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 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 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 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 5

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 6

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 7

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 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 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

Question 10

What are `const` type parameters (`<const T>`) and why are they better than `as const`?

Accepted Answer

**Target Audience:** Developers writing helpers such as `defineConfig`, `defineRoutes` or typed event lists, who want callers to get exact literal types without extra syntax.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 and `--strict`. `const` type parameters arrived in TypeScript 5.0 ([PR #51865](https://github.com/microsoft/TypeScript/pull/51865), released March 2023).

> **How to read this doc:** Every idea is explained in plain language first, then tagged with

TYPESCRIPTDECODED.

Easy

How does type inference work in TypeScript?

What are tuple types?

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?

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

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

What are `const` type parameters (`<const T>`) and why are they better than `as const`?

How do you safely work with JSON of unknown shape?

Hard

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`?

What are template literal types?

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

What does the infer keyword do?

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

What are conditional types?