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

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

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 4

What are assertion functions (asserts)?

Accepted Answer

**Target Audience:** Developers who know type guards and want to write validation helpers that narrow types without wrapping everything in `if` blocks.
**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 pr

Question 5

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 6

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 7

What is ReadonlyArray and readonly tuple?

Accepted Answer

**Target Audience:** Developers who type function parameters and want to promise callers that their arrays will not be changed.
**Difficulty:** Medium
**TypeScript version:** Every compiler message and output below was produced with TypeScript 5.4.5 (`--strict`, target ES2022, lib ES2022 + DOM).

> **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 8

How does TypeScript 5.5 infer type predicates for `Array.filter` automatically?

Accepted Answer

**Target Audience:** Developers who use `Array.filter` and type guards daily and want to know exactly when TypeScript 5.5+ narrows the result for them.
**Difficulty:** Medium
**TypeScript version:** Inferred type predicates shipped in TypeScript 5.5 ([TypeScript 5.5 announcement](https://devblogs.microsoft.com/typescript/announcing-typescript-5-5/); the feature was implemented by Dan Vanderkam, see [Effective TypeScript: TS 5.5](https://effectivetypescript.com/2024/07/02/ts-55/)). The page runs

Question 9

What are mapped types?

Accepted Answer

**Target Audience:** Developers comfortable with `keyof` and indexed access types who want to transform whole object types.
**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

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

How does type inference work in TypeScript?

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

Medium

What is type narrowing and what are type guards?

What are assertion functions (asserts)?

What do ReturnType and Parameters do?

What are generics and why are they useful?

What is ReadonlyArray and readonly tuple?

How does TypeScript 5.5 infer type predicates for `Array.filter` automatically?

What are mapped types?

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

Hard

How would you build a DeepPartial type?

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 are recursive types?

What does the infer keyword do?