TypeScript 5.5 Released: Inferred Type Predicates and Regex Checking

What happened

TypeScript 5.5 was released on June 20, 2024, as announced on Microsoft's official TypeScript blog. It is a practical update focused on making the type system smarter about patterns developers already write, particularly around narrowing and regular expressions.

Inferred type predicates

The headline feature is inferred type predicates. Previously, filtering an array to remove undefined values, for example with a filter callback checking that a value is not undefined, did not narrow the resulting array's type, so later code would still see possibly-undefined elements. TypeScript 5.5 can now infer a type predicate for such functions, so the filtered array gets the more precise type automatically. This removes a long-standing friction point in everyday TypeScript.

Regular-expression syntax checking

TypeScript historically skipped over regular expressions. Version 5.5 adds basic syntax checking for regex literals, catching mistakes such as an unbalanced parenthesis, a backreference to a group that does not exist, or a named group that is not defined. It also warns when a regex feature is newer than the configured ECMAScript target. This catches a class of bugs that previously only surfaced at runtime.

The JSDoc @import tag and isolated declarations

For JavaScript users, 5.5 adds a JSDoc @import tag, giving a clean way to import types for checking without a runtime import. The release also introduces the isolatedDeclarations option, which reports errors when a file's exports are not annotated well enough for a tool to generate declaration files without a full type-checker, enabling faster and more parallel declaration-file generation in large codebases.

Why it mattered

TypeScript 5.5 reduced everyday friction: smarter narrowing means less manual type juggling around filters, and regex checking turns silent runtime bugs into compile-time errors. The isolated-declarations work, meanwhile, targeted build performance for large monorepos. Together they continued TypeScript's pattern of steadily improving both correctness and developer experience.

Related on Skillo

See also: TypeScript 5.4 and its narrowing improvements.

Sources

• Announcing TypeScript 5.5 (official Microsoft blog)

Published date reflects the original event date (2024-06-20). This article is original Skillo editorial written from the sources above; facts were verified in September 2026.