javascript

Google JavaScript Style Guide

Official Google JavaScript coding standards for ES6+ code.

Golden Rules

1. Use const by default — let only when reassignment needed, never var
2. Arrow functions for callbacks — traditional functions for methods
3. Template literals for string interpolation
4. Destructuring where it improves readability
5. Named exports over default exports
6. JSDoc for public APIs — document parameters and return types
7. 2-space indentation — consistent formatting

Quick Reference

Naming Conventions

Element	Convention	Example
Classes	UpperCamelCase	UserService
Functions/Methods	lowerCamelCase	getUserById
Variables	lowerCamelCase	userCount
Constants	UPPER_SNAKE_CASE	MAX_SIZE
Private fields	#privateField	#userId
Files	lower-kebab-case	user-service.js

Variables

// ✓ CORRECT
const greeting = 'Hello';
let count = 0;

// ✗ INCORRECT
var greeting = 'Hello';  // never use var

Functions

// ✓ CORRECT - arrow functions for callbacks
const double = (x) => x * 2;
array.map(item => item.id);

// ✓ CORRECT - traditional functions for methods
class User {
  getName() {
    return this.name;
  }
}

// ✓ CORRECT - default parameters
function greet(name, greeting = 'Hello') {
  return `${greeting}, ${name}!`;
}

Strings

// ✓ CORRECT - template literals
const name = 'Alice';
const greeting = `Hello, ${name}!`;

// ✗ INCORRECT
const greeting = 'Hello, ' + name + '!';

Destructuring

// ✓ CORRECT - object destructuring
const {id, name} = user;
const {x, y, ...rest} = coordinates;

// ✓ CORRECT - array destructuring
const [first, second] = items;
const [head, ...tail] = list;

// ✓ CORRECT - function parameters
function processUser({id, name, email}) {
  // ...
}

Modules

// ✓ CORRECT - named exports
export function myFunction() { /* ... */ }
export class MyClass { /* ... */ }

// ✓ CORRECT - named imports
import {myFunction, MyClass} from './my-module.js';

// ✗ INCORRECT - default exports (avoid)
export default function() { /* ... */ }

Classes

// ✓ CORRECT
class Animal {
  #name;  // private field

  constructor(name) {
    this.#name = name;
  }

  speak() {
    return `${this.#name} makes a sound.`;
  }
}

JSDoc

/**
 * Fetches user data from the API.
 * @param {number} userId - The user ID to fetch.
 * @param {Object} options - Optional configuration.
 * @param {number} options.timeout - Request timeout in ms.
 * @return {Promise<Object>} The user data.
 */
function fetchUser(userId, {timeout = 5000} = {}) {
  // ...
}

Promises and Async/Await

// ✓ CORRECT - async/await
async function fetchUser(id) {
  const response = await fetch(`/api/users/${id}`);
  if (!response.ok) {
    throw new Error(`HTTP error: ${response.status}`);
  }
  return response.json();
}

// ✓ CORRECT - error handling
try {
  const user = await fetchUser(1);
} catch (error) {
  console.error('Failed to fetch user:', error);
}

Common Mistakes

Mistake	Correct Approach
Using var	Use const / let
String concatenation	Use template literals
Default exports	Use named exports
Missing JSDoc	Document public APIs
Traditional functions for callbacks	Use arrow functions
Ignoring destructuring	Use where it helps readability

When to Use This Guide

• Writing new JavaScript code
• Refactoring existing JavaScript
• Code reviews
• Setting up ESLint rules
• Onboarding new team members

Install

npx skills add testdino-hq/google-styleguides-skills/javascript

Full Guide

See javascript.md for complete details, examples, and edge cases.