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TypeScript 6.0 RC Released: Last JS-Based Compiler, Major Breaking Changes Before Native Go Port

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Microsoft released the TypeScript 6.0 Release Candidate on March 6, 2026, marking the final TypeScript version built on the existing JavaScript codebase before TypeScript 7.0 ships as a native Go port with parallel type-checking. The RC introduces significant breaking changes to defaults: `strict` is now `true` by default, `types` defaults to `[]` (empty array) instead of auto-loading all `@types/*` packages, `module` defaults to `esnext`, `--target es5` is deprecated, and `--outFile` has been removed entirely. Developers upgrading from TypeScript 5.9 should audit tsconfig.json defaults, add explicit `"types": ["node"]` where needed, and use the new `--stableTypeOrdering` flag to preview TS 7.0's deterministic type ordering behavior.

Key Takeaways

  • Install via `npm install -D typescript@rc`; breaking defaults: `strict: true`, `types: []`, `module: esnext`, `target: es2025` — projects relying on old defaults must now set these explicitly in tsconfig.json
  • Deprecations that become hard errors in TS 7.0: `--target es5`, `--outFile`, `--moduleResolution node/classic`, `--module amd/umd/systemjs`, `--baseUrl`, `esModuleInterop: false`, `--downlevelIteration`
  • New features: Temporal API built-in types (`esnext`), `Map.getOrInsert`/`getOrInsertComputed` (Stage 4), `RegExp.escape` (ES2025), `#/` subpath imports, `--stableTypeOrdering` flag for TS 7.0 migration prep

Original source: Microsoft TypeScript Blog