TypeScript 5.9: Leaner, Faster, Smarter

With the release of TypeScript 5.9, the language has slimmed down and gotten smarter. This update introduces several improvements that make everyday development smoother and builds faster.

1. A Cleaner tsc --init

Running tsc --init no longer produces a massive 200-line tsconfig.json full of commented-out options. The new default is much leaner and ships with only the most useful settings, like:

{
  "moduleDetection": "force",
  "target": "esnext"
}

2. Deferred Imports with import defer

The new import defer syntax lets you declare a module upfront but only execute it when one of its exports is actually used. This is especially handy for:

Large modules with heavy initialization costs

Conditional, platform-specific code

Dependencies that aren’t always required

3. Friendlier Editor Tooltips

In VS Code, hover tooltips can now be expanded and collapsed inline with +/- buttons, giving you a quick way to peek deeper into complex types.

Many DOM APIs now include short, MDN-sourced descriptions right in the tooltip—no more context switching to the browser for a quick reminder.

4. Under-the-Hood Performance Gains

TypeScript now caches more intermediate type computations, a big performance win for codebases that rely heavily on libraries like Zod or tRPC.

File existence checks have been reduced, shaving off additional compilation time.

Bottom Line

TypeScript 5.9 is leaner, faster, and more developer-friendly. Cleaner configs, deferred imports, smarter tooltips, and performance boosts make this a solid update for any project.