TypeScript 6.0

Apply these conventions when working with TypeScript 6.0+ (released 2026-03-23). 6.0 is a bridge release — the final version on the JavaScript-based compiler before the stable Go-native TypeScript 7 port. Most 6.0 changes are about modern defaults and deprecations; the goal is to make the 7.0 jump uneventful.

Do not start new projects on TypeScript 6. Use TypeScript 7 unless a compiler-API consumer, custom transformer, framework, or embedded-language tool still requires TypeScript 6. For existing projects, use this skill to clear 6.0 deprecations, then follow the TypeScript 7 skill's porting section.

New Compiler Defaults (these change build behavior)

• strict: true — was false. Implicit any, null checks, function types, bind/call/apply, property init, etc. all enforced by default.
• module: "esnext" — was "commonjs". Output is ESM unless overridden.
• target — floats to the latest stable; in 6.0 it resolves to "es2025".
• types: [] — was implicitly "all @types/* in node_modules". Now you must opt-in: "types": ["node", "jest"]. Big perf and correctness win.
• noUncheckedSideEffectImports: true — bare import "./foo" now type-checks the imported module's existence.
• rootDir: "." — was inferred from the input file set. Output layout in monorepos is now stable across edits.

If a project breaks on upgrade, the cause is almost always one of these defaults — pin the old value explicitly in tsconfig.json and migrate one flag at a time.

New Language / Inference Features