TypeScript 5.5 New Features - Inference Improvements and Performance Boost

TypeScript 5.5 Overview

TypeScript 5.5 is an update that significantly improves developer experience and performance. Many practical features have been added, including improved type inference, regular expression syntax checking, and faster editor response.

flowchart TB
    subgraph TS55["TypeScript 5.5 Major New Features"]
        subgraph TypeSystem["Type System Improvements"]
            TS1["Inferred Type Predicates"]
            TS2["Constant index access narrowing"]
            TS3["typeof operator narrowing improvement"]
        end

        subgraph Regex["Regular Expression Support"]
            R1["Regular expression literal syntax checking"]
            R2["Group name completion"]
            R3["Invalid flag detection"]
        end

        subgraph Perf["Performance"]
            P1["Editor response speed 10-20% improvement"]
            P2["Reduced build time"]
            P3["Reduced memory usage"]
        end
    end

Inferred Type Predicates

In TypeScript 5.5, functions can automatically infer that they return Type Predicates.

// TypeScript 5.4 and earlier: Explicit Type Predicate required
function isString(value: unknown): value is string {
  return typeof value === 'string';
}

// TypeScript 5.5: Auto-inference
function isString(value: unknown) {
  return typeof value === 'string';
}

// Usage example
const values: (string | number)[] = ['hello', 42, 'world'];

// In 5.5, type is automatically narrowed
const strings = values.filter(isString);
// strings type: string[]

// Utilization in array methods
const mixedArray = [1, 'two', null, 3, undefined, 'four'];

const numbers = mixedArray.filter(
  (item): item is number => typeof item === 'number'
);

// This is also OK in 5.5 (auto-inference)
const validItems = mixedArray.filter(item => item != null);
// validItems type: (string | number)[]

More Complex Examples

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

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

interface Guest {
  role: 'guest';
  sessionId: string;
}

type Person = User | Admin | Guest;

// Auto-inferred Type Predicate
function isAdmin(person: Person) {
  return 'role' in person && person.role === 'admin';
}

function isUser(person: Person) {
  return 'email' in person;
}

const people: Person[] = [
  { id: '1', name: 'Alice', email: 'alice@example.com' },
  { id: '2', name: 'Bob', email: 'bob@example.com', role: 'admin', permissions: ['read', 'write'] },
  { role: 'guest', sessionId: 'abc123' },
];

// Type is automatically narrowed
const admins = people.filter(isAdmin);
// admins type: Admin[]

const users = people.filter(isUser);
// users type: (User | Admin)[]

Constant Index Access Narrowing

// TypeScript 5.5: Improved type narrowing with constant index

interface Config {
  database?: {
    host: string;
    port: number;
  };
  cache?: {
    enabled: boolean;
    ttl: number;
  };
}

function processConfig(config: Config) {
  // 5.4 and earlier: Not narrowed
  // 5.5: Correctly narrowed
  if (config['database'] !== undefined) {
    console.log(config['database'].host); // OK in 5.5
    console.log(config['database'].port); // OK in 5.5
  }

  // Array example
  const items: (string | undefined)[] = ['a', undefined, 'b'];

  if (items[0] !== undefined) {
    console.log(items[0].toUpperCase()); // OK in 5.5
  }
}

// Tuple improvement
type Tuple = [string, number, boolean?];

function processTuple(tuple: Tuple) {
  if (tuple[2] !== undefined) {
    const flag: boolean = tuple[2]; // OK in 5.5
  }
}

Regular Expression Syntax Checking

// TypeScript 5.5: Regular expression literal syntax checking

// Error: Invalid escape sequence
const invalid1 = /\p/; // Error: Invalid escape sequence

// Error: Unclosed group
const invalid2 = /hello(/; // Error: Unterminated group

// Error: Invalid flag
const invalid3 = /test/xyz; // Error: Unknown flag 'x', 'y', 'z'

// Error: Invalid back reference
const invalid4 = /(\w+) \2/; // Error: Reference to non-existent group

// Valid regular expressions
const valid1 = /\d+/g;
const valid2 = /(?<name>\w+)/;
const valid3 = /hello world/i;

// Named capture group completion
const emailRegex = /(?<user>\w+)@(?<domain>\w+\.\w+)/;
const match = 'test@example.com'.match(emailRegex);

if (match?.groups) {
  console.log(match.groups.user);   // Completion works
  console.log(match.groups.domain); // Completion works
}

// Unicode property escapes
const unicodeRegex = /\p{Script=Hiragana}+/u;
const japaneseText = 'こんにちは'.match(unicodeRegex);

jsxImportSource Auto-Detection

// TypeScript 5.5: JSX import source auto-detection

// tsconfig.json
{
  "compilerOptions": {
    "jsx": "react-jsx",
    // jsxImportSource often no longer needs explicit specification
  }
}

// Auto-detected from package.json dependencies
// - react → "react"
// - preact → "preact"
// - @emotion/react → "@emotion/react"

// Manual override is also possible
/** @jsxImportSource @emotion/react */
import { css } from '@emotion/react';

const style = css`
  color: blue;
`;

Performance Improvements

Optimization Details

// 1. Type caching improvements
// Reduced recalculation of the same types

type DeepReadonly<T> = {
  readonly [P in keyof T]: T[P] extends object ? DeepReadonly<T[P]> : T[P];
};

// In 5.5, recursive type caching is improved,
// processing large object types faster

// 2. Conditional type lazy evaluation
type ConditionalType<T> = T extends string
  ? { type: 'string'; value: string }
  : T extends number
  ? { type: 'number'; value: number }
  : never;

// Evaluation is deferred until needed

// 3. Import resolution optimization
// Type checking of unused imports is skipped

isolatedDeclarations Mode

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

// This mode requires explicit type annotations for exports

// OK: Explicit return type
export function add(a: number, b: number): number {
  return a + b;
}

// Error: Return type required
export function multiply(a: number, b: number) {
  return a * b;
}

// OK: Explicit type
export const config: { port: number; host: string } = {
  port: 3000,
  host: 'localhost',
};

// Error: Type annotation required
export const settings = {
  debug: true,
};

Benefits

isolatedDeclarations Benefits:

New Set Method Type Support

// ECMAScript 2025 Set Methods Type Definitions

const setA = new Set([1, 2, 3]);
const setB = new Set([2, 3, 4]);

// Union
const union = setA.union(setB);
// Set { 1, 2, 3, 4 }

// Intersection
const intersection = setA.intersection(setB);
// Set { 2, 3 }

// Difference
const difference = setA.difference(setB);
// Set { 1 }

// Symmetric difference
const symmetricDifference = setA.symmetricDifference(setB);
// Set { 1, 4 }

// Subset check
const isSubset = setA.isSubsetOf(setB); // false
const isSuperset = setA.isSupersetOf(setB); // false
const isDisjoint = setA.isDisjointFrom(setB); // false

Upgrade Method

# Update with npm
npm install -D typescript@5.5

# Update with yarn
yarn add -D typescript@5.5

# Update with pnpm
pnpm add -D typescript@5.5

Breaking Change Verification

// 1. Impact of Type Predicate inference
// Unexpected type narrowing may occur in existing code

// 2. Regular expression checking
// Previously valid invalid regex will now error

// Workaround: First build with strict: false and check warnings

Summary

Reference Links

• TypeScript 5.5 Release Notes
• TypeScript Handbook
• TypeScript GitHub
• TypeScript Playground