TypeScript 5.9+ Knowledge Patch

Claude's baseline knowledge covers TypeScript through 5.8. This skill provides features from 5.9 (May 2025) onwards.

5.9 (2025-05-22)

import defer Syntax

New ECMAScript proposal for deferred module evaluation. The module's code isn't executed until you access one of its exports, enabling lazy loading without dynamic import().

import defer * as feature from "./some-feature.js";

// Module not executed yet
console.log("Starting up...");

// Module executes NOW when property is accessed
console.log(feature.specialConstant);

Constraints:

• •Only namespace imports allowed (import defer * as name)
• •No named imports (import defer { x }) or default imports
• •Requires --module preserve or --module esnext

--module node20

Stable module option modeling Node.js 20 behavior. Unlike nodenext (which tracks latest Node.js):

• •Remains stable—won't change with future Node.js releases
• •Implies --target es2023 (not esnext)
• •Supports CommonJS requiring ESM modules
• •Rejects deprecated import assertions

Use node20 for predictable behavior; use nodenext to track latest Node.js features.

Updated tsc --init Defaults

Generated tsconfig.json is now minimal with modern prescriptive defaults:

{
  "compilerOptions": {
    "target": "esnext",
    "module": "nodenext",
    "strict": true,
    "moduleDetection": "force",
    "noUncheckedSideEffectImports": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true
  }
}

Commented-out options removed in favor of editor autocompletion.

Breaking Change: ArrayBuffer Type Narrowing

ArrayBuffer no longer supertypes TypedArray buffer variants. Code passing typed array buffers may need updates:

// ❌ May error in 5.9
function process(buf: ArrayBuffer) { /* ... */ }
const arr = new Uint8Array(10);
process(arr); // Error: Uint8Array not assignable to ArrayBuffer

// ✅ Fix: access the underlying buffer
process(arr.buffer);

// ✅ Or: explicitly type the TypedArray
const arr = new Uint8Array<ArrayBuffer>(10);

Update @types/node if encountering Buffer compatibility issues.

Breaking Change: Type Argument Inference

Changes to prevent type variable "leaks" during inference. Some code may require explicit generic type arguments:

// Before: type inferred (possibly incorrectly)
const result = someGenericFunction(value);

// After: may need explicit type argument
const result = someGenericFunction<ExpectedType>(value);

TypeScript 7 (Native Port)

TS7 rewrites the compiler in Go. TS6 is the last JS-based release. Details: See references/typescript-7-transition.md