TypeScript 6.0, a planned update to Microsoft’s strongly typed JavaScript variant, has reached the release candidate (RC) stage, with the RC adding type checking for function expressions in generic calls.

The last TypeScript release based on the JavaScript codebase, before TypeScript 7.0 introduces a compiler and language service based on the Go language for better performance, TypeScript 6.0 reached the RC stage on March 6. General availability of the production release has been set for March 17, although the RC was supposed to be released February 24, meaning it was 10 days late. The TypeScript 6.0 RC, which follows the February 11 beta release, can be installed via NPM by running the command npm install -D typescript@rc.

New in the RC is an adjustment in type checking for function expressions in generic calls, especially those occurring in generic JSX expressions, according to Microsoft. Aimed at aligning TypeScript 6.0 with the planned behavior of Go-based TypeScript 7.0, this adjustment will typically catch more bugs in existing code, though developers may find that some generic calls may need an explicit type argument.

Also, Microsoft has extended its deprecation of import assertion syntax (i.e. import ... assert {...}) to import() calls like import(..., { assert: {...}}). And DOM types have been updated to reflect the latest web standards, including some adjustments to Temporal APIs.

Other changes in TypeScript 6.0 include the RegExp.escape function for escaping regular expression characters such as *, ?, and +. Based on an ECMAScript proposal that has reached stage 4, RegExp.escape is now available in the es2025 library. Also, the contents of lib.dom.iterable.d.ts and lib.dom.asynciterable.d.ts are now included in lib.dom.d.ts. TypeScript’s lib option lets developers specify which global declarations a target runtime has.

Now feature-complete, TypeScript 6.0 also deprecates the asserts syntax. The asserts keyword was proposed to the JavaScript language via the import assertions proposal; however, the proposal eventually morphed into the import attributes proposal, which uses the with keyword instead of asserts.

Microsoft expects TypeScript 7.0 to follow soon after TypeScript 6.0, with the goal of maintaining continuity while enabling a faster feedback loop for migration issues discovered during adoption.