Answer each in 13 sentences. Time yourself: 30 seconds per question. Q1: What is structural typing? TypeScript checks compatibility based on shape (properties and types), not name. Two types with the same structure are assignable to each other even if they have different names. Q2: vs — main difference? Both declare object shapes. is open (can be merged/extended via declaration merging); is closed (no merging, but supports unions, intersections, mapped types, and conditional types that can't). Use for public API shapes, for complex compositions. Q3: What is the TypeScript Temporal Dead Zone (TDZ)? TypeScript doesn't have a TDZ — that's a JavaScript runtime concept. TypeScript's type checking happens at compile time, not runtime. However, TypeScript will error if you use a variable before it's declared. Q4: What does vs do? disables type checking entirely — you can do anything with it. is typesafe — you must narrow it before using it. Prefer for values whose type you don't know (e.g., API responses, clauses). Q5: What is a discriminated union? A union type where each member has a common literal property (discriminant) that TypeScript can use to narrow the type. E.g., — checking narrows the type automatically. Q6: What is ? The bottom type — a type with no values. Represents values that never occur (e.g., function that always throws, exhaustive switch default). Useful for exhaustiveness checks: if you reach , TypeScript tells you you've missed a case. Q7: What does do? Produces a union of all keys of a type as string/symbol literals. → . Used in generic constraints: . Q8: What is used for? Extracts a type from within a conditional type. — captures whatever the return type is and binds it to . Q9: What does do in a mapped type? Removes the optional modifier from all properties. — the makes every property required by removing the . Q10: What is the difference between and ? makes only the toplevel properties optional. recursively makes all nested properties optional. TypeScript doesn't have builtin — you implement it with a recursive conditional type. Q11: What is excess property checking? TypeScript only performs extra property checking on fresh object literals assigned directly to a typed variable. If you pass through an intermediate variable, the check is bypassed. errors; doesn't. Q12: What does do? How is it different from a type annotation? validates that a value matches a type but keeps the inferred literal/narrow type. A regular annotation widens to the annotation's type. — TypeScript still knows is , not . Q13: What is a type predicate? A return type annotation that tells TypeScript a function narrows the type of . — after calling this, TypeScript knows is a in the truthy branch. Q14: What is ? A builtin utility type that extracts the return type of a function type. → . Implemented as . Q15: What is an assertion function? A function whose signature uses that tells TypeScript a condition is true after the function returns (throws if not). — after calling, TypeScript knows is . Q16: What is declaration merging? Multiple declarations with the same name are merged into one. Two declarations merge their properties. A can merge with a or . Useful for augmenting thirdparty types (). Q17: What does do? How is it implemented? Creates a new type with keys K removed from T. — removes key. Implemented as . Q18: What is a conditional type? A type that evaluates differently based on a condition: . If is assignable to , resolves to , otherwise . Can be recursive. Used for , , , etc. Q19: What is distributive conditional type? When in is a naked type parameter, TypeScript distributes over unions. → . Wrap in tuple to prevent distribution. Q20: What are template literal types? Types built from string template literals: `on${Capitalize

Advanced Patterns