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📢 TypeScript 5.7 is here! Now with ✅ Better checks for uninitialized variables ✅ Smarter tsconfig lookup in editors ✅ Improved startup time ✅ Stricter checks for JSON imports ✅ Extension rewrites for relative imports and more! Try it today! devblogs.microsoft.com/typescript/a...

Honest question, must --target and --lib be in sync? Just from reading the docs I would have thought I could set lib to 2024 and target to 2022 and get to use new features while giving browsers the chance to catch up. If the two settings must be in sync, why isn’t it just a single setting?

Sounds reasonable! But then I find this paragraph from the blog post somewhat confusing “TypeScript 5.7 now supports --target es2024, which allows users to target ECMAScript 2024 runtimes. This target primarily enables specifying the new --lib es2024 which contains…” Emphasis on “enables”.

(Not to mention the people setting target=ES5 to downlevel let/const to var for performance, but I would argue that TypeScript is the wrong tool to do that job)

Is there a meaningful boost to using var?

The TDZ (temporal dead zone) created by let/const can have a high runtime cost, so sometimes, yes; used to be some 30% difference in TS, so we manually changed the important ones over to get most of that speedup I think V8 optimized some of it after this discovery but not all