JavaScript and TypeScript Best Practices
Comprehensive coding standards and performance optimization guide for JavaScript and TypeScript applications, designed for AI agents and LLMs working with modern JavaScript/TypeScript codebases.
When to Use This Skill
This skill provides expert-level patterns for JavaScript and TypeScript code. Load AGENTS.md to scan rule summaries and identify relevant optimizations for your task.
How to Use
This skill uses a progressive disclosure structure to minimize context usage:
1. Start with the Overview (AGENTS.md)
Read AGENTS.md for a concise overview of all rules with one-line summaries organized by category.
2. Load Specific Rules as Needed
When you identify a relevant pattern or issue, load the corresponding reference file for detailed implementation guidance:
Quick Start:
- quick-start.md - Complete workflow examples with before/after code
General Best Practices:
- naming-conventions.md - Descriptive names, qualifier ordering, boolean prefixes
- functions.md - Function size, parameters, explicit values
- control-flow.md - Early returns, flat structure, block style
- state-management.md - const vs let, immutability, pure functions
- return-values.md - Return zero values instead of null/undefined
- misc.md - Line endings, defensive programming, technical debt
- code-duplication.md - Extract common patterns, DRY principle, when to consolidate
TypeScript:
- any.md - Avoid any, use unknown or generics
- enums.md - Use as const objects instead of enum
- type-vs-interface.md - Prefer type over interface
Safety:
- input-validation.md - Validate external data with schemas
- assertions.md - Split assertions, include values
- error-handling.md - Handle all errors explicitly
- error-messages.md - User-friendly vs developer-specific messages
Performance:
- reduce-branching.md - Convert conditionals to lookups, hoist invariants, early returns
- reduce-looping.md - Single-pass operations, O(1) lookups, typed arrays
- memoization.md - Hoist invariants, precompute constants, cache expensive operations
- batching.md - Batch I/O operations
- predictable-execution.md - Sequential access, cache locality, grouped data
- bounded-iteration.md - Set limits on loops and queues
- defer-await.md - Move await into branches that need it
- cache-property-access.md - Cache lookups, eliminate aliases, avoid unnecessary destructuring
- cache-storage-api.md - Cache localStorage/sessionStorage/cookie reads
- object-operations.md - Safe mutation, shallow clones, preallocate shapes
- avoid-allocations.md - Inline simple computations, avoid needless variables, reduce GC pressure
- currying.md - Curry to precompute constant parameters, optimize hot paths
- performance-misc.md - Strings, regex, async overhead, closures, try/catch
Documentation:
- jsdoc.md - Well-formed JSDoc for exports
- comment-markers.md - TODO, FIXME, HACK, NOTE markers
- comments-to-remove.md - Commented code, edit history
- comments-to-preserve.md - Markers, linter directives, business logic
- comments-placement.md - Move end-of-line comments above code
3. Apply the Pattern
Each reference file contains:
- ❌ Incorrect examples showing the anti-pattern
- ✅ Correct examples showing the optimal implementation
- Explanations of why the pattern matters