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

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 3

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 4

What are literal types?

Accepted Answer

**Target Audience:** Beginners who know basic TypeScript types and want to model "only these exact values".
**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**

Question 5

What is type narrowing and what are type guards?

Accepted Answer

**Target Audience:** Developers who use unions every day and want to know exactly when TypeScript lets them treat a value as one specific member.
**Difficulty:** Medium
**TypeScript version:** Every narrowing result and compiler message below comes from TypeScript 5.4.5 with `--strict`. The 5.5 feature mentioned at the end 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 int

Question 6

What are union and intersection types?

Accepted Answer

**Target Audience:** Developers who know the basic types and want to combine them into real-world shapes.
**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:** 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 **Ver

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 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 10

How do you safely work with JSON of unknown shape?

Accepted Answer

**Target Audience:** Developers who parse API responses, request bodies, config files or `localStorage` and want those values typed without lying to the compiler.
**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:** 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 s

TYPESCRIPTDECODED.

Easy

What is the difference between any and unknown?

What are the basic types in TypeScript?

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

What are literal types?

Medium

What is type narrowing and what are type guards?

What are union and intersection types?

What do ReturnType and Parameters do?

What are generics and why are they useful?

What are indexed access types?

How do you safely work with JSON of unknown shape?

How does `satisfies` validate shape without widening literal types?

How do you get a union of an object's values as a type?

Hard

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