JavaScript Style Guide Skill

Activation

This skill activates when the user mentions or implies JavaScript in their request. Once activated, it:

Responds with best-practice guidance
Generates JavaScript code that strictly conforms to the style guide
Provides explanations for why each convention is recommended

Complete Style Rules

1. References

Use const for all references; avoid var.
If you must reassign references, use let instead of var.
Both const and let are block-scoped, whereas var is function-scoped.

// bad
var count = 1;

// good
const count = 1;
let mutableValue = 1;
mutableValue += 1;

2. Objects

Use literal syntax for object creation.
Use computed property names when creating objects with dynamic property names.
Use object method shorthand and property value shorthand.
Group shorthand properties at the beginning of the object declaration.
Only quote properties that are invalid identifiers.
Prefer the object spread syntax (...) over Object.assign to shallow-copy objects.

// bad
const item = new Object();

// good
const item = {};

// computed property names
const key = 'name';
const obj = { [key]: 'value' };

// method & property shorthand
const name = 'Alice';
const atom = {
  name,
  value: 1,
  addValue(val) {
    return atom.value + val;
  },
};

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

3. Arrays

Use literal syntax for array creation.
Use Array.from or the spread operator to convert array-like objects.
Use return statements in array method callbacks.
Use line breaks after the opening bracket and before the closing bracket if the array has multiple lines.

// bad
const items = new Array();

// good
const items = [];

// convert iterable
const nodes = Array.from(document.querySelectorAll('.item'));
const uniqueValues = [...new Set(arr)];

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

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

4. Destructuring

Use object destructuring when accessing multiple properties of an object.
Use array destructuring.
Use object destructuring for multiple return values, not array destructuring.

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

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

// array destructuring
const [first, , third] = [1, 2, 3];

// multiple return values — use object destructuring
function processInput(input) {
  return { left, right, top, bottom };
}
const { left, top } = processInput(input);

5. Strings

Use single quotes '' for strings.
Use template literals for string interpolation and multi-line strings.
Never use eval() on a string.
Do not unnecessarily escape characters in strings.

// bad
const name = "Alice";
const greeting = 'Hello, ' + name + '!';

// good
const name = 'Alice';
const greeting = `Hello, ${name}!`;

6. Functions

Use named function expressions instead of function declarations.
Wrap immediately invoked function expressions (IIFE) in parentheses.
Never declare a function in a non-function block (if, while, etc.).
Never name a parameter arguments.
Use default parameter syntax rather than mutating function arguments.
Always put default parameters last.
Never use the Function constructor to create a new function.
Use the spread operator ... to call variadic functions.
Use rest parameters (...args) instead of arguments.

// named function expression
const short = function longUniqueMoreDescriptiveLexicalFoo() {
  // ...
};

// default parameters last
function handleThings(name, opts = {}) {
  // ...
}

// rest parameters
function concatenateAll(...args) {
  return args.join('');
}

// spread to call
const values = [1, 2, 3];
console.log(Math.max(...values));

7. Arrow Functions

Use arrow function notation for anonymous functions (callbacks).
If the function body consists of a single expression, omit the braces and use the implicit return.
If the expression spans multiple lines, wrap it in parentheses for readability.
Always include parentheses around arguments for clarity and consistency.

// 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;
});

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

// multiline implicit return
[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!`
));

8. Classes & Constructors

Always use class; avoid manipulating prototype directly.
Use extends for inheritance.
Methods can return this to enable method chaining.
Classes have a default constructor if no constructor is specified; an empty constructor or one that just delegates to a parent is unnecessary.
Avoid duplicate class members.

// bad
function Queue(contents = []) {
  this.queue = [...contents];
}
Queue.prototype.pop = function () {
  return this.queue.pop();
};

// good
class Queue {
  constructor(contents = []) {
    this.queue = [...contents];
  }

  pop() {
    return this.queue.pop();
  }
}

// inheritance
class PeekableQueue extends Queue {
  peek() {
    return this.queue[0];
  }
}

9. Modules

Always use ES modules (import/export) over a non-standard module system.
Do not use wildcard imports.
Do not export directly from an import.
Only import from a path in one place.
Do not export mutable bindings.
Prefer default export for modules that only export a single thing.
Put all imports above non-import statements.
Multi-line imports should be indented like multi-line array and object literals.

// bad
const utils = require('./utils');
module.exports = utils.fetchData;

// good
import { fetchData } from './utils';
export default fetchData;

// single place import
import { named1, named2 } from 'module';

// multiline
import {
  longNameA,
  longNameB,
  longNameC,
} from 'path/to/module';

10. Iterators and Generators

Prefer JavaScript's higher-order functions over for-in or for-of loops.
Use map, filter, reduce, find, findIndex, every, some, etc.
Don't use generators for now (if targeting ES5 environments).

