TypeScript Deep-Dive Quiz

This quiz works through the parts of TypeScript that interviewers actually probe: how the compiler infers types when you do not annotate them, how generics let a function stay type-safe across many input shapes, and how the type system computes new types from existing ones with conditional types, mapped types, and the infer keyword. It also covers narrowing, how TypeScript uses control flow, typeof, instanceof, discriminated unions, and user-defined type guards to shrink a union down to a single type inside a branch.

A distinctive feature is that every answer includes the compiled JavaScript. Types, interfaces, and type-only imports are erased at build time and produce no runtime code, while enums, class parameter properties, and decorators do emit something. Seeing the emitted output next to the source makes it obvious which constructs are purely a compile-time contract and which carry a runtime cost, a distinction that comes up constantly in interviews about bundle size and performance.

The utility types, Partial, Required, Readonly, Pick, Omit, Record, ReturnType, Parameters, Awaited, and the rest, are treated as things you should be able to reimplement, not just call. Several questions ask you to predict the result of a hand-written mapped type that mirrors one of the built-ins, which is a common whiteboard exercise.

Beginner covers the everyday type system: primitive and literal types, arrays and tuples, union and intersection types, interfaces versus type aliases, optional and readonly properties, function types, and the difference between any, unknown, and never. These are the questions a screening call is most likely to open with.

Intermediate moves into generics with constraints, keyof and indexed access types, typeof used as a type query, basic mapped and conditional types, and control-flow narrowing. Advanced pushes into recursive conditional types, the infer keyword, template literal types, variadic tuple types, the satisfies operator, const type parameters, NoInfer, and module augmentation, the material that distinguishes a senior TypeScript answer.