prefer-next-typescript-performance-milestone

What it flags

Flags a repository that depends on TypeScript 5.x but is still below the next notable 5.x performance milestone.

Current milestone sequence:

This rule is repo-aware. It reads the repository TypeScript version from package.json.

TypeScript 5.x has a few especially relevant performance milestones.

5.2: improved some heavy type relation checks, with the TypeScript team showing over 33% speed-up on a drizzle case
5.5: improved language service and public API performance, with the TypeScript team citing 5-8% faster API-based builds and 10-20% faster language service operations
5.9:
- Caching intermediate type instantiations (PR #61505): reduces redundant work and allocations during type parameter substitution. Beneficial for type-heavy libraries like Zod and tRPC.
- Optimizing file existence checks (PR #61822): removes unnecessary closure allocations. ~11% speed-up in larger projects.

5.7 is also relevant as a secondary milestone on Node 22 because TypeScript can benefit from compile caching at process startup.

The rule requires:

a detectable typescript dependency in package.json
the repository also appears JavaScript-heavy (used as a gate so the diagnostic only surfaces when relevant CI work is present)

Ignore this finding when:

the repo is pinned to an older TypeScript for compatibility reasons
upgrade validation cost is currently higher than the expected runtime gain
the visible workflow job is not meaningfully affected by TypeScript compiler or API performance

Compare type-check time before and after the upgrade
If relevant, compare API-based tool timings such as loaders, test transforms, or editor-facing build integrations