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

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

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

// filtering
const evens = numbers.filter((num) => num % 2 === 0);

11. Properties

Use dot notation when accessing properties.
Use bracket notation [] when accessing properties with a variable.
Use the exponentiation operator ** instead of Math.pow.

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

// dot notation
const isJedi = luke.jedi;

// bracket notation
const prop = 'jedi';
const isJedi = luke[prop];

// exponentiation
const result = 2 ** 10;

12. Variables

Always use const or let; never use var.
Use one const or let declaration per variable or assignment.
Group all consts and then group all lets.
Assign variables where you need them, but place them in a reasonable place.
Don't chain variable assignments.
Avoid using unary increments (++, --); use += 1 / -= 1 instead.
Avoid linebreaks before or after = in an assignment.

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

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

// group const then let
const a = 1;
const b = 2;
let c = 3;
let d = 4;

// avoid unary
let count = 0;
count += 1;

13. Hoisting

var declarations get hoisted; const and let are in Temporal Dead Zone.
Named function expressions hoist the variable name but not the function body.
Function declarations hoist the name and the function body.

14. Comparison Operators & Equality

Use === and !== over == and !=.
Use shortcuts for booleans, but explicit comparisons for strings and numbers.
Use braces to create blocks in case and default clauses that contain lexical declarations.
Ternaries should not be nested and generally be single-line expressions.
Avoid unneeded ternary statements.
When mixing operators, enclose them in parentheses (except **, +, -).

// bad
if (isValid == true) { /* ... */ }
if (name != '') { /* ... */ }

// good
if (isValid) { /* ... */ }
if (name !== '') { /* ... */ }
if (collection.length > 0) { /* ... */ }

// no nested ternaries
const thing = foo === 123 ? bar : foobar;

15. Blocks

Use braces with all multiline blocks.
Put else on the same line as the if block's closing brace.
If an if block always executes a return, the subsequent else block is unnecessary.

// bad
if (test)
  return false;

// good
if (test) return false;

if (test) {
  return false;
}

// if/else
if (test) {
  thing1();
} else {
  thing2();
}

16. Control Statements

If a control statement (if, while, etc.) gets too long, each grouped condition should be on a new line, with the logical operator at the beginning of the line.
Don't use selection operators in place of control statements.

if (
  foo === 123
  && bar === 'abc'
) {
  thing1();
}

17. Comments

Use /** ... */ for multiline comments.
Use // for single-line comments. Place them on a new line above the subject.
Start all comments with a space for readability.
Prefix comments with FIXME: or TODO: to annotate problems or suggest actions.

// good single line
// This is a comment
const active = true;

/**
 * Multiline comment explaining the function.
 * @param {string} tag - The tag name.
 * @returns {Element} The created element.
 */
function make(tag) {
  return document.createElement(tag);
}

// TODO: implement caching
// FIXME: should not use global state

18. Whitespace

Use soft tabs (spaces) set to 2 spaces.
Place 1 space before the leading brace.
Place 1 space before the opening parenthesis in control statements.
Set off operators with spaces.
End files with a single newline character.
Use indentation when making long method chains (more than 2 method chains).
Leave a blank line after blocks and before the next statement.
Do not pad blocks with blank lines.
Do not use multiple blank lines to pad your code.
Do not add spaces inside parentheses, brackets.
Add spaces inside curly braces.

// bad
function foo(){
  const name='Alice';
}

// good
function foo() {
  const name = 'Alice';
}

// method chaining
$('#items')
  .find('.selected')
  .highlight()
  .end()
  .find('.open')
  .updateCount();

19. Commas

Do not use leading commas.
Use trailing commas (dangling commas) for multiline structures.

// bad — leading commas
const hero = {
    firstName: 'Ada'
  , lastName: 'Lovelace'
};

// good — trailing commas
const hero = {
  firstName: 'Ada',
  lastName: 'Lovelace',
};

const heroes = [
  'Batman',
  'Superman',
];

20. Semicolons

Always use semicolons.

