TypeScript 5.5: Game-Changing Features for Modern Development

TypeScript 5.5 marks another significant milestone in the evolution of JavaScript's type-safe superset. Released with a focus on developer experience and type inference improvements, this version introduces features that make TypeScript smarter, faster, and more intuitive to use.

Inferred Type Predicates

One of the most exciting features in TypeScript 5.5 is inferred type predicates. Previously, you had to explicitly annotate functions with type predicates. Now, TypeScript can infer them automatically.

Before TypeScript 5.5

// Had to explicitly define the type predicate
function isString(value: unknown): value is string {
  return typeof value === 'string';
}

const values = ['hello', 42, true, 'world'];
const strings = values.filter(isString); // string[]

With TypeScript 5.5

// TypeScript now infers the type predicate automatically
function isString(value: unknown) {
  return typeof value === 'string';
}

const values = ['hello', 42, true, 'world'];
const strings = values.filter(isString); // string[] - Still works!

// Even works with arrow functions
const isNumber = (value: unknown) => typeof value === 'number';
const numbers = values.filter(isNumber); // number[]

This feature extends to more complex scenarios:

interface User {
  id: number;
  name: string;
  email?: string;
}

interface AdminUser extends User {
  role: 'admin';
  permissions: string[];
}

// TypeScript infers this as a type predicate
const isAdmin = (user: User) => {
  return 'role' in user && user.role === 'admin';
};

const users: User[] = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob', role: 'admin', permissions: ['read', 'write'] } as AdminUser
];

const admins = users.filter(isAdmin); // AdminUser[]

Control Flow Narrowing for Const

TypeScript 5.5 introduces improved control flow analysis for constant conditions, making the type system even smarter about understanding your code's logic.

const DEBUG = true;

function processData(data: string | number) {
  if (DEBUG) {
    console.log('Processing:', data);
  }
  
  // TypeScript now understands that this code is reachable
  // even with const conditions
  if (typeof data === 'string') {
    return data.toUpperCase();
  } else {
    return data * 2;
  }
}

// Feature flags pattern
const FEATURE_FLAGS = {
  NEW_UI: true,
  LEGACY_API: false,
} as const;

function renderComponent() {
  if (FEATURE_FLAGS.NEW_UI) {
    // TypeScript knows this branch is always taken
    return <ModernComponent />;
  } else {
    // TypeScript knows this is dead code
    return <LegacyComponent />;
  }
}

Regular Expression Syntax Checking

TypeScript 5.5 now validates regular expression literals at compile time, catching errors before they hit production:

// TypeScript now catches regex syntax errors
const invalidRegex = /[a-Z]/; // Error: Invalid regular expression

// Valid patterns are fine
const validEmail = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
const validPhone = /^\d{3}-\d{3}-\d{4}$/;

// Works with RegExp constructor when pattern is literal
const pattern = new RegExp('[a-Z]'); // Error: Invalid regular expression

// Template literals with regex
const createPattern = (flag: string) => {
  return new RegExp(`^${flag}:[a-z]+$`); // Validated when possible
};

Improved JSDoc Support

TypeScript 5.5 significantly enhances JSDoc support, making it easier to add types to JavaScript files:

@import Tag

// math.js
/**
 * @import { Vector3 } from './types'
 * @import * as MathUtils from './utils'
 */

/**
 * Calculates the magnitude of a 3D vector
 * @param {Vector3} vector - The input vector
 * @returns {number} The magnitude
 */
function magnitude(vector) {
  return Math.sqrt(
    vector.x ** 2 + 
    vector.y ** 2 + 
    vector.z ** 2
  );
}

Better Template Literal Types in JSDoc

/**
 * @typedef {'small' | 'medium' | 'large'} Size
 * @typedef {`${Size}-${number}`} SizeWithNumber
 */

/**
 * @param {SizeWithNumber} size
 */
function processSize(size) {
  const [sizeType, number] = size.split('-');
  console.log(`Size type: ${sizeType}, Number: ${number}`);
}

processSize('medium-42'); // Valid
processSize('huge-10');   // Error: not a valid size

Isolated Declarations Mode

TypeScript 5.5 introduces --isolatedDeclarations mode, which ensures that each file can have its declarations generated independently:

// tsconfig.json
{
  "compilerOptions": {
    "isolatedDeclarations": true
  }
}

// user.ts
// Error: Need explicit type annotation for exported declaration
export const createUser = (name: string) => ({ 
  id: Math.random(), 
  name 
});

// Fixed version
interface User {
  id: number;
  name: string;
}

export const createUser = (name: string): User => ({ 
  id: Math.random(), 
  name 
});

This mode helps with:

  • Faster builds in large projects
  • Better parallelization
  • Clearer API contracts

Performance Improvements

TypeScript 5.5 brings substantial performance enhancements:

Faster Object Literal Type Checking

// Complex object literals are now checked up to 15% faster
const config = {
  server: {
    port: 3000,
    host: 'localhost',
    ssl: {
      enabled: true,
      cert: '/path/to/cert',
      key: '/path/to/key'
    }
  },
  database: {
    type: 'postgres',
    connection: {
      host: 'db.example.com',
      port: 5432,
      database: 'myapp'
    }
  },
  // ... many more nested properties
};

