TypeScript 5.5: Inferred Type Predicates and Isolated Declarations

Inferred Type Predicates

TypeScript 5.5 can now automatically infer type guards in filter callbacks. For example, [1, null, 2].filter(x => x !== null) correctly returns number[] without manual type predicate functions.

Isolated Declarations

The new --isolatedDeclarations option requires explicit type annotations on all exports. This enables tools like esbuild and swc to generate declaration files without running full TypeScript type checking, significantly accelerating large project builds.

Regex Syntax Checking

TypeScript 5.5 performs basic syntax checking on regex literals, catching common regex errors at compile time.

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