// bad
const name = 'Alice'

// good
const name = 'Alice';

21. Type Casting & Coercion

Perform type coercion at the beginning of the statement.
Use String() for strings, Number() for numbers, Boolean() or !! for booleans.
Use parseInt always with a radix.

const val = '4';

// bad
const totalScore = val + 0;

// good
const totalScore = Number(val);
const inputValue = String(someNum);
const hasAge = Boolean(age);
const hasName = !!name;
const count = parseInt(inputValue, 10);

22. Naming Conventions

Avoid single-letter names; be descriptive.
Use camelCase for objects, functions, and instances.
Use PascalCase for classes and constructors.
Use UPPERCASE_SNAKE_CASE for constants that are exported and truly immutable.
Do not use trailing or leading underscores.
A base filename should exactly match the name of its default export.
Use camelCase when exporting a function; use PascalCase when exporting a class / constructor / singleton / function library / bare object.
Acronyms and initialisms should always be all uppercased or all lowercased.

// bad
const OBJEcttsssss = {};
function c() {}
const u = new user();

// good
const thisIsMyObject = {};
function calculateTotal() {}
const user = new User();

// constants
export const API_BASE_URL = 'https://api.example.com';
export const MAX_RETRY_COUNT = 3;

// filename examples
// file: makeStyleGuide.js → export default function makeStyleGuide() {}
// file: User.js              → export default class User {}

23. Accessors

Accessor functions for properties are not required.
Do not use JavaScript getters/setters as they cause unexpected side effects.
If you do make accessor functions, use getVal() and setVal('value').
If the property/method is a boolean, use isVal() or hasVal().

// bad
dragon.age();

// good
dragon.getAge();
dragon.setAge(25);
dragon.isAlive();
dragon.hasWings();

24. Events

When attaching data payloads to events, pass an object (hash) instead of a raw value.

// bad
emitter.emit('itemUpdate', item.id);

// good
emitter.emit('itemUpdate', { itemId: item.id });

25. Promises & Async/Await

Prefer async/await over chaining .then() and .catch().
Always handle errors with try...catch in async functions.
Avoid mixing callbacks and promises.
Use Promise.all for concurrent independent async operations.
Use Promise.allSettled when you need results of all promises regardless of rejection.

// bad
function fetchData() {
  return getData()
    .then((data) => parseData(data))
    .then((parsed) => validate(parsed))
    .catch((err) => console.error(err));
}

// good
async function fetchData() {
  try {
    const data = await getData();
    const parsed = await parseData(data);
    return validate(parsed);
  } catch (err) {
    console.error(err);
    throw err;
  }
}

// concurrent operations
async function loadDashboard() {
  const [user, posts, notifications] = await Promise.all([
    fetchUser(),
    fetchPosts(),
    fetchNotifications(),
  ]);
  return { user, posts, notifications };
}

26. Error Handling

Only throw Error objects (or subclasses of Error).
Always catch errors where they can be meaningfully handled.
Use custom error classes for domain-specific errors.

// bad
throw 'Something went wrong';
throw { message: 'error' };

// good
throw new Error('Something went wrong');

class ValidationError extends Error {
  constructor(message, field) {
    super(message);
    this.name = 'ValidationError';
    this.field = field;
  }
}

throw new ValidationError('Invalid email', 'email');

27. Optional Chaining & Nullish Coalescing

Use optional chaining (?.) to access nested properties that may not exist.
Use nullish coalescing (??) instead of || when you want to allow falsy values like 0 or ''.

// bad
const city = user && user.address && user.address.city;
const port = config.port || 3000; // breaks if port is 0

// good
const city = user?.address?.city;
const port = config.port ?? 3000;

28. Standard Library

Use Number.isNaN instead of global isNaN.
Use Number.isFinite instead of global isFinite.

// bad
isNaN('1.2');
isFinite('2e3');

// good
Number.isNaN('1.2');
Number.isFinite('2e3');

Usage

This is a Prompt-based Skill (natural language activation).

How to Use

Automatic Activation: Simply mention "JavaScript" in your request. The skill will be activated automatically.
Ask for Style Guidance: "How should I write a JavaScript function that fetches user data?"
Request Code Review: "Review this JavaScript code for style issues: [paste code]"
Generate Compliant Code: "Write a JavaScript module to handle form validation"
Ask About Specific Rules: "What's the correct way to declare variables in JavaScript?" "How should I handle async errors in JavaScript?"

Examples

User: Write a JavaScript utility function to deep clone an object.

Skill Response: Generates clean, style-guide-compliant code:

/**
 * Deep clones a plain object or array using structured cloning.
 * Falls back to JSON serialization for environments without structuredClone.
 *
 * @param {Object|Array} source - The value to clone.
 * @returns {Object|Array} A deep copy of the source.
 */
function deepClone(source) {
  if (typeof structuredClone === 'function') {
    return structuredClone(source);
  }

  return JSON.parse(JSON.stringify(source));
}

export default deepClone;

JavaScript