Question 1

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 2

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 3

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 4

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 5

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

Question 6

What do keyof and typeof do in TypeScript?

Accepted Answer

**Target Audience:** Developers who know generics and want to derive types from existing types and values instead of writing them twice.
**Difficulty:** Medium
**TypeScript version:** Every compiler message and output below was produced with TypeScript 5.4.5 (`--strict`, target ES2022). `keyof` and indexed access types were introduced in TypeScript 2.1 ([TypeScript 2.1 release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-1.html)).

> **How to read this doc:** Ea

Question 7

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

Accepted Answer

**Target Audience:** Library authors moving their build to tsdown, Rolldown or another oxc-based tool, and anyone asked to make an existing codebase pass `isolatedDeclarations`.
**Difficulty:** Hard
**TypeScript version:** The page examples were checked with TypeScript 5.4.5 `--strict` (which has no `isolatedDeclarations` flag). Every `isolatedDeclarations` error and `.d.ts` file below came from TypeScript 5.9.3 and from oxc's isolated-declaration transform as bundled in rolldown 1.0.3 (`isolate

TYPESCRIPTDECODED.

Medium