Bun, the JavaScript runtime developed by Anthropic, released a new version, Bun v1.4.3, on October 10, 2026. Its headline feature is a TypeScript type checker available through the built-in “bun check” command. Bun says it is more than three times faster than TypeScript 7, which is implemented in Go, while using less memory.
Bun Takes On Both Execution and Checking
First, some context. Type checking is the process of finding mistakes in how code is used before a program runs—for example, passing a string where a number is expected. It is one of TypeScript’s major selling points and an essential step in larger-scale development.
In addition to JavaScript, Bun can load and transform TypeScript files and run them directly. Separately, it can check TypeScript types by passing files to the “bun check” command. In this update, the speed of the latter feature is front and center.
When tools for running and checking code are bundled in a single environment, developers can reduce both the number of tools they need to install and the effort required to configure them. This development reflects a broader shift in which runtimes are expanding their role from places to run code to environments that also provide safety checks during development.
Compatibility Numbers Matter, Not Just Speed
A type checker is of little use if its results differ from those of the original, no matter how fast it is. On this point, the release notes say Bun passed 100% of the TypeScript 7.0.2 conformance tests.
Bun also checked 72 major open-source TypeScript files—including files with errors intentionally introduced by the Bun team—with both TypeScript and Bun, then compared the results. It says there was just one discrepancy. The results suggest an effort to quantify how closely Bun’s checks align with those of the original.
For developers considering a switch, this is likely to matter as much as speed. If the warnings change even as checks get faster, that could affect how a team works.
How to Interpret the “More Than 3x” Claim
Bun says its type checker is more than three times faster than TypeScript 7 and uses between one-half and one-quarter or less of the memory. TypeScript 7 has already been sped up through its implementation in Go, so Bun is claiming a substantial lead over an already fast alternative.
However, these are comparison results reported by Bun. Type-checking times vary depending on the size and style of the code and the machine running it. The time savings for a particular project will only become clear after testing it in practice.
Even so, shorter check times could make it easier to run checks every time a file is saved or frequently check code while editing. Lower memory use should also make the tool easier to run in resource-constrained environments such as CI. The speed gains could affect how development itself is carried out.
Other Updates Beyond Type Checking
Version 1.4.3 includes many other changes. The official announcement says the release fixes 166 issues and cuts server CPU usage while idle by 67%.
Other changes include improved Node.js compatibility, faster JSON.parse, and numerous bug fixes. The type checker is the headline feature, but the update also improves the runtime as a whole. See the release notes on Bun’s official blog for details.
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