? Since `import`s are hoisted, keeping them all at the top prevents surprising behavior.

```typescript
// bad
import foo from 'foo';
foo.init();

import bar from 'bar';

// good
import foo from 'foo';
import bar from 'bar';

foo.init();
```

---

<a name="modules--multiline-imports-over-newlines"></a>
[**11.5**](#modules--multiline-imports-over-newlines) ‣ Multiline imports should be indented just like multiline array and object literals.

<img src="../prettier.svg" height="18" align="center"/> **Enforced by Prettier**

<img src="../eslint.svg" height="18" align="center"/> [`object-curly-newline`](https://eslint.org/docs/rules/object-curly-newline)

> Why? The curly braces follow the same indentation rules as every other curly brace block in the style guide, as do the trailing commas.

```typescript
// bad
import { longNameA, longNameB, longNameC,
  longNameD, longNameE, longNameF } from 'path';

// good
import {
  longNameA,
  longNameB,
  longNameC,
  longNameD,
  longNameE,
} from 'path';
```

---

<a name="modules--no-webpack-loader-syntax"></a>
[**11.6**](#modules--no-webpack-loader-syntax) ‣ Disallow Webpack loader syntax in module import statements.

<img src="../eslint.svg" height="18" align="center"/> [`import/no-webpack-loader-syntax`](https://github.com/benmosher/eslint-plugin-import/blob/master/docs/rules/no-webpack-loader-syntax.md)

> Why? Since using Webpack syntax in the imports couples the code to a module bundler. Prefer using the loader syntax in `webpack.config.js`.

```typescript
// bad
import fooSass from 'css!sass!foo.scss';
import barCss from 'style!css!bar.css';

// good
import fooSass from 'foo.scss';
import barCss from 'bar.css';
```

---

<a name="modules--import-extensions"></a>
[**11.7**](#modules--import-extensions) ‣ Do not include JavaScript filename extensions

<img src="../eslint.svg" height="18" align="center"/> [`import/extensions`](https://github.com/benmosher/eslint-plugin-import/blob/master/docs/rules/extensions.md)

  > Why? Including extensions inhibits refactoring, and inappropriately hardcodes implementation details of the module you're importing in every consumer.

  ```typescript
  // bad
  import foo from './foo.js';
  import bar from './bar.jsx';
  import baz from './baz/index.jsx';

  // good
  import foo from './foo';
  import bar from './bar';
  import baz from './baz';
  ```

**[⬆  back to top](#table-of-contents)**

## Iterators and Generators

<a name="iterators--prefer-functional"></a>
[**12.1**](#iterators--prefer-functional) ‣ Prefer JavaScript’s higher-order functions instead of `for`/`for-of` loops, particularly when iterating to build up a value.

> Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side effects.

> Sometimes you genuinely will need to mutate or do something side-effecty in a loop, and in that case `for-of` or a traditional `for` loop are acceptable. But avoid this whenerever possible.

> Use `map()` / `every()` / `filter()` / `find()` / `findIndex()` / `reduce()` / `some()` / ... to iterate over arrays, and `Object.keys()` / `Object.values()` / `Object.entries()` to produce arrays so you can iterate over objects.

```typescript
const numbers = [1, 2, 3, 4, 5];

// bad
let sum = 0;
for (let num of numbers) {
  sum += num;
}
sum === 15;

// bad
let sum = 0;
numbers.forEach((num) => {
  sum += num;
});
sum === 15;

// good
const sum = numbers.reduce((total, num) => total + num, 0);
sum === 15;

// bad
const increasedByOne = [];
for (let i = 0; i < numbers.length; i++) {
  increasedByOne.push(numbers[i] + 1);
}

// bad
const increasedByOne = [];
numbers.forEach((num) => {
  increasedByOne.push(num + 1);
});

// good
const increasedByOne = numbers.map((num) => num + 1);
```

---

<a name="iterators--no-for-in"></a>
[**12.2**](#iterators--no-for-in) ‣ Do not use `for-in`.

<img src="../eslint.svg" height="18" align="center"/> [`no-restricted-syntax`](https://eslint.org/docs/rules/no-restricted-syntax)

> Why? `for-in` has confusing and unexpected behavior. For this reason, `for-of` was added to the language and should be used instead.

```typescript
const obj = { a: 'foo', b: 'bar' };

// bad
for (const key in obj) {
  console.log(key, obj[key]);
}

// avoids the inherited properties issue, but still bad
for (const key in obj) {
  if (obj.hasOwnProperty(key)) {
    console.log(key, obj[key]);
  }
}

// good
for (const key of Object.keys(obj)) {
  console.log(key, obj[key]);
}

// best, if you're working on both the keys and values
for (const [key, value] of Object.entries(obj)) {
  console.log(key, value);
}
```

---

<a name="generators--spacing"></a>
[**12.3**](#generators--spacing) ‣ A generator's function signature should be spaced with `function*` as a single unit surrounded by spaces.

<img src="../prettier.svg" height="18" align="center"/> **Enforced by Prettier**

<img src="../eslint.svg" height="18" align="center"/> [`generator-star-spacing`](https://eslint.org/docs/rules/generator-star-spacing)

> Why? `function` and `*` are part of the same conceptual keyword - `*` is not a modifier for `function`, `function*` is a unique construct, different from `function`.

```typescript
// bad
function * foo() {
  // ...
}

// bad
const bar = function * () {
  // ...
};

// bad
const baz = function *() {
  // ...
};

// bad
const quux = function*() {
  // ...
};

// bad
function*foo() {
  // ...
}

// bad
function *foo() {
  // ...
}

// very bad
function
*
foo() {
  // ...
}

// very bad
const wat = function
*
() {
  // ...
};

// good
function* foo() {
  // ...
}

// good
const foo = function* () {
  // ...
};
```

**[⬆  back to top](#table-of-contents)**

## Properties

<a name="properties--dot"></a>
[**13.1**](#properties--dot) ‣ Use dot notation when accessing properties.

<img src="../eslint.svg" height="18" align="center"/> [`dot-notation`](https://eslint.org/docs/rules/dot-notation.html)

```typescript
const luke = {
  jedi: true,
  age: 28,
};

// bad
const isJedi = luke['jedi'];

// good
const isJedi = luke.jedi;
```


**[⬆  back to top](#table-of-contents)**

## Variables

<a name="variables--const"></a>
[**14.1**](#variables--const) ‣ Always use `const` or `let` to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that.

<img src="../eslint.svg" height="18" align="center"/> [`no-undef`](https://eslint.org/docs/rules/no-undef), [`prefer-const`](https://eslint.org/docs/rules/prefer-const)

```typescript
// bad
superPower = new SuperPower();

// good
const superPower = new SuperPower();
```

---

<a name="variables--one-const"></a>
[**14.2**](#variables--one-const) ‣ Use one `const` or `let` declaration per variable or assignment.

<img src="../eslint.svg" height="18" align="center"/> [`one-var`](https://eslint.org/docs/rules/one-var.html)

> Why? It’s easier to add new variable declarations this way, and you never have to worry about swapping out a `;` for a `,` or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.

```typescript
// bad
const items = getItems(),
    goSportsTeam = true,
    dragonball = 'z';

// bad
// (compare to above, and try to spot the mistake)
const items = getItems(),
    goSportsTeam = true;
    dragonball = 'z';

// good
const items = getItems();
const goSportsTeam = true;
const dragonball = 'z';
```

---

<a name="variables--const-let-group"></a>
[**14.3**](#variables--const-let-group) ‣ Group all your `const`s and then group all your `let`s.

> Why? This is helpful when later on you might need to assign a variable depending on one of the previously assigned variables.

```typescript
// bad
let i, len, dragonball,
    items = getItems(),
    goSportsTeam = true;

// bad
let i: number;
const items = getItems();
let dragonball: Dragonball;
const goSportsTeam = true;
let len: number;

// good
const goSportsTeam = true;
const items = getItems();
let dragonball: Dragonball;
let i: number;
let length: number;
```

---

<a name="variables--define-where-used"></a>
[**14.4**](#variables--define-where-used) ‣ Assign variables where you need them, but place them in a reasonable place.

> Why? `let` and `const` are block scoped and not function scoped.

```typescript
// bad - unnecessary function call
function checkName(hasName) {
  const name = getName();

  if (hasName === 'test') {
    return false;
  }

  if (name === 'test') {
    this.setName('');
    return false;
  }

  return name;
}

// good
function checkName(hasName) {
  if (hasName === 'test') {
    return false;
  }

  const name = getName();

  if (name === 'test') {
    this.setName('');
    return false;
  }

  return name;
}
```

<a name="variables--no-chain-assignment"></a>
[**14.5**](#variables--no-chain-assignment) ‣ Don’t chain variable assignments.

<img src="../eslint.svg" height="18" align="center"/> [`no-multi-assign`](https://eslint.org/docs/rules/no-multi-assign)

> Why? Chaining variable assignments creates implicit global variables.

```typescript
// bad
(function example() {
  // JavaScript interprets this as
  // let a = ( b = ( c = 1 ) );
  // The let keyword only applies to variable a; variables b and c become
  // global variables.
  let a = b = c = 1;
}());

console.log(a); // throws ReferenceError
console.log(b); // 1
console.log(c); // 1

// good
(function example() {
  let a = 1;
  let b = a;
  let c = a;
}());

console.log(a); // throws ReferenceError
console.log(b); // throws ReferenceError
console.log(c); // throws ReferenceError

// the same applies for `const`
```

---

<a name="variables--unary-increment-decrement"></a>
[**14.6**](#variables--unary-increment-decrement) ‣ Avoid using unary increments and decrements (`++`, `--`).

<img src="../eslint.svg" height="18" align="center"/> [`no-plusplus`](https://eslint.org/docs/rules/no-plusplus)

> Why? Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like `num += 1` instead of `num++` or `num ++`. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.

```typescript
// bad

const array = [1, 2, 3];
let num = 1;
num++;
--num;

let sum = 0;
let truthyCount = 0;
for (let i = 0; i < array.length; i++) {
  let value = array[i];
  sum += value;
  if (value) {
    truthyCount++;
  }
}

// good

const array = [1, 2, 3];
let