SKILL.md
TypeScript Best Practices
Comprehensive performance optimization guide for TypeScript applications. Contains 45 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.
When to Apply
Reference these guidelines when:
- Configuring tsconfig.json for a new or existing project
- Writing complex type definitions or generics
- Optimizing async/await patterns and data fetching
- Organizing modules and managing imports
- Reviewing code for compilation or runtime performance
Rule Categories by Priority
Priority
Category
Impact
Prefix
1
Type System Performance
CRITICAL
type-
2
Compiler Configuration
CRITICAL
tscfg-
3
Async Patterns
HIGH
async-
4
Module Organization
HIGH
module-
5
Type Safety Patterns
MEDIUM-HIGH
safety-
6
Memory Management
MEDIUM
mem-
7
Runtime Optimization
LOW-MEDIUM
runtime-
8
Advanced Patterns
LOW
advanced-
Table of Contents
- Type System Performance — CRITICAL
- 1.1 Add Explicit Return Types to Exported Functions — CRITICAL (30-50% faster declaration emit)
- 1.2 Avoid Deeply Nested Generic Types — CRITICAL (prevents exponential instantiation cost)
- 1.3 Avoid Large Union Types — CRITICAL (quadratic O(n²) comparison cost)
- 1.4 Extract Conditional Types to Named Aliases — CRITICAL (enables compiler caching, prevents re-evaluation)
- 1.5 Limit Type Recursion Depth — HIGH (prevents exponential type expansion when applicable)
- 1.6 Prefer Interfaces Over Type Intersections — CRITICAL (2-5× faster type resolution)
- 1.7 Simplify Complex Mapped Types — HIGH (reduces type computation by 50-80% when applicable)
- Compiler Configuration — CRITICAL
- 2.1 Configure Include and Exclude Properly — CRITICAL (prevents scanning thousands of unnecessary files)
- 2.2 Enable Incremental Compilation — CRITICAL (50-90% faster rebuilds)
- 2.3 Enable isolatedDeclarations for Parallel Declaration Emit — CRITICAL (enables parallel .d.ts generation without type-checker)
- 2.4 Enable skipLibCheck for Faster Builds — CRITICAL (20-40% faster compilation)
- 2.5 Enable strictFunctionTypes for Faster Variance Checks — CRITICAL (enables optimized variance checking)
- 2.6 Use erasableSyntaxOnly for Node.js Native TypeScript — HIGH (prevents 100% of Node.js type-stripping runtime errors)
- 2.7 Use isolatedModules for Single-File Transpilation — CRITICAL (80-90% faster transpilation with bundlers)
- 2.8 Use Project References for Large Codebases — CRITICAL (60-80% faster incremental builds)
- Async Patterns — HIGH
- 3.1 Annotate Async Function Return Types — HIGH (prevents runtime errors, improves inference)
- 3.2 Avoid await Inside Loops — HIGH (N× faster for N iterations, 10 users = 10× improvement)
- 3.3 Avoid Unnecessary async/await — HIGH (eliminates trivial Promise wrappers and improves stack traces)
- 3.4 Defer await Until Value Is Needed — HIGH (enables implicit parallelization)
- 3.5 Use Promise.all for Independent Operations — HIGH (2-10× improvement in I/O-bound code)
- Module Organization — HIGH
- 4.1 Avoid Barrel File Imports — HIGH (200-800ms import cost, 30-50% larger bundles)
- 4.2 Avoid Circular Dependencies — HIGH (prevents runtime undefined errors and slow compilation)
- 4.3 Control @types Package Inclusion — HIGH (prevents type conflicts and reduces memory usage)
- 4.4 Use Dynamic Imports for Large Modules — HIGH (reduces initial bundle by 30-70%)
- 4.5 Use Type-Only Imports for Types — HIGH (eliminates runtime imports for type information)
- Type Safety Patterns — MEDIUM-HIGH
- 5.1 Enable noUncheckedIndexedAccess — MEDIUM-HIGH (prevents 100% of unchecked index access errors at compile time)
- 5.2 Enable strictNullChecks — MEDIUM-HIGH (prevents null/undefined runtime errors)
- 5.3 Prefer unknown Over any — MEDIUM-HIGH (forces type narrowing, prevents runtime errors)
- 5.4 Use Assertion Functions for Validation — MEDIUM-HIGH (reduces validation boilerplate by 50-70%)
- 5.5 Use const Assertions for Literal Types — MEDIUM-HIGH (preserves literal types, enables better inference)
- 5.6 Use Exhaustive Checks for Union Types — MEDIUM-HIGH (prevents 100% of missing case errors at compile time)
- 5.7 Use Type Guards for Runtime Type Checking — MEDIUM-HIGH (eliminates type assertions, catches errors at boundaries)
- Memory Management — MEDIUM
- 6.1 Avoid Closure Memory Leaks — MEDIUM (prevents retained references in long-lived callbacks)
- 6.2 Avoid Global State Accumulation — MEDIUM (prevents unbounded memory growth)
- 6.3 Clean Up Event Listeners — MEDIUM (prevents unbounded memory growth)
- 6.4 Clear Timers and Intervals — MEDIUM (prevents callback retention and repeated execution)
- 6.5 Use WeakMap for Object Metadata — MEDIUM (prevents memory leaks, enables automatic cleanup)
- Runtime Optimization — LOW-MEDIUM
- 7.1 Avoid Object Spread in Hot Loops — LOW-MEDIUM (reduces object allocations by N×)
- 7.2 Cache Property Access in Loops — LOW-MEDIUM (reduces property lookups by N× in hot paths)
- 7.3 Prefer Native Array Methods Over Lodash — LOW-MEDIUM (eliminates library overhead, enables tree-shaking)
- 7.4 Use for-of for Simple Iteration — LOW-MEDIUM (reduces iteration boilerplate by 30-50%)
- 7.5 Use Modern String Methods — LOW-MEDIUM (2-5× faster than regex for simple patterns)
- 7.6 Use Set/Map for O(1) Lookups — LOW-MEDIUM (O(n) to O(1) per lookup)
- Advanced Patterns — LOW
- 8.1 Use Branded Types for Type-Safe IDs — LOW (prevents mixing incompatible ID types)
- 8.2 Use satisfies for Type Validation with Inference — LOW (prevents property access errors, enables 100% autocomplete accuracy)
- 8.3 Use Template Literal Types for String Patterns — LOW (prevents 100% of string format errors at compile time)