JavaScript Style Guide

Relevant source files

• README.md
• packages/eslint-config-airbnb-base/rules/best-practices.js
• packages/eslint-config-airbnb-base/rules/style.js

Purpose and Scope

This document describes the core JavaScript style guide for the Airbnb JavaScript Style Guide repository, as documented in README.md1-4098 The guide defines fundamental language patterns, syntax conventions, and best practices for writing modern JavaScript (ES6+). It assumes the use of Babel transpilation via babel-preset-airbnb and browser polyfills via airbnb-browser-shims.

For details on specific topics, see the following child pages:

• Types, Variables, and References
• Objects, Arrays, and Destructuring
• Strings and Template Literals
• Functions and Arrow Functions
• Classes, Modules, and Iterators
• Comparison Operators and Control Flow
• Code Style and Formatting

Sources: README.md1-10

Style Guide Organization

The JavaScript style guide is organized into 30 major sections covering language fundamentals through testing and performance considerations. The core rules are enforced via ESLint configurations in eslint-config-airbnb and eslint-config-airbnb-base packages.

Language Architecture Overview

The following diagram maps high-level style categories to the specific code entities and rules that govern them.

Sources: README.md21-61 README.md63-3820

Core Language Fundamentals

Types and References

JavaScript distinguishes between primitive types (pass-by-value) and complex types (pass-by-reference). The guide mandates const for all references that are not reassigned and let for reassignable references, strictly prohibiting var.

For details, see Types, Variables, and References.

Sources: README.md63-162

Objects, Arrays, and Destructuring

The guide enforces literal syntax for objects and arrays, discouraging constructors. It promotes modern ES6+ features like computed properties, method shorthand, and the spread operator for immutability. Destructuring is required for accessing multiple properties.

For details, see Objects, Arrays, and Destructuring.

Sources: README.md164-576

Strings and Template Literals

Single quotes are the standard for strings. Template literals are required for interpolation or multi-line strings to avoid messy concatenation.

For details, see Strings and Template Literals.

Sources: README.md578-659

Execution and Structure

Functions and Arrow Functions

The guide prioritizes named function expressions over declarations to control hoisting. Arrow functions are required for anonymous callbacks and cases where lexical this is needed.

For details, see Functions and Arrow Functions.

Sources: README.md661-1107

Classes, Modules, and Iterators

ES6 class syntax is mandatory over direct prototype manipulation. The module system relies on import/export rather than CommonJS require. Functional iteration methods (like map and reduce) are preferred over for-in or for-of loops.

For details, see Classes, Modules, and Iterators.

Sources: README.md1109-1591

Logic and Formatting

Comparison and Control Flow

Strict equality (===) is required. The guide provides specific truthiness rules and dictates the structure of blocks, including "cuddled" else statements and mandatory braces for multi-line conditions.

For details, see Comparison Operators and Control Flow.

Sources: README.md2022-2419

Code Style and Formatting

This covers the "look and feel" of the code: 2-space indentation, 100-character line limits, mandatory semicolons, and trailing commas. It also defines naming conventions like camelCase for variables and PascalCase for classes.

For details, see Code Style and Formatting.

Sources: README.md2562-3575

ESLint Rule Integration

The style guide is implemented through a series of ESLint rules defined in the configuration packages.

Code Entity to ESLint Rule Mapping

The following diagram bridges natural language style requirements to the specific ESLint rules used in the codebase to enforce them.

Sources: packages/eslint-config-airbnb-base/rules/best-practices.js53-54 packages/eslint-config-airbnb-base/rules/style.js124-147 packages/eslint-config-airbnb-base/rules/style.js523-524

Standard Library and Testing

The guide extends to the usage of the JavaScript standard library and testing practices:

• Standard Library: Prefer Number.isNaN() over the global isNaN() and Number.isFinite() over global isFinite() README.md3781-3819
• Testing: Recommends small, pure functions to facilitate testing with frameworks like Jest or Mocha README.md3821-3841

Sources: README.md3781-3841