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name: typescript-performance-best-practices description: TypeScript performance optimization guidelines for build times, type-checking, declaration emit, and editor responsiveness. This skill should be used when writing, reviewing, or optimizing TypeScript code and tsconfig settings. Triggers on tasks involving slow builds, slow type-checking, CI timeouts, editor lag, tsserver memory issues, monorepo configuration, project references, incremental builds, declaration emit, or any TypeScript performance investigation.

TypeScript Performance Best Practices

Comprehensive performance optimization guide for TypeScript codebases. Contains 43 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.

When to Apply

Reference these guidelines when:

• Writing or refactoring TypeScript code
• Tuning build times, type-checking performance, declaration emit, or editor responsiveness
• Investigating performance regressions
• Reviewing code for performance issues
• Configuring tsconfig.json for new projects or monorepos

Quick Diagnostic Workflow

When TypeScript feels slow, follow this diagnostic sequence:

# Step 1: Get baseline metrics
tsc --extendedDiagnostics --noEmit

# Key metrics to check:
# - Files: Should match expected source count
# - Check time: Usually the bottleneck (>10s is high)
# - Instantiations: High count indicates complex generics

# Step 2: If file count is high
tsc --listFiles | wc -l
tsc --explainFiles 2>&1 | grep -v "@types" | head -50

# Step 3: If Check time is high
tsc --generateTrace ./trace --noEmit
# Open chrome://tracing and load trace/trace.json

# Step 4: If module resolution is slow
tsc --traceResolution 2>&1 | head -200

Rule Categories by Priority

Priority	Category	Impact	Quick Win
1	Build Graph & Incremental	CRITICAL	Enable incremental: true
2	Program Scope & Inputs	HIGH	Narrow include patterns
3	Module Resolution & Imports	HIGH	Use moduleResolution: "bundler"
4	Type Acquisition & Lib Checks	MEDIUM-HIGH	Enable skipLibCheck: true
5	Type System Complexity	CRITICAL	Annotate function return types
6	Declaration Emit & Public API	MEDIUM-HIGH	Enable isolatedDeclarations
7	Editor/tsserver Performance	MEDIUM-HIGH	Use project references
8	Diagnostics & Profiling	DIAGNOSTIC	Run tsc --extendedDiagnostics

Common Scenarios

Scenario: Slow CI Builds (>60s)

Symptoms: CI takes minutes, incremental doesn't help

Diagnostic:

tsc --extendedDiagnostics --noEmit
# Check: Files count, Check time, Memory used

Likely fixes:

1. Enable incremental: true with cached .tsbuildinfo
2. Enable skipLibCheck: true (10-50% improvement)
3. Split into project references (60-70% memory reduction)
4. Narrow include patterns

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Scenario: Slow Editor/IntelliSense

Symptoms: Autocomplete lags, hover takes seconds, high memory

Diagnostic:

# Check tsserver memory
ps aux | grep tsserver | awk '{print $6/1024 " MB"}'

Likely fixes:

1. Enable disableReferencedProjectLoad: true
2. Enable disableSolutionSearching: true
3. Enable disableSourceOfProjectReferenceRedirect: true
4. Split into project references

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Scenario: Type Checking Takes >10s

Symptoms: Check time dominates in diagnostics

Diagnostic:

tsc --generateTrace ./trace --noEmit
# Open chrome://tracing, find slow checkSourceFile operations

Likely fixes:

1. Add explicit return type annotations to exported functions
2. Simplify large unions (50+ members)
3. Replace deep intersections with interface extension
4. Name complex types with type aliases

---

Scenario: Unexpected Files in Build

Symptoms: More files than expected, node_modules source compiled

Diagnostic:

tsc --explainFiles 2>&1 | grep -v "@types" | grep "node_modules"

Likely fixes:

1. Add "exclude": ["node_modules", "dist"]
2. Narrow include to ["src"]
3. Remove broad **/* patterns

Quick Reference

1. Build Graph & Incremental (CRITICAL)

Impact: 50-80% faster rebuilds; essential for monorepos

Rule	Impact	Description
build-project-references	CRITICAL	Split large repos into referenced projects
build-composite	CRITICAL	Enable composite for project reference builds
build-incremental	CRITICAL	Reuse prior type-checking work
build-tsbuildinfo	HIGH	Cache incremental state in stable location
build-tsc-build	HIGH	Use tsc --build for reference graphs
build-assume-direct-deps	MEDIUM	Speed up watch in very large repos

2. Program Scope & Inputs (HIGH)

Impact: Reduces files parsed; every extra file costs time

Rule	Impact	Description
scope-tight-include	HIGH	Restrict include to source directories
scope-exclude-build-output	HIGH	Exclude dist/build/node_modules
scope-avoid-broad-globs	MEDIUM-HIGH	Avoid **/* in large repos
scope-use-files-array	MEDIUM	Explicit file lists for small packages
scope-separate-tests	MEDIUM	Keep tests in separate tsconfig

3. Module Resolution & Imports (HIGH)

Impact: Reduces resolution overhead; wrong mode = excessive lookups

Rule	Impact	Description
resolve-moduleResolution-modern	HIGH	Use node16/nodenext/bundler
resolve-bundler-mode	HIGH	Use bundler mode for bundled apps
resolve-keep-paths-tight	MEDIUM-HIGH	Avoid catch-all paths patterns
resolve-use-packagejson-exports	MEDIUM	Use package.json exports/imports
resolve-baseurl-minimize	MEDIUM	Use baseUrl only when needed

4. Type Acquisition & Lib Checks (MEDIUM-HIGH)

Impact: 10-50% faster type-checking; skips redundant work

Rule	Impact	Description
types-limit-types	MEDIUM-HIGH	Include only needed @types
types-restrict-typeRoots	MEDIUM	Limit global type search paths
types-skip-lib-check	HIGH	Skip .d.ts validation (major win)
types-typeacquisition-control	MEDIUM	Control automatic type acquisition
types-disable-filename-based-acquisition	LOW	Prevent filename-based type downloads

5. Type System Complexity (CRITICAL)

Impact: Complex types cause O(n^2) checking; simplification = big wins

Rule	Impact	Description
type-annotate-exports	CRITICAL	Named exported types reduce emit cost
type-annotate-returns	CRITICAL	Return annotations reduce inference
type-name-complex-types	HIGH	Named types reduce expansions
type-avoid-deep-intersections	HIGH	Interfaces 4x faster than intersections
type-avoid-large-unions	HIGH	Large unions cause O(n*m) checking
type-export-named-types	MEDIUM-HIGH	Named exports reduce .d.ts size

6. Declaration Emit & Public API (MEDIUM-HIGH)

Impact: Up to 3x faster with isolatedDeclarations; parallel emit

Rule	Impact	Description
dts-emit-declaration-only	MEDIUM-HIGH	Skip JS emit when bundler handles it
dts-declarationDir	MEDIUM	Organize .d.ts output separately
dts-strip-internal	MEDIUM	Remove @internal from .d.ts
dts-isolated-declarations	CRITICAL	Enable parallel .d.ts emit
dts-noCheck-fast-emit	HIGH	Skip type-check for .d.ts emit

7. Editor/tsserver Performance (MEDIUM-HIGH)

Impact: ~3GB to <1GB memory; faster startup, navigation

Rule	Impact	Description
tsserver-disable-referenced-project-load	MEDIUM-HIGH	Load projects on demand
tsserver-disable-solution-searching	MEDIUM-HIGH	Stop upward config search
tsserver-disable-source-redirect	MEDIUM-HIGH	Use .d.ts across boundaries
tsserver-use-project-references	HIGH	Split projects for memory savings
tsserver-solution-config	MEDIUM-HIGH	Root tsconfig with files:[]

8. Diagnostics & Profiling (DIAGNOSTIC)

Purpose: Tools for finding performance issues, not optimizations themselves

Rule	Purpose	When to Use
diag-extended-diagnostics	Detailed timing breakdown	First diagnostic to run
diag-diagnostics	Quick timing overview	CI summaries
diag-explain-files	Why files are included	Unexpected file count
diag-trace-resolution	Module resolution steps	"Cannot find module" errors
diag-generate-trace	Chrome DevTools trace	Deep type-checking analysis
diag-list-files	All files in compilation	Scope audits

Recommended Base Configuration

For most TypeScript projects:

{
  "compilerOptions": {
    // Performance essentials
    "incremental": true,
    "skipLibCheck": true,
    "moduleResolution": "bundler",

    // For libraries
    "declaration": true,
    "declarationMap": true,
    "isolatedDeclarations": true,

    // Standard strictness
    "strict": true,
    "noUncheckedIndexedAccess": true,

    // Output
    "target": "ES2022",
    "module": "ESNext"
  },
  "include": ["src"],
  "exclude": ["node_modules", "dist"]
}

For monorepos, add to each package:

{
  "compilerOptions": {
    "composite": true,
    "disableReferencedProjectLoad": true,
    "disableSolutionSearching": true,
    "disableSourceOfProjectReferenceRedirect": true
  }
}

How to Use

Read individual rule files for detailed explanations and code examples:

rules/build-project-references.md
rules/type-annotate-returns.md
rules/diag-generate-trace.md

Each rule file contains:

• Impact rating with quantified description
• Why it matters with numbered benefits
• Incorrect/Correct code examples
• When NOT to apply (important for avoiding over-optimization)
• Common mistakes to avoid
• Diagnostic commands for verification
• References to official documentation

References

• TypeScript Performance Wiki
• TypeScript tsconfig Reference
• TypeScript Project References
• TypeScript 5.5 isolatedDeclarations