1,920
Learners
verifying this skill
3,874
Sessions
completed to date
70
Avg SCI
across all levels
4
Tiers
claimed → peer-endorsed
In a peer session, work through type system challenges: refa
In a peer session, work through type system challenges: refactor untyped code, design type-safe APIs, or implement generics.
Your partner evaluates your use of discriminated unions, uti
Your partner evaluates your use of discriminated unions, utility types, type narrowing, and structural typing.
Sessions build evidence across Proof and Practice components
Sessions build evidence across Proof and Practice components of your SCI.
TypeScript — facts worth knowing.
TypeScript was created at Microsoft by Anders Hejlsberg – the same person who designed C#, Delphi, and Turbo Pascal.
Did you know?
TypeScript's structural type system means two completely unrelated types are compatible if they have the same shape – a deliberate design choice that aligns with JavaScript's duck-typing tradition.
Did you know?
The TypeScript compiler (tsc) itself is written in TypeScript – one of the most ambitious dog-fooding exercises in open-source history.
Did you know?
Why TypeScript matters.
TypeScript adds static types to JavaScript, enabling richer editor tooling, fewer runtime bugs, and more confident refactoring at scale. It has become the default choice for serious frontend and backend teams. Lemma sessions test your type system fluency – generics, conditional types, discriminated unions, and real-world application.
TypeScript is the fastest-growing language on GitHub and is now used in the majority of large JavaScript codebases. Companies routinely require it for senior roles. Verified TypeScript proficiency signals that you think about type safety as an engineering discipline, not just a tool.
1,920 people are learning this on Lemma
3,874 peer sessions completed
Avg SCI of 70 — tier 3 practitioners
avg SCI on Lemma
TypeScript practitioners
Related skills
TypeScript verification — common questions
During a peer session, you work through type system challenges, refactor untyped code, or design type-safe APIs. Your partner evaluates your use of generics, utility types, discriminated unions, and type narrowing.
Your TypeScript SCI reflects mastery of the type system including generics, conditional types, mapped types, declaration merging, and practical application in real codebases – not just whether you can add type annotations.
Peers evaluate your ability to design type-safe interfaces, use generics effectively, handle union types with narrowing, and leverage utility types like Partial, Required, Pick, and Omit. Advanced sessions cover conditional types and template literal types.
Absolutely. The type system adds a distinct layer of expertise that JavaScript proficiency does not imply. Many JavaScript experts struggle with TypeScript's structural type system, generics, and declaration files.
Microsoft's certifications test knowledge through multiple-choice exams. Lemma verification captures your ability to write, refactor, and reason about TypeScript in real time with a peer witness who evaluates your decisions.
Start verifying your TypeScript skills.
Join the waitlist. Your first peer session is free.