Bun 1.4.3 adds a built-in Rust TypeScript checker

Bun 1.4.3 adds bun check, a built-in TypeScript type checker written in Rust, which Bun says is faster and uses less memory than tsc. The release also brings new runtime APIs and performance improvements for developers using Bun.

Watch Desk analysis

What happened

The new checker can be run on its own or with bun –check to type-check a file before executing it. Its errors can also use GitHub Actions formatting, making them easier to surface in pull requests. The Bun 1.4.3 release notes also introduce Bun.FetchSession for isolated connection management and Bun.ModuleGraph for multi-instance process execution.

Bun lists additional performance improvements across its runtime, HTTP and JavaScriptCore components. The release description gives no figures for those gains or for the checker’s speed and memory comparisons.

Why it matters

Type-checking is a routine part of working in TypeScript, but it often means running a separate tool from the runtime that executes the code. Bun is bringing that job closer to its existing workflow, including a command that checks a file before it runs and output designed for pull requests.

That could make Bun a more complete option for developers who want fewer separate tools in their build process. The useful question is how the checker fares on real projects, not just whether it has arrived with a tidy command name.

Our read

This is a practical developer release, not just a version number with a fresh coat of paint. Bun’s checker and GitHub Actions support make the change easy to try; the performance claims are promising, though the release provides no comparative figures. TypeScript teams can test bun check on a project and see whether it fits their existing checks before making it part of the build.

What to watch

• How bun check performs on real-world TypeScript projects compared with tsc.
• Whether teams adopt it in local development and continuous integration.
• What the new FetchSession and ModuleGraph APIs enable in practice.

Discussion spark: Would you switch from `tsc` to Bun’s built-in checker if it proved faster on your codebase, or is keeping type-checking separate from the runtime more valuable?

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