TypeScript Best Practices

When to Apply

• 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

Table of Contents

1. 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 — CRITICAL (prevents exponential type expansion)
   • 1.6 Prefer Interfaces Over Type Intersections — CRITICAL (2-5× faster type resolution)
   • 1.7 Simplify Complex Mapped Types — CRITICAL (reduces type computation by 50-80%)
2. 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 skipLibCheck for Faster Builds — CRITICAL (20-40% faster compilation)
   • 2.4 Enable strictFunctionTypes for Faster Variance Checks — CRITICAL (enables optimized variance checking)
   • 2.5 Use isolatedModules for Single-File Transpilation — CRITICAL (80-90% faster transpilation with bundlers)
   • 2.6 Use Project References for Large Codebases — CRITICAL (60-80% faster incremental builds)
3. 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 microtask queue overhead)
   • 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)
4. 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)
5. Type Safety Patterns — MEDIUM-HIGH
   • 5.1 Enable strictNullChecks — MEDIUM-HIGH (prevents null/undefined runtime errors)
   • 5.2 Prefer unknown Over any — MEDIUM-HIGH (forces type narrowing, prevents runtime errors)
   • 5.3 Use Assertion Functions for Validation — MEDIUM-HIGH (reduces validation boilerplate by 50-70%)
   • 5.4 Use const Assertions for Literal Types — MEDIUM-HIGH (preserves literal types, enables better inference)
   • 5.5 Use Exhaustive Checks for Union Types — MEDIUM-HIGH (prevents 100% of missing case errors at compile time)
   • 5.6 Use Type Guards for Runtime Type Checking — MEDIUM-HIGH (eliminates type assertions, catches errors at boundaries)
6. 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)
7. 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×)
   • 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)
8. 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)

References

1. https://github.com/microsoft/TypeScript/wiki/Performance
2. https://www.typescriptlang.org/docs/handbook/
3. https://v8.dev/blog
4. https://nodejs.org/en/learn/diagnostics/memory