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.

javascript
// 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.

javascript
// 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.

javascript
// 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.

javascript
// 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.

javascript
// 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.

javascript
// 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.

javascript
// 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.

javascript
// 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.

javascript
// 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).

javascript
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.

javascript
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.

javascript
// 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 **, +, -).

javascript
// 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.

javascript
// 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.

javascript
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.

javascript
// 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.

javascript
// 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.

javascript
// 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.

javascript
// 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.

javascript
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.

javascript
// 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().

javascript
// 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.

javascript
// 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.

javascript
// 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.

javascript
// 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 ''.

javascript
// 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.

javascript
// 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

1. Automatic Activation: Simply mention "JavaScript" in your request. The skill will be activated automatically.

1. Ask for Style Guidance:

"How should I write a JavaScript function that fetches user data?"

1. Request Code Review:

"Review this JavaScript code for style issues: [paste code]"

1. Generate Compliant Code:

"Write a JavaScript module to handle form validation"

1. 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:

javascript
/**
 * 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;