JavaScript Best Practices

Comprehensive guide for modern JavaScript development covering performance, code quality, and best practices.

When to Apply

Reference these guidelines when:

• Writing new JavaScript code
• Reviewing code for performance or quality issues
• Refactoring existing JavaScript code
• Optimizing application performance
• Ensuring code maintainability

Rule Categories by Priority

Quick Reference

1. Performance (CRITICAL)

• perf-avoid-memory-leaks – Always cleanup event listeners, timers, intervals, subscriptions, and observers

2. Async Patterns (HIGH)

• async-promise-chains – Prefer async/await over .then()/.catch() chains for better readability

3. Error Handling (HIGH)

• error-defensive-programming – Use guard clauses and validate input at system boundaries
• error-explicit-validation – Don’t hide data errors with optional chaining – validate explicitly

4. Code Quality (HIGH)

• quality-strict-equality – Use === instead of == (except for null checks)
• quality-explicit-boolean-checks – Don’t rely on truthy/falsy for non-boolean values
• quality-small-functions – Keep functions focused on one task (5-15 lines ideal)
• quality-no-magic-numbers – Extract numbers to named constants
• quality-immutability – Prefer creating new objects/arrays over mutating existing ones
• quality-pure-functions – Write functions with no side effects when possible
• quality-naming-conventions – Use meaningful, consistent names (camelCase, PascalCase, UPPER_SNAKE_CASE)
• quality-commenting – Comments explain “why”, not “what” – code should be self-documenting
• quality-avoid-console-log – Use structured logging with levels in production

5. Modern Syntax (MEDIUM)

• modern-const-let-no-var – Never use var; prefer const, use let only when reassignment is needed

6. Arrays (MEDIUM)

• array-mutation-safety – Avoid in-place mutations (push, sort, reverse) on shared arrays

How to Use

Read individual rule files for detailed explanations and code examples:

rules/perf-avoid-memory-leaks.md
rules/async-promise-chains.md
rules/error-defensive-programming.md
rules/quality-strict-equality.md

Each rule file contains:

• Brief explanation of why it matters
• Incorrect code example with explanation
• Correct code example with explanation
• Additional context and references

Full Compiled Document

For the complete guide with all rules expanded: AGENTS.md