TypeScript 6.0: The Breaking Changes Nobody Is Talking About

TypeScript 6 shipped with stricter inference, a new type-level module system, and several long-standing behaviours finally corrected. Most of it is additive — but three changes will break existing codebases silently. Here's what to audit before you upgrade.

The TypeScript team landed over 60 fixes and a handful of new features in 6.0, with the stated goal of making the type system more predictable — even where that means stricter errors on previously accepted code.

• Released: March 2025
• Node.js minimum: 18.x
• tsconfig target default raised to ES2022

These don't throw errors at compile time in all cases — which makes them the most dangerous. They change runtime behaviour or silent type widening that your tests may not catch.

• Narrowing of in operator now respects optional properties strictly
• Template literal types no longer widen to string in certain inference contexts
• Decorator metadata is now opt-in — existing decorator libraries may break

Run tsc --noEmit after upgrading before touching anything else. Treat every new error as a bug that was already in your code.

Beyond the fixes, TypeScript 6 ships several quality-of-life additions that reduce boilerplate and close gaps that developers have worked around for years.

• Named tuple element inference: labels preserved through spreads
• satisfies operator improvement: now works in type positions
• Isolated declarations mode: faster incremental builds in monorepos

• Bump typescript to ^6.0.0 and run noEmit check
• Audit all packages that use decorator metadata (NestJS, TypeORM, inversify)
• Update tsconfig target and lib if you support older runtimes