Question 1

What is TypeScript and why use it over JavaScript?

Accepted Answer

In short: A typed superset of JavaScript that compiles to plain JavaScript. Static types catch errors at compile time, give IntelliSense and safe refactoring, document APIs and scale better for large teams. Types are erased at runtime, so there is no runtime cost — and no runtime checking.

Question 2

What is the difference between interface and type?

Accepted Answer

In short: Both describe object shapes. type can also name unions, intersections, tuples, primitives and mapped/conditional types; interface supports declaration merging and extends , and is often preferred for public object contracts. Interfaces can t express unions; types can t be reopened.

Question 3

any vs unknown vs never?

Accepted Answer

In short: any disables type checking for that value (errors pass silently). unknown accepts any value but must be narrowed (typeof/guards/schema) before use — the safe choice for external data and catch errors. never represents values that never occur — functions that always throw, or the impossible branch in an exhaustive switch.

Question 4

What are union types and how do you narrow them?

Accepted Answer

In short: A union A | B means the value is one of several types. Narrow with typeof , instanceof , in , equality on a discriminant property ( kind ), truthiness, or custom type guards ( x is T ) so TypeScript knows which member you have in each branch.

Question 5

What is type inference?

Accepted Answer

In short: The compiler works out types without annotations: let n = 5 is number , a function s return type comes from its return statements, generic arguments come from the values passed. Annotate function parameters and public APIs; let inference handle locals.

Question 6

What are generics? Give an example.

Accepted Answer

In short: Type parameters that make code reusable while keeping types: function first T (xs: T[]): T | undefined . With constraints ( T extends { id: number } ) and keyof ( K extends keyof T ) you get type-safe helpers such as pluck(items, name ) . Common built-ins: Array T , Promise T , Record K,V .

Question 7

Explain Partial, Pick, Omit and Record with use cases.

Accepted Answer

In short: Partial T makes all props optional (update/PATCH payloads). Pick T,K keeps chosen keys (summary DTO). Omit T,K removes keys (create payload without id ). Record K,V builds an object type with keys K (lookup tables like Record Status, string ).

Question 8

What does strict mode enable and why should it be on?

Accepted Answer

In short: strict: true enables noImplicitAny , strictNullChecks , strictFunctionTypes , strictBindCallApply , strictPropertyInitialization , alwaysStrict , useUnknownInCatchVariables and others. It catches the most common bugs — especially null/undefined access. New projects should always use it; legacy projects can turn flags on gradually.

Question 9

What is a discriminated union?

Accepted Answer

In short: A union of object types that share a literal tag property ( kind , type , status ). Switching on the tag narrows to the exact member, and a never check in default makes the switch exhaustive — adding a new member produces compile errors wherever it isn t handled. Ideal for state ( { status: loading } | { status: error ; error: string } | { status: ok ; data: T } ) and Redux/NgRx actions.

Question 10

What is the difference between as (type assertion) and satisfies?

Accepted Answer

In short: as T tells the compiler trust me, it s T — no runtime check and it can hide mistakes. satisfies T checks that a value matches T but keeps the more precise inferred type, so you get validation without losing literal types. Prefer satisfies for config objects and avoid as except at well-validated boundaries.

Question 11

What are declaration files (.d.ts)?

Accepted Answer

In short: Files containing only type declarations that describe JavaScript code — they let TypeScript type-check calls into JS libraries. Libraries ship their own ( types in package.json) or come from DefinitelyTyped ( npm i -D @types/lodash ). You can write declare module x or augment globals (e.g. Window ) in your own .d.ts .

Question 12

What are mapped and conditional types?

Accepted Answer

In short: Mapped types transform each property of another type: { [K in keyof T]: Readonly T[K] } (with as to rename keys). Conditional types choose a type by condition: `T extends U ? X : Y , with infer to extract parts ( T extends Promise ? R : T`). Built-in utilities like Partial , ReturnType and Awaited are implemented with them.

Question 13

Structural vs nominal typing — what does TypeScript use?

Accepted Answer

In short: Structural: types are compatible if their shapes match, regardless of names, so { id: 1, name: x } is assignable to any interface with those members. C# and Java are nominal. To simulate nominal types (e.g. UserId vs OrderId both numbers), use branded types: type UserId = number { readonly __brand: UserId } .

Question 14

The API returned a different JSON shape and the app crashed even though it compiled. Why, and how do you prevent it?

Accepted Answer

In short: Types are erased at runtime; http.get Order[] () or as Order[] only told the compiler what to expect. Prevent: generate client types from the backend OpenAPI document in the build (so contract changes show up as compile errors), validate responses at the boundary with Zod/Valibot, version the API, and add contract/end-to-end tests.

Question 15

You are migrating a large JavaScript project to TypeScript. How do you do it?

Accepted Answer

In short: Incrementally: add tsconfig with allowJs and checkJs off, rename files to .ts module by module starting from leaves (utilities, models), add @types packages, use any / unknown temporarily with TODOs, then turn on strict flags one by one ( noImplicitAny , then strictNullChecks ). Add type-checking ( tsc --noEmit ) to CI so coverage only grows.

Question 16

Write a type-safe function that groups an array by a key.

Accepted Answer

typescript Copy function groupBy T, K extends PropertyKey ( items : T[], key : ( item : T ) = K): Record K, T[] { return items. reduce ( ( acc, item ) = { const k = key (item); (acc[k] ??= []). push (item); return acc; }, {} as Record K, T[] ); } const byStatus = groupBy (orders, o = o. status ); // Record OrderStatus, Order[] (Modern runtimes also have Object.groupBy(items, fn) from ES2024.)

TypeScript — Types, Generics and Real-World Patterns