Overview
- 1TypeScript 5.5 (June 2024) delivers inferred type predicates — the most requested feature in years.
- 2Regex syntax checking catches common mistakes in regular expressions at compile time.
- 3Isolated Declarations mode enables parallel TypeScript compilation for monorepo performance.
- 4TypeScript underpins every major framework in 2026 — React, Angular, Vue, and Next.js all ship types first.
- 5The 5.x series focuses on inference improvements and compiler performance over new syntax.
Key Features in 5.5
Inferred Type Predicates — TypeScript infers `x is T` return types from function bodies
Regular Expression Syntax Checking — invalid regex flagged at compile time
Isolated Declarations — isolatedDeclarations flag for parallel type-checking in monorepos
const type parameters — `<const T>` infers literal types instead of widening
using and await using — explicit resource management via Symbol.dispose
Variadic Tuple Types, Mapped Types, Template Literal Types
satisfies operator — validates expression type without widening
Use Cases
- →Type-safe codebases at any scale — from one developer to 100+
- →API contracts between frontend and backend (shared types)
- →Refactoring safety — compiler catches breaking changes
- →IDE intelligence: autocomplete, go-to-definition, rename across files
What's New in TypeScript 5.5
- Inferred Type Predicates: `function isString(x: unknown) { return typeof x === "string"; }` — return type inferred as `x is string`
- Regex syntax checking: `/[a-z]/g` is valid; `/?/` throws a compile error
- isolatedDeclarations: true — each file must have explicit return types; enables parallel builds
- Performance: type-checking speed improved 10-15% on large projects via internal optimisations
- editor.jsxImportSource now reads from tsconfig.json directly
- Control flow narrowing improvements for discriminated unions with optional properties
Type System Essentials
- Primitive types: string, number, boolean, null, undefined, bigint, symbol
- Union types (string | number), intersection types (A & B), literal types ("left" | "right")
- Generics: <T> for reusable typed functions, classes, and interfaces
- Utility types: Partial<T>, Required<T>, Readonly<T>, Pick<T, K>, Omit<T, K>, Record<K, V>
- Template literal types: `${Verb}${Noun}` for string manipulation at the type level
- Conditional types: `T extends U ? X : Y` for type-level logic
- infer keyword: extract types from conditional type clauses
Advanced Patterns
- Discriminated unions — a shared literal property narrows type in switch/if blocks
- satisfies operator — `const palette = { red: [255,0,0] } satisfies Record<string, Color>`
- const assertions — `as const` freezes an object and infers narrowest types
- Declaration merging — interfaces can be augmented across modules (module augmentation)
- Mapped types with `as` remapping: `type Getters<T> = { [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K] }`
- Variance annotations — `in` and `out` modifiers for explicit generic variance
Tooling & Configuration
- tsconfig.json: strict: true enables all strict checks (strictNullChecks, noImplicitAny, etc.)
- Project references: composite projects with incremental builds for monorepos
- tsc --noEmit for type-check-only CI runs without emitting output
- skipLibCheck: true — skip type-checking node_modules for faster builds
- paths mapping for import aliases (@ → src/)
- ESLint + typescript-eslint replaces TSLint for linting TypeScript code
Frequently Asked Questions
What are inferred type predicates in TypeScript 5.5?
TypeScript 5.5 can automatically infer type predicate return types (`x is string`) from function bodies that perform type narrowing. Previously you had to write the predicate manually: `function isString(x: unknown): x is string`. Now TypeScript infers it from the implementation.
What is the satisfies operator in TypeScript?
The `satisfies` operator (TypeScript 4.9+) validates that an expression matches a type without widening it. `const config = { port: 3000 } satisfies ServerConfig` checks that config matches ServerConfig but preserves the literal type 3000 rather than widening to number.
Should I use interface or type alias?
Both are largely interchangeable. Use `interface` for object shapes you may extend or that participate in declaration merging. Use `type` for unions, intersections, tuples, mapped types, and conditional types. In practice, consistency matters more than which one you pick.