TypeScript 5.9 Iteration Plan

[DanielRosenwasser]

Date	Event
2025-02-25	TypeScript 5.8 Release
2025-05-23	Create 5.9 Beta (5.9.0) Build for Testing
2025-05-26	TypeScript 5.9 Beta Release
2025-06-13	Create 5.9 RC (5.9.1) Build for Testing
2025-06-17	TypeScript 5.9 RC Release
2025-07-18	Create 5.9 Final (5.9.2) Build for Testing
2025-07-22	TypeScript 5.9 Final Release 🚀

Compiler and Language

• tsc --init Updates
• 6.0 Deprecation Plans
• Investigate Cyclic Project References
• node20 Resolution
• lib.d.ts Updates

Editor and Language Service

• Expandable Hovers

Performance

• Port to Native
• Cache Mapper Instantiations

[controversial]

Is the “Checked Returns for Conditional and Indexed Access Types” feature (originally #56941, now #61359) planned to re-land in 5.9?

(this feature was included in 5.8 beta, but reverted before the RC; @gabritto had mentioned the feature could re-land in 5.9)

[DanielRosenwasser]

We're not planning on bringing that in for 5.9. We want to be more conservative about larger features that are more involved while so much of our attention is currently on the port.

[controversial]

understood, thanks for clarifying @DanielRosenwasser

should we expect that that feature (or others of similar scope) won’t land in Typescript until 7.x?

i.e. should we expect it will never land in Strada—in order to avoid the additional development burden of trying to port a “moving target”—but instead the team will hold off until Corsa is ready to be the target of new features like this one?

(to be clear, that approach would not be at all unreasonable to me, just curious what to expect!)

[ltdu]

There is a typo in planning table - all dates are for previous year :)

[DanielRosenwasser]

@ltdu fixed, thank you!

@controversial I can't say that for sure - we generally won't plan on new features, but it depends on priority, finding improvements that need to be backported during the port, etc. We just don't want to split our focus too much.

[nicolo-ribaudo]

Hi! Is there any plan to include #59391, which is already at Stage 3?

(I understand if no given your other comments, but still asking because maybe JS features have higher priority)

[petamoriken]

I hope #61236 will be included too.

[nnnnoel]

Any updates about release date? It seems RC version not released.

[Renegade334]

Presumably we're too late for an es2025 target?

[DanielRosenwasser]

Sorry for the delay all. It has been a busy while and so we're off schedule. Time to get things rolling again though!

@typescript-bot create release-5.9

[typescript-bot]

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
create release-5.9	✅ Started