Question 1

What are optional properties and optional chaining in TypeScript?

Accepted Answer

**Target Audience:** Beginners modelling data where some fields are sometimes missing.
**Difficulty:** Easy
**TypeScript version:** Compiler behaviour checked with TypeScript 5.4.5 (`tsc --strict`, plus one check with `--exactOptionalPropertyTypes`); emitted JavaScript checked with the 5.4.5 `ts.transpile`.

> **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 me

Question 2

How do you make values immutable in TypeScript?

Accepted Answer

**Target Audience:** Developers who want to stop accidental mutation of config, state and shared data, and who need to know which guarantee each tool really gives.
**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 sh

Question 3

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 4

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 5

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 6

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 7

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 8

What are tuple types?

Accepted Answer

**Target Audience:** Beginners who know arrays and want to describe fixed-shape data such as pairs and records.
**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 *

Question 9

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 10

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**

TYPESCRIPTDECODED.

Easy

What are optional properties and optional chaining in TypeScript?

How do you make values immutable in TypeScript?

How does type inference work in TypeScript?

What is the difference between interface and type?

What is the difference between any and unknown?

What are the basic types in TypeScript?

What is TypeScript and why use it over JavaScript?

What are tuple types?

Medium

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

What is the definite assignment assertion (!)?

What is type narrowing and what are type guards?

What are assertion functions (asserts)?

What are generics and why are they useful?

What is ReadonlyArray and readonly tuple?

What are type assertions and when should you use them?

What are enums and what are their trade-offs?

Hard

What is key remapping in mapped types?

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

What does `NoInfer<T>` do and when should you block inference?

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

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

What are template literal types?

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

What are recursive types?