Ensuring Typescript Code Quality With expect-type

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TypeScript's static typing catches many errors, but how do we ensure our type definitions themselves are correct? This talk introduces type testing and demonstrates its implementation using expect-type, a lightweight library for exactly this.

We'll cover:

- The concept of type testing and its importance in TypeScript projects
- A deep dive into expect-type's features and API
- Practical examples of using expect-type to catch subtle type errors
- Integrating expect-type into existing projects and CI pipelines
- Advanced use cases and best practices

Drawing from real-world scenarios, we'll explore how expect-type can be used as a standalone library, or through Vitest which bundles it natively, to enhance type safety and code quality.

Suitable for TypeScript developers of all levels, this talk will equip you with the tools and knowledge to write more robust, type-safe code using expect-type.

This talk has been presented at JSNation US 2024, check out the latest edition of this JavaScript Conference.

FAQ

More information about Expect Type is available on its NPM page, GitHub repository, and VTest documentation. The author is also available on X under mmcalc.

Expect Type error messages are integrated with the TypeScript CLI, providing detailed messages that help identify mismatches between expected and actual types.

Expect Type can help improve your TypeScript skills by allowing you to evaluate how TypeScript interacts with your code base and by documenting the type behavior within your team.

No, Expect Type does not require any additional build steps or IDE extensions, and it has no dependencies, ensuring no bloat is added to your node modules.

Yes, Expect Type can be used in production code without any performance concerns since it introduces no runtime overhead.

Expect Type supports testing for various TypeScript features, including complex types, type guard functions, and generic type testing.

Expect Type can be used with any test framework that supports TypeScript, including Jest, Mocha, and Playwright, with built-in support available for VTest.

Expect Type is a library for type-level testing in TypeScript, allowing developers to write tests for their types similar to how they write tests for runtime behavior.

When using Expect Type, focus on testing edge cases and be selective about what you test. It's more useful for libraries than applications and should be run as part of your CI pipelines.

Misha Kaletsky
Misha Kaletsky
9 min
21 Nov, 2024

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Video Summary and Transcription
Expect Type is a library for type-level testing in TypeScript. It allows you to write tests for your types, ensuring that they stay what you want them to be as your code base evolves. The library supports a wide range of TypeScript features, including type guards, advanced features, and generic type testing. It also integrates with various test frameworks, including VTest. Best practices include testing only helpful things, making generics specific, and running tests as part of CI pipelines. Understanding error messages and fixing type errors are important aspects of using the library.
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