Question 1

How does type inference work in TypeScript?

Accepted Answer

**Target Audience:** Beginners who wonder how much TypeScript they actually have to write by hand.
**Difficulty:** Easy
**TypeScript version:** Every inferred type below is proven by a compile-time check that TypeScript 5.4.5 (`--strict`) accepts; outputs are from a real 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. Inferred types are not guessed from memory: each

Question 2

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 3

What are the basic types in TypeScript?

Accepted Answer

**Target Audience:** Beginners writing their first TypeScript annotations.
**Difficulty:** Easy
**TypeScript version:** Every compiler message and output below was produced with TypeScript 5.4.5 (`--strict`, target ES2022).

> **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. All output shown was produced by actually running the code; the **Verified** section quotes it in full

Question 4

What is TypeScript and why use it over JavaScript?

Accepted Answer

**Target Audience:** Beginners who know some JavaScript and keep hearing "just use TypeScript".
**Difficulty:** Easy
**TypeScript version:** Compiler behaviour checked with TypeScript 5.4.5 (`tsc --strict`); release history checked against the sources 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 message and every output shown was produced

Question 5

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 6

What are generic constraints?

Accepted Answer

**Target Audience:** Developers who know what `<T>` means and want to make generics that can actually do something with `T`.
**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 cod

Question 7

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 8

What are generics and why are they useful?

Accepted Answer

**Target Audience:** Developers who can write basic TypeScript types and keep meeting `<T>` in library code.
**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 code; the **Verifie

Question 9

What are enums and what are their trade-offs?

Accepted Answer

**Target Audience:** Developers deciding how to model a fixed set of values (statuses, roles, directions) and wanting the real behaviour of enums, not folklore.
**Difficulty:** Medium
**TypeScript version:** Behaviour checked with TypeScript 5.4.5 and `--strict`; the 5.0 and 5.8 changes are cited from release coverage.

> **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

Question 10

How do you type async functions and Promises?

Accepted Answer

**Target Audience:** Developers who use `async`/`await` daily and want the types to be precise — return types, errors, and concurrent calls.
**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 `t

TYPESCRIPTDECODED.

Easy

How does type inference work in TypeScript?

What is the difference between any and unknown?

What are the basic types in TypeScript?

What is TypeScript and why use it over JavaScript?

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

Medium

What are generic constraints?

What do ReturnType and Parameters do?

What are generics and why are they useful?

What are enums and what are their trade-offs?

How do you type async functions and Promises?

What are declaration files (.d.ts)?

How do you migrate a JavaScript codebase to TypeScript?

What does the 'strict' flag in tsconfig enable?

Hard

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

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

How do you author library types that pass `isolatedDeclarations` for `oxc` / `tsdown` builds?

What does the infer keyword do?

What are covariance and contravariance?

What is `isolatedDeclarations` and how does it enable single-file transpilation for declaration emit?

What are conditional types?