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 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 3

How do you safely work with JSON of unknown shape?

Accepted Answer

**Target Audience:** Developers who parse API responses, request bodies, config files or `localStorage` and want those values typed without lying to the compiler.
**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 s

Question 4

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 5

What does as const do?

Accepted Answer

**Target Audience:** Developers who have seen `as const` in codebases and want to know exactly what it changes and when to use it.
**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 actually ru

Question 6

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

Accepted Answer

**Target Audience:** Developers who write generic functions (helpers, hooks, library APIs) and want callers to get errors in the right place.
**Difficulty:** Hard
**TypeScript version:** `NoInfer` shipped in TypeScript 5.4 ([TypeScript 5.4 announcement](https://devblogs.microsoft.com/typescript/announcing-typescript-5-4/)). 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

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

Question 8

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

Accepted Answer

**Target Audience:** Developers who build or publish TypeScript libraries or large monorepos and want to understand why `.d.ts` generation is slow and what `isolatedDeclarations` changes.
**Difficulty:** Hard
**TypeScript version:** `isolatedDeclarations` arrived in TypeScript 5.5. The page compiler is TypeScript 5.4.5, which rejects the flag (`error TS5023: Unknown compiler option '--isolatedDeclarations'.`, checked here), so the page examples only show the underlying idea. Every `isolatedDecla

Question 9

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

Accepted Answer

**Target Audience:** Developