Question 1

What is the difference between interface and type?

Accepted Answer

**Target Audience:** Beginners who have seen both keywords in the same codebase and want to know when the choice matters.
**Difficulty:** Easy
**TypeScript version:** Every error and output below comes from TypeScript 5.4.5 with `--strict`; release and guidance facts are cited.

> **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 output was produced

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

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 5

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 6

What is the definite assignment assertion (!)?

Accepted Answer

**Target Audience:** Developers who meet `field!: Type` in Angular, ORM or class-heavy code and want to know what it promises and what it costs.
**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

Question 7

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 8

How do you do exhaustiveness checking?

Accepted Answer

**Target Audience:** Developers who `switch` over unions and want the compiler to flag every place that must change when a new variant is added.
**Difficulty:** Medium
**TypeScript version:** Every error message below was produced by TypeScript 5.4.5 with `--strict`; the `satisfies` version fact is cited from the 4.9 release notes.

> **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 e

Question 9

What is declaration merging?

Accepted Answer

**Target Audience:** Developers who have seen the same interface name declared twice, or `namespace` blocks next to a function, and want the rules.
**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 a

Question 10

What is the difference between void, undefined and never as return types?

Accepted Answer

**Target Audience:** Developers who annotate return types and want to know exactly what `void`, `undefined` and `never` promise to the caller.
**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

TYPESCRIPTDECODED.

Easy

What is the difference between interface and type?

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 the definite assignment assertion (!)?

What are union and intersection types?

How do you do exhaustiveness checking?

What is declaration merging?

What is the difference between void, undefined and never as return types?

How do you type async functions and Promises?

What do Exclude, Extract, and NonNullable do?

How do `noUncheckedIndexedAccess`, `exactOptionalPropertyTypes` and TS 5.6+ checks harden 2025 codebases?

Hard

What is key remapping in mapped types?

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

Why are conditional types distributive and how do you control it with `infer`?

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

What is module augmentation?

What are conditional types?