Typescript

TypeScript Best Practices

Comprehensive performance optimization guide for TypeScript applications. Contains 42 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

1. [Type System Performance](references/_sections.md#1-type-system-performance) — CRITICAL

• 1.1 [Add Explicit Return Types to Exported Functions](references/type-explicit-return-types.md) — CRITICAL (30-50% faster declaration emit)
• 1.2 [Avoid Deeply Nested Generic Types](references/type-avoid-deep-generics.md) — CRITICAL (prevents exponential instantiation cost)
• 1.3 [Avoid Large Union Types](references/type-avoid-large-unions.md) — CRITICAL (quadratic O(n²) comparison cost)
• 1.4 [Extract Conditional Types to Named Aliases](references/type-extract-conditional-types.md) — CRITICAL (enables compiler caching, prevents re-evaluation)
• 1.5 [Limit Type Recursion Depth](references/type-limit-recursion-depth.md) — CRITICAL (prevents exponential type expansion)
• 1.6 [Prefer Interfaces Over Type Intersections](references/type-interfaces-over-intersections.md) — CRITICAL (2-5× faster type resolution)
• 1.7 [Simplify Complex Mapped Types](references/type-simplify-mapped-types.md) — CRITICAL (reduces type computation by 50-80%)

1. [Compiler Configuration](references/_sections.md#2-compiler-configuration) — CRITICAL

• 2.1 [Configure Include and Exclude Properly](references/tscfg-exclude-properly.md) — CRITICAL (prevents scanning thousands of unnecessary files)
• 2.2 [Enable Incremental Compilation](references/tscfg-enable-incremental.md) — CRITICAL (50-90% faster rebuilds)
• 2.3 [Enable skipLibCheck for Faster Builds](references/tscfg-skip-lib-check.md) — CRITICAL (20-40% faster compilation)
• 2.4 [Enable strictFunctionTypes for Faster Variance Checks](references/tscfg-strict-function-types.md) — CRITICAL (enables optimized variance checking)
• 2.5 [Use isolatedModules for Single-File Transpilation](references/tscfg-isolate-modules.md) — CRITICAL (80-90% faster transpilation with bundlers)
• 2.6 [Use Project References for Large Codebases](references/tscfg-project-references.md) — CRITICAL (60-80% faster incremental builds)

1. [Async Patterns](references/_sections.md#3-async-patterns) — HIGH

• 3.1 [Annotate Async Function Return Types](references/async-explicit-return-types.md) — HIGH (prevents runtime errors, improves inference)
• 3.2 [Avoid await Inside Loops](references/async-avoid-loop-await.md) — HIGH (N× faster for N iterations, 10 users = 10× improvement)
• 3.3 [Avoid Unnecessary async/await](references/async-avoid-unnecessary-async.md) — HIGH (eliminates microtask queue overhead)
• 3.4 [Defer await Until Value Is Needed](references/async-defer-await.md) — HIGH (enables implicit parallelization)
• 3.5 [Use Promise.all for Independent Operations](references/async-parallel-promises.md) — HIGH (2-10× improvement in I/O-bound code)

1. [Module Organization](references/_sections.md#4-module-organization) — HIGH

• 4.1 [Avoid Barrel File Imports](references/module-avoid-barrel-imports.md) — HIGH (200-800ms import cost, 30-50% larger bundles)
• 4.2 [Avoid Circular Dependencies](references/module-avoid-circular-dependencies.md) — HIGH (prevents runtime undefined errors and slow compilation)
• 4.3 [Control @types Package Inclusion](references/module-control-types-inclusion.md) — HIGH (prevents type conflicts and reduces memory usage)
• 4.4 [Use Dynamic Imports for Large Modules](references/module-dynamic-imports.md) — HIGH (reduces initial bundle by 30-70%)
• 4.5 [Use Type-Only Imports for Types](references/module-use-type-imports.md) — HIGH (eliminates runtime imports for type information)

1. [Type Safety Patterns](references/_sections.md#5-type-safety-patterns) — MEDIUM-HIGH

• 5.1 [Enable strictNullChecks](references/safety-strict-null-checks.md) — MEDIUM-HIGH (prevents null/undefined runtime errors)
• 5.2 [Prefer unknown Over any](references/safety-prefer-unknown-over-any.md) — MEDIUM-HIGH (forces type narrowing, prevents runtime errors)
• 5.3 [Use Assertion Functions for Validation](references/safety-assertion-functions.md) — MEDIUM-HIGH (reduces validation boilerplate by 50-70%)
• 5.4 [Use const Assertions for Literal Types](references/safety-const-assertions.md) — MEDIUM-HIGH (preserves literal types, enables better inference)
• 5.5 [Use Exhaustive Checks for Union Types](references/safety-exhaustive-checks.md) — MEDIUM-HIGH (prevents 100% of missing case errors at compile time)
• 5.6 [Use Type Guards for Runtime Type Checking](references/safety-use-type-guards.md) — MEDIUM-HIGH (eliminates type assertions, catches errors at boundaries)

1. [Memory Management](references/_sections.md#6-memory-management) — MEDIUM

• 6.1 [Avoid Closure Memory Leaks](references/mem-avoid-closure-leaks.md) — MEDIUM (prevents retained references in long-lived callbacks)
• 6.2 [Avoid Global State Accumulation](references/mem-avoid-global-state.md) — MEDIUM (prevents unbounded memory growth)
• 6.3 [Clean Up Event Listeners](references/mem-cleanup-event-listeners.md) — MEDIUM (prevents unbounded memory growth)
• 6.4 [Clear Timers and Intervals](references/mem-clear-timers.md) — MEDIUM (prevents callback retention and repeated execution)
• 6.5 [Use WeakMap for Object Metadata](references/mem-use-weakmap-for-metadata.md) — MEDIUM (prevents memory leaks, enables automatic cleanup)

1. [Runtime Optimization](references/_sections.md#7-runtime-optimization) — LOW-MEDIUM

• 7.1 [Avoid Object Spread in Hot Loops](references/runtime-avoid-object-spread-in-loops.md) — LOW-MEDIUM (reduces object allocations by N×)
• 7.2 [Cache Property Access in Loops](references/runtime-cache-property-access.md) — LOW-MEDIUM (reduces property lookups by N×)
• 7.3 [Prefer Native Array Methods Over Lodash](references/runtime-prefer-array-methods.md) — LOW-MEDIUM (eliminates library overhead, enables tree-shaking)
• 7.4 [Use for-of for Simple Iteration](references/runtime-use-for-of-for-iteration.md) — LOW-MEDIUM (reduces iteration boilerplate by 30-50%)
• 7.5 [Use Modern String Methods](references/runtime-use-string-methods.md) — LOW-MEDIUM (2-5× faster than regex for simple patterns)
• 7.6 [Use Set/Map for O(1) Lookups](references/runtime-use-set-for-lookups.md) — LOW-MEDIUM (O(n) to O(1) per lookup)

1. [Advanced Patterns](references/_sections.md#8-advanced-patterns) — LOW

• 8.1 [Use Branded Types for Type-Safe IDs](references/advanced-branded-types.md) — LOW (prevents mixing incompatible ID types)
• 8.2 [Use satisfies for Type Validation with Inference](references/advanced-satisfies-operator.md) — LOW (prevents property access errors, enables 100% autocomplete accuracy)
• 8.3 [Use Template Literal Types for String Patterns](references/advanced-template-literal-types.md) — 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

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: thongdn-it
• Source: thongdn-it/react-agent-skills
• License: MIT

Install and usage instructions live in the source repository linked above.