Biome partners with Vercel to improve type inference

Apr 2, 2025

Back at the start of 2024, Biome added an ambitious goal to its roadmap: integrate a subset of the TypeScript type system directly into Biome so that type-informed lint rules can work out of the box.

In order to make this feasible, we first needed better infrastructure. The main blocker for this was multi-file analysis, which is coming with Biome 2.0.

Today, we finally have the technical means to implement this goal, but, as a project, we need more than only technical means. Which is why we are grateful to announce Vercel as a partner to help us achieve this goal and push the web forward.

Vercel has contracted me (one of the Biome lead developers) to work on our type inference effort for many months. Vercel’s goal with this partnership is to both help improve their own internal linter DX (as they have recently standardized on Biome) and share those improvements with the rest of the JavaScript ecosystem.

We aim to have a fully functioning(*) versions of the noFloatingPromises rule and a similar noMisusedPromises rule.

Frequently Asked Questions

Because type inference and type checkers are popular topics in our community, I have collected a few common questions on this topic.

Are you reimplementing a type checker in Biome?

No. TypeScript’s tsc is a complex and fully-featured type checker, and we have no intention to rebuild it. For type checking you are expected to continue to use tsc.

This work focuses on type inference which is a small subset of the functionality of a full type checker. The goal is to be able to write lint rules that act on type information, without needing to prove that this type information is correct.

How can you write a type-informed lint rule if you don’t know the type information is correct?

Linters have different aims than type checkers. Where a type checker aims to detect misuse of types, a linter aims to detect common mistakes in general. But the goal of detecting common mistakes doesn’t require 100% correctness when it comes to the types it operates on. In fact, even the TypeScript Design Goals state they are willing to compromise correctness in favor of productivity.

For a linter, what’s most important is that we don’t flag false positives, instances where our lint rules may think there’s an issue when really there’s not. False positives are a source of frustration for developers, because they take time to analyze; time that was wasted when it turns out there really wasn’t an issue to begin with.

On the other hand, false negatives are less problematic. They represent situations where we wish the lint rule would flag an issue, but didn’t. They can also represent lost productivity (by not flagging the issue early in the pipeline), but they are not productivity lost due to the rule itself.

Hypothetically, a lint rule that has no false positives, but has 20% false negatives, still offers 80% of the value of a rule that is always correct. So our goal is simply: Create lint rules that don’t flag false positives, while trying to get the amount of false negatives down as much as we can.

We cannot say upfront which percentage we will actually achieve, but since Vercel is sponsoring this work, we’ll use their repositories as a benchmark and try to optimise towards their use cases.

How does Microsoft’s announcement of a Go port for `tsc` influence your work?

It’s too early to tell. The Go port is not available to users yet, and the APIs that should allow it to be used by other tooling aren’t expected until the end of the year. Additionally, even though the Go port is supposedly much faster than the Node.js version, it will likely still be significantly slower than an implementation built into Biome’s core due to inter-process communication and the need to do duplicate work such as parsing files in separate processes.

For now, that gives us an opportunity to pursue our own type inference implementation and see how it goes. When the Go version is available for integration into other tools, we can always reevaluate our approach.

Can we follow your progress?

Yes! There’s a public project overview that people can inspect: https://github.com/orgs/biomejs/projects/4/views/2

Note that issues may be continuously added to the project as false negatives keep popping up.