Optimized Union Type Handling

// Union types with many members are handled more efficiently
type HTTPMethod = 
  | 'GET' 
  | 'POST' 
  | 'PUT' 
  | 'DELETE' 
  | 'PATCH' 
  | 'HEAD' 
  | 'OPTIONS' 
  | 'CONNECT' 
  | 'TRACE';

type APIEndpoint = 
  | '/users'
  | '/posts'
  | '/comments'
  // ... 50+ more endpoints

type APIRoute = `${HTTPMethod} ${APIEndpoint}`;

Const Type Parameters

TypeScript 5.5 improves const type parameter inference:

// Generic function with const type parameter
function createTuple<const T extends readonly unknown[]>(...args: T): T {
  return args;
}

// Infers exact tuple type
const tuple = createTuple('a', 1, true); // readonly ['a', 1, true]

// Works with object literals too
function createConfig<const T extends Record<string, unknown>>(config: T): T {
  return config;
}

const config = createConfig({
  name: 'MyApp',
  version: '1.0.0',
  features: ['auth', 'api'] as const
}); // Exact type is preserved

Editor Experience Enhancements

Auto-Import Improvements

// TypeScript 5.5 better handles barrel exports
// utils/index.ts
export * from './string-utils';
export * from './array-utils';
export * from './date-utils';

// main.ts
// Typing 'formatDate' now suggests the correct import
formatDate(new Date()); // Auto-imports from 'utils'

Better Snippet Completions

// Enhanced completions for common patterns
class UserService {
  constructor(
    // Better parameter completions
    private readonly db: Database,
    private readonly logger: Logger
  ) {}
  
  // Method snippet completions are more intelligent
  async getUser(id: string) {
    try {
      const user = await this.db.users.findById(id);
      this.logger.info(`Retrieved user ${id}`);
      return user;
    } catch (error) {
      this.logger.error(`Failed to get user ${id}:`, error);
      throw error;
    }
  }
}

Configuration Improvements

New Compiler Options

{
  "compilerOptions": {
    // New in 5.5
    "isolatedDeclarations": true,
    "allowImportingTsExtensions": true,
    "noUncheckedSideEffectImports": true,
    
    // Improved behavior
    "moduleResolution": "bundler",
    "resolveJsonModule": true
  }
}

Real-World Applications

Type-Safe API Client

// Leveraging new features for a type-safe API client
type APIResponse<T> = {
  data: T;
  error: null;
} | {
  data: null;
  error: string;
};

class APIClient {
  private baseURL: string;
  
  constructor(baseURL: string) {
    this.baseURL = baseURL;
  }
  
  // Using inferred type predicates
  private isErrorResponse<T>(response: APIResponse<T>) {
    return response.error !== null;
  }
  
  async get<T>(endpoint: string): Promise<T> {
    const response = await fetch(`${this.baseURL}${endpoint}`);
    const data: APIResponse<T> = await response.json();
    
    // TypeScript narrows the type automatically
    if (this.isErrorResponse(data)) {
      throw new Error(data.error);
    }
    
    return data.data;
  }
}

Configuration Builder Pattern

// Using const type parameters for builder pattern
class ConfigBuilder<const T extends Record<string, unknown> = {}> {
  private config: T;
  
  constructor(initial: T = {} as T) {
    this.config = initial;
  }
  
  set<K extends string, V>(
    key: K, 
    value: V
  ): ConfigBuilder<T & Record<K, V>> {
    return new ConfigBuilder({
      ...this.config,
      [key]: value
    });
  }
  
  build(): Readonly<T> {
    return Object.freeze(this.config);
  }
}

// Type-safe configuration building
const config = new ConfigBuilder()
  .set('port', 3000)
  .set('host', 'localhost')
  .set('ssl', { enabled: true })
  .build();

// config type is inferred as:
// Readonly<{
//   port: number;
//   host: string;
//   ssl: { enabled: boolean };
// }>

Migration Guide

Updating to TypeScript 5.5

  1. Update TypeScript:
npm install -D [email protected]
  1. Review Breaking Changes:
  • Stricter type predicate inference might reveal existing issues
  • Regular expression validation may flag invalid patterns
  1. Enable New Features:
{
  "compilerOptions": {
    "target": "ES2022",
    "module": "NodeNext",
    "isolatedDeclarations": true
  }
}
  1. Leverage New Features:
  • Remove explicit type predicates where inference works
  • Use const type parameters for precise types
  • Take advantage of improved JSDoc support

Best Practices

  1. Let TypeScript Infer:
// Don't over-annotate
const isPositive = (n: number) => n > 0; // Let TS infer the return type
  1. Use Const Assertions:
const ROUTES = {
  HOME: '/',
  ABOUT: '/about',
  CONTACT: '/contact'
} as const;
  1. Embrace Isolated Declarations:
// Always export types for public APIs
export interface UserData {
  id: string;
  name: string;
}

export function processUser(data: UserData): UserData {
  return { ...data, name: data.name.trim() };
}

Conclusion

TypeScript 5.5 continues the tradition of making TypeScript more powerful while reducing the friction of using it. With inferred type predicates, improved const handling, and better JSDoc support, TypeScript is becoming smarter at understanding your intent without requiring verbose annotations.

These improvements, combined with significant performance enhancements, make TypeScript 5.5 a compelling upgrade for any TypeScript project. Whether you're building a small library or a large-scale application, these features will help you write safer, more maintainable code with less effort.