TypeScript 5.5 Highlights: New Set API, Faster Builds, and Regex Checks

Microsoft has officially released TypeScript 5.5, the latest version of its strongly‑typed JavaScript superset.

The update brings a range of improvements, including inferred type predicates, more permissive yet standards‑compliant regular‑expression checks, and performance and size optimizations.

New Set methods such as union, intersection, difference accept another Set and return a new Set; the isSubsetOf method accepts a Set and returns a boolean.

Regular‑expression checking is now slightly relaxed but still flags suspicious escapes that do not conform to ECMAScript Annex B.

Performance gains include skipping work in transpileModule and optimizing filtered contextual types, which shortens build and iteration times. Monomorphization of the language service and public API reduces the overall package size from 30.2 MB to 20.4 MB and the bundle size from 5.5 MB to 3.7 MB. The isolatedDeclarations improvement also increases the frequency of directly copying input source code when generating declaration files.

Monomorphization means editors and build tools using the TypeScript API will experience faster performance, similar to the work done for Node and Symbol objects in TypeScript 5.0. npm -D typescript TypeScript 5.5 also adds basic syntax checking for regular expressions and now infers type predicates for functions under certain conditions, narrowing constant indexed accesses and for‑loop expressions when both sides are valid constants. obj[key] ECMAScript module API usage is now easier: named imports from the typescript package work in Node.js, and the new @import annotation uses the same syntax as ECMAScript imports.

A new transpileDeclaration API generates a single declaration file from source text, mirroring the functionality of transpileModule for compiling individual TypeScript files.