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TypeScript 5.9: smarter hovers, faster builds
Canonical as of Feb 10, 2026
TypeScript 5.9: smarter hovers, faster builds
TS 5.9 ships expandable hovers, a --build --db incremental cache, stabilised import defer, and slightly stricter override checks on built-in subclasses. Low-risk upgrade.
TypeScript 5.9
As of: 2026-02
What's in 5.9
- Expandable hovers (preview) — hover popouts expand long union/object types instead of truncating.
--build --db— persistent incremental build cache; composite monorepo builds get dramatically faster on warm runs.import deferstabilised — evaluation-deferred imports for startup-sensitive paths.- Empty-loop-body checks return — empty
for/whilebodies warn again. - Behavioural change: subclassing built-ins with
declared fields checks override compatibility more strictly; decorator-heavy codebases may surface new errors.
Should you upgrade?
Yes — low-risk from 5.8. The one commonly-hit issue is the stricter override check; fix with explicit override keywords rather than suppressing.