JavaScript and TypeScript Best Practices

Comprehensive coding standards for JavaScript and TypeScript applications, designed for AI agents and LLMs working with modern JavaScript/TypeScript codebases.

Note: This skill focuses on general best practices, TypeScript patterns, and safety. For performance optimization, use the accelint-ts-performance skill instead.

When to Use This Skill

This skill provides expert-level patterns for JavaScript and TypeScript code. Load [AGENTS.md](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](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
• [bundler-paths.md](references/bundler-paths.md) - Use statically analyzable import and file-system paths for optimal bundling

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:

• For performance optimization tasks, use the accelint-ts-performance skill for comprehensive profiling workflows and optimization patterns

Documentation:

• For documentation tasks, use the accelint-ts-documentation skill for comprehensive JSDoc and comment guidance

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

4. Use the Report Template

When this skill is invoked, use the standardized report format:

Template: [assets/output-report-template.md](assets/output-report-template.md)

The report format provides:

• Executive Summary with impact assessment
• Severity levels (Critical, High, Medium, Low) for prioritization
• Impact analysis (potential bugs, type safety, maintainability, runtime failures)
• Categorization (Type Safety, Safety, State Management, Return Values, Code Quality)
• Pattern references linking to detailed guidance in references/
• Phase 2 summary table for tracking all issues

When to use the audit template:

• Skill invoked directly via /accelint-ts-best-practices <path>
• User asks to "review code quality" or "audit code" across file(s), invoking skill implicitly

When NOT to use the report template:

• User asks to "fix this type error" (direct implementation)
• User asks "what's wrong with this code?" (answer the question)
• User requests specific fixes (apply fixes directly without formal report)