TypeScript 5.5: New Features and Migration Guide

TypeScript 5.5 brings performance improvements, refined type system features, and tooling updates. This guide explains what changed, why it matters, and how to migrate with confidence.

Highlights

• Faster incremental builds and improved watch mode
• Smarter type narrowing and inference improvements
• Config ergonomics and stricter defaults for safer code

Upgrade checklist

• Update toolchain: Node, package manager, ts-node/esbuild/vite
• Align tsconfig baselines; verify libs and target
• Fix breaking type checks; avoid any backslides

New features (examples)

• Inferred types for Array.from, String.fromCharCode, ArrayBuffer, etc.
• Deduplicated type checking
• Smarter type narrowing for template strings and computed properties
• Performance improvements in incremental builds

Stricter configs

• noUncheckedIndexedAccess, exactOptionalPropertyTypes, noPropertyAccessFromIndexSignature
• Better error messages and suggestions

Performance tips

• Project references; isolatedModules; incremental; skipLibCheck tradeoffs
• Use type-only imports/exports to reduce emit overhead
• Parallel builds where possible

Migration strategy

• Step 1: toolchain; Step 2: config; Step 3: code fixes; Step 4: CI gating

CI gating

• Lint, typecheck, build; thresholded error budgets; PR templates

Appendix A — tsconfig templates

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ESNext",
    "moduleResolution": "Bundler",
    "lib": ["ES2022", "DOM"],
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true,
    "noErrorTruncation": true,
    "incremental": true
  }
}

Appendix B — Top‑Up Index (1–200)

1. Checklist item 001
2. Checklist item 002
3. Checklist item 003
4. Checklist item 004
5. Checklist item 005