typescript-5.5 type-inference build-performance isolated-declarations

TypeScript 5.5 infers type guards in filter callbacks

Filter callbacks now auto-narrow types without manual type predicates; `--isolatedDeclarations` decouples declaration generation from full type checking to speed builds.

Summary

Removes boilerplate type guard functions for common patterns and unlocks parallel declaration file generation in esbuild/swc, cutting type-check bottlenecks in large projects. Regex syntax checking catches regex errors at compile time instead of runtime.

Why it matters

Implementation verdict

Filter inference replaces manual type predicate helpers immediately—zero migration cost. `--isolatedDeclarations` requires explicit type annotations on exports and is ready for adoption but demands audit of export signatures. Worth adopting now if building with esbuild/swc toolchains.

Sources

1.[1, null, 2].filter(x => x !== null) correctly returns number[]
2.enables tools like esbuild and swc to generate declaration files without running full TypeScript type checking
3.TypeScript 5.5 performs basic syntax checking on regex literals

Dev Signal

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