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 Activate This Skill

Use this skill when the task involves:

Writing JavaScript/TypeScript Code

• Creating new functions, classes, or modules
• Implementing control flow logic (conditionals, loops, early returns)
• Managing state with variables (const vs let)
• Writing TypeScript type definitions, interfaces, or generics
• Defining enums or constant objects
• Structuring code with proper naming conventions

TypeScript-Specific Tasks

• Fixing type errors or improving type safety
• Replacing any with proper types (unknown, generics)
• Converting enum to as const objects
• Choosing between type and interface
• Adding type annotations to untyped code
• Implementing generic functions or utilities

Code Quality & Style

• Refactoring nested conditionals to early returns
• Simplifying complex control flow
• Improving variable naming (descriptive names, proper prefixes for booleans)
• Reducing code duplication (extracting common patterns)
• Applying consistent code style
• Removing commented-out code or outdated comments

Safety & Validation

• Adding input validation at system boundaries
• Implementing assertions for programmer error detection
• Improving error handling patterns
• Writing better error messages (user-facing or developer-facing)
• Validating external data with schemas (Zod, etc.)
• Preventing null/undefined propagation

Performance Optimization

• Reducing unnecessary branching or conditionals
• Optimizing array operations (reduce instead of chained filter/map)
• Implementing efficient data structures (Set vs Array for lookups)
• Caching expensive computations or API calls
• Optimizing loops (cache property access, array length)
• Deferring await operations to avoid blocking
• Caching localStorage, sessionStorage, or cookie reads

Documentation

• Adding JSDoc comments to exported functions
• Using proper comment markers (TODO, FIXME, HACK, NOTE, PERF)
• Removing unnecessary comments
• Preserving important comments (business logic, linter directives)
• Documenting function parameters, return types, and examples

Code Review

• Reviewing code for style violations
• Identifying performance anti-patterns
• Checking for safety issues (missing validation, poor error handling)
• Ensuring TypeScript best practices
• Verifying proper documentation

When NOT to Use This Skill

Do not activate for:

• Framework-specific code (React, Vue, Angular) - use framework-specific skills instead
• Node.js-specific APIs or server-side concerns (unless general JS/TS patterns apply)
• Build tooling configuration (webpack, vite, tsconfig.json) unless code quality-related
• Testing code (use a testing-specific skill if available)
• Database queries or ORM usage
• CSS, HTML, or styling (unless embedded in JS/TS files)
• Package management (npm, yarn, pnpm)

How to Use

This skill uses a progressive disclosure structure to minimize context usage:

1. Start with the Overview (AGENTS.md)

Read [AGENTS.md](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](references/quick-start.md) - Complete workflow examples with before/after code

General Best Practices:

• [naming-conventions.md](references/naming-conventions.md) - Descriptive names, qualifier ordering, boolean prefixes
• [functions.md](references/functions.md) - Function size, parameters, explicit values
• [control-flow.md](references/control-flow.md) - Early returns, flat structure, block style
• [state-management.md](references/state-management.md) - const vs let, immutability, pure functions
• [return-values.md](references/return-values.md) - Return zero values instead of null/undefined
• [misc.md](references/misc.md) - Line endings, defensive programming, technical debt
• [code-duplication.md](references/code-duplication.md) - Extract common patterns, DRY principle, when to consolidate

TypeScript:

• [any.md](references/any.md) - Avoid any, use unknown or generics
• [enums.md](references/enums.md) - Use as const objects instead of enum
• [type-vs-interface.md](references/type-vs-interface.md) - Prefer type over interface

Safety:

• [input-validation.md](references/input-validation.md) - Validate external data with schemas
• [assertions.md](references/assertions.md) - Split assertions, include values
• [error-handling.md](references/error-handling.md) - Handle all errors explicitly
• [error-messages.md](references/error-messages.md) - User-friendly vs developer-specific messages

Performance:

• [reduce-branching.md](references/reduce-branching.md) - Convert conditionals to lookups, hoist invariants, early returns
• [reduce-looping.md](references/reduce-looping.md) - Single-pass operations, O(1) lookups, typed arrays
• [memoization.md](references/memoization.md) - Hoist invariants, precompute constants, cache expensive operations
• [batching.md](references/batching.md) - Batch I/O operations
• [predictable-execution.md](references/predictable-execution.md) - Sequential access, cache locality, grouped data
• [bounded-iteration.md](references/bounded-iteration.md) - Set limits on loops and queues
• [defer-await.md](references/defer-await.md) - Move await into branches that need it
• [cache-property-access.md](references/cache-property-access.md) - Cache lookups, eliminate aliases, avoid unnecessary destructuring
• [cache-storage-api.md](references/cache-storage-api.md) - Cache localStorage/sessionStorage/cookie reads
• [object-operations.md](references/object-operations.md) - Safe mutation, shallow clones, preallocate shapes
• [avoid-allocations.md](references/avoid-allocations.md) - Inline simple computations, avoid needless variables, reduce GC pressure
• [currying.md](references/currying.md) - Curry to precompute constant parameters, optimize hot paths
• [performance-misc.md](references/performance-misc.md) - Strings, regex, async overhead, closures, try/catch

Documentation:

• [jsdoc.md](references/jsdoc.md) - Well-formed JSDoc for exports
• [comment-markers.md](references/comment-markers.md) - TODO, FIXME, HACK, NOTE markers
• [comments-to-remove.md](references/comments-to-remove.md) - Commented code, edit history
• [comments-to-preserve.md](references/comments-to-preserve.md) - Markers, linter directives, business logic
• [comments-placement.md](references/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