TypeScript 5.5: Advanced Features and Performance Enhancements

TypeScript 5.5 brings significant improvements to type checking, module resolution, and developer experience. This release focuses on enhanced performance and more precise type inference.

Advanced Module Resolution

TypeScript 5.5 introduces smarter module resolution strategies that better handle complex project structures and monorepo setups.

Conditional Exports Support

// package.json

{
  "exports": {
    ".": {
      "import": "./dist/esm/index.js",
      "require": "./dist/cjs/index.js",
      "types": "./dist/types/index.d.ts"
    },
    "./utils": {
      "import": "./dist/esm/utils.js",
      "require": "./dist/cjs/utils.js",
      "types": "./dist/types/utils.d.ts"
    }
  }
}

Improved Path Mapping

// tsconfig.json

{
  "compilerOptions": {
    "baseUrl": ".",
    "paths": {
      "@/": ["src/"],
      "@/components/": ["src/components/"],
      "@/utils/": ["packages/utils/src/"]
    }
  }
}

Enhanced Type Inference

Contextual Type Inference Improvements

// Before: Required explicit typing

const users: User[] = [
  { id: 1, name: 'John' },
  { id: 2, name: 'Jane' }
]
// TypeScript 5.5: Better inference from context
const users = [
  { id: 1, name: 'John' },
  { id: 2, name: 'Jane' }
] as const satisfies User[]

Generic Constraints Enhancement

type ExtractKeys = keyof {

  [K in keyof T as T[K] extends U ? K : never]: T[K]
}
interface Person {
  name: string
  age: number
  email: string
}
type StringKeys = ExtractKeys // 'name' | 'email'

Performance Optimizations

Faster Compilation Times

Incremental Type Checking: Only re-checks changed files and their dependencies

Parallel Type Checking: Utilizes multiple CPU cores for large projects

Memory Optimization: Reduced memory usage for large codebases

Build Performance Metrics

| Project Size | TypeScript 5.4 | TypeScript 5.5 | Improvement |

|-------------|----------------|----------------|-------------|

| Small (< 100 files) | 2.3s | 1.8s | 22% faster |

| Medium (100-1000 files) | 12.5s | 9.2s | 26% faster |

| Large (> 1000 files) | 45.8s | 32.1s | 30% faster |

Advanced Utility Types

New Utility Types

// Exact - Ensures exact type matching

type Exact = T extends infer U ? U : never
// Writable - Makes all properties writable
type Writable = {
  -readonly [P in keyof T]: T[P]
}
// NonNullableDeep - Recursively removes null and undefined
type NonNullableDeep = T extends null | undefined
  ? never
  : T extends object
  ? { [K in keyof T]: NonNullableDeep }
  : T

Template Literal Types Enhancement

type Routes = '/users' | '/users/:id' | '/posts' | '/posts/:id'
// Extract parameter names
type RouteParams = T extends ${string}:${infer P}${string}
  ? P | RouteParams<...>
  : never
type UserRouteParams = RouteParams<'/users/:id'> // 'id'

Improved Error Messages

Contextual Error Reporting

// More helpful error messages

const user = {
  name: 'John',
  age: 30
}
// Error: Property 'email' does not exist on type '{ name: string; age: number; }'
// Suggestion: Did you mean to add 'email' property?
user.email = 'john@example.com'

Quick Fixes and Suggestions

Auto-import suggestions for missing modules

Type assertion suggestions for type mismatches

Interface merging hints for conflicting declarations

Module Augmentation Improvements

Declaration Merging Enhancements

// module.d.ts

declare module 'express' {
  interface Request {
    user?: User
  }
}
// Usage
app.use((req, res, next) => {
  req.user = authenticatedUser // TypeScript knows about user property
})

Configuration and Tooling

Enhanced tsconfig.json Options

{

  "compilerOptions": {
    "exactOptionalPropertyTypes": true,
    "noImplicitOverride": true,
    "noPropertyAccessFromIndexSignature": true,
    "noUncheckedIndexedAccess": true,
    "allowImportingTsExtensions": true
  }
}

Better IDE Integration

Faster IntelliSense with improved language service

Enhanced refactoring tools for large codebases

Better debugging support with source maps

Migration Guide

Upgrading from TypeScript 5.4

Update TypeScript

npm install typescript@5.5 --save-dev
Run migration tool

npx tsc --noEmit --strict

Breaking Changes

Stricter type checking for optional properties

Changes to module resolution algorithm

Updated error message formats

Best Practices for TypeScript 5.5

1. Enable Strict Mode: Use all strict compiler options for better type safety

2. Leverage Utility Types: Use built-in and custom utility types for complex scenarios

3. Optimize Build Performance: Configure incremental builds and parallel checking

4. Use Declaration Merging: Extend third-party libraries safely

5. Implement Proper Error Handling: Use discriminated unions for better error types

Future Roadmap

TypeScript 5.5 lays the groundwork for future releases focusing on:

Even faster compilation through advanced caching strategies

Better JavaScript interop with improved type inference

Enhanced metaprogramming capabilities

Improved tooling ecosystem integration

Performance Benchmarks

Real-World Project Analysis

Redux Toolkit: 15% faster type checking

Next.js Applications: 20% reduction in build times

Monorepo Projects: 25% improvement in incremental builds

Conclusion

TypeScript 5.5 represents a significant step forward in type-safe JavaScript development. With improved performance, better type inference, and enhanced developer experience, it empowers developers to build more reliable and maintainable applications.

The focus on performance optimizations and advanced type system features makes TypeScript 5.5 an essential upgrade for teams working on large-scale applications.