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 are generic constraints?

Accepted Answer

**Target Audience:** Developers who know what `<T>` means and want to make generics that can actually do something with `T`.
**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 cod

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 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 7

What does the 'strict' flag in tsconfig enable?

Accepted Answer

**Target Audience:** Developers who have `"strict": true` in their config and want to know exactly what it switched on.
**Difficulty:** Medium
**TypeScript version:** The flag list and every error below come from TypeScript 5.4.5 (its option table and real `tsc` runs); the 5.6 and 6.0 changes are cited from release coverage and were not run here.

> **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 a

Question 8

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

Question 9

What are function overloads?

Accepted Answer

**Target Audience:** Developers who type functions that accept several argument shapes and want precise return types, explained from first principles.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 and `--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. Type claims are checked by `tsc` (an `Equal<...>` line or a `// @ts-expect-error`

Question 10

Can generics have default types?

Accepted Answer

**Target Audience:** Developers who write generic types and want them easy to use without type arguments everywhere.
**Difficulty:** Medium
**TypeScript version:** Every compiler message and output below was produced with TypeScript 5.4.5 (`--strict`, target ES2022). Defaults themselves arrived in TypeScript 2.3 ([TypeScript 2.3 release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-3.html)).

> **How to read this doc:** Each idea is explained in plain language fi

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 are generic constraints?

What do ReturnType and Parameters do?

What is ReadonlyArray and readonly tuple?

What does the 'strict' flag in tsconfig enable?

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

What are function overloads?

Can generics have default types?

What does the satisfies operator do?

Hard

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

What does the infer keyword do?

What are covariance and contravariance?

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

How do you choose between `NoInfer<T>`, `satisfies`, and `const` type params when designing APIs?