Question 1

What is the difference between any and unknown?

Accepted Answer

**Target Audience:** Beginners who have seen `any` everywhere and want to know why reviewers keep asking for `unknown`.
**Difficulty:** Easy
**TypeScript version:** Compiler behaviour checked with TypeScript 5.4.5 (`tsc --strict`); the history of `unknown` checked against the release notes linked inline.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Every compiler messa

Question 2

How do you type functions: optional, default and rest parameters?

Accepted Answer

**Target Audience:** Beginners writing their first typed functions with flexible arguments.
**Difficulty:** Easy
**TypeScript version:** Compiler behaviour checked with TypeScript 5.4.5 (`tsc --strict`).

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Every compiler message and every output shown was produced by actually running the code; the **Verified** section quotes i

Question 3

How does `using` / `await using` with `Symbol.dispose` work in TypeScript 5.2+?

Accepted Answer

**Target Audience:** Developers who write cleanup code (closing connections, releasing locks, deleting temp files) and want to understand TypeScript's `using` declarations from zero.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 `--strict` (the page compiler; `using` has been supported since 5.2). The page compiles to ES2022, so it runs TypeScript's downlevel helpers. As a cross-check, both examples were also compiled to ESNext (native `using` syntax) and run in Bu

Question 4

What do ReturnType and Parameters do?

Accepted Answer

**Target Audience:** Developers who keep writing the same type twice — once on a function, once next to it — and want to derive it instead.
**Difficulty:** Medium
**TypeScript version:** Every type claim below was checked by TypeScript 5.4.5 with `--strict`; release versions are cited, not run.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Type claims are checked by `ts

Question 5

What are indexed access types?

Accepted Answer

**Target Audience:** Developers who know basic object types and want to reuse parts of existing types instead of copying them.
**Difficulty:** Medium
**TypeScript version:** Every compiler message and output below was produced with TypeScript 5.4.5 (`--strict`, target ES2022).

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. All output was produced by actually running the c

Question 6

What does `verbatimModuleSyntax` enforce and why did it replace `importsNotUsedAsValues`?

Accepted Answer

**Target Audience:** Developers who write `import` statements in TypeScript and want to know exactly which of them survive into the JavaScript, and why a modern tsconfig insists on `import type`.
**Difficulty:** Medium
**TypeScript version:** Every compiler message and emitted file below came from TypeScript 5.4.5 on a scratch project. `verbatimModuleSyntax` arrived in TypeScript 5.0, which deprecated `importsNotUsedAsValues` and `preserveValueImports` in its favour ([TypeScript 5.0 RC announcem

Question 7

How do you type and compose multiple disposables with `DisposableStack` and `AsyncDisposableStack`?

Accepted Answer

**Target Audience:** Developers who already know `using` and need to manage several resources together: setup that can fail half-way, objects that own other resources, and async cleanup.
**Difficulty:** Hard
**TypeScript version:** Types checked with TypeScript 5.4.5 `--strict` (`DisposableStack` and `AsyncDisposableStack` are declared in `lib.esnext.disposable`, available since 5.2). The page runtime has no built-in stacks, so Examples 1 and 2 use the built-in class when present and otherwise a

Question 8

How do you use `infer` to build deep `Awaited`, `ReturnType` and `Unwrap` utilities?

Accepted Answer

**Target Audience:** Developers who know what `infer` does and want to write their own recursive utility types for real code.
**Difficulty:** Hard
**TypeScript version:** Every type claim below was checked by TypeScript 5.4.5 with `--strict`; the 4.5 release fact is cited, not run.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Type claims are checked by `tsc` (an `Equal

Question 9

How do TypeScript 5.0 Stage 3 decorators differ from legacy `experimentalDecorators`?

Accepted Answer

**Target Audience:** Developers who use or write decorators (Angular, NestJS, TypeORM, MobX, Lit, or their own) and need to know which of the two systems they are on.
**Difficulty:** Hard
**TypeScript version:** Standard decorators shipped in TypeScript 5.0 ([Announcing TypeScript 5.0](https://devblogs.microsoft.com/typescript/announcing-typescript-5-0/)). Every compiler message and output below was produced with TypeScript 5.4.5 (`--strict`, target ES2022). The legacy examples were compiled sep

Question 10

How do you choose between `NoInfer<T>`, `satisfies`, and `const` type params when designing APIs?

Accepted Answer

**Target Audience:** Developers who already know what each of the three features does on its own and now have to pick between them when designing a function or library API.
**Difficulty:** Hard
**TypeScript version:** `satisfies` arrived in TypeScript 4.9 ([4.9 release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-9.html)), `const` type parameters in 5.0 ([5.0 release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-5-0.html)) and `NoI

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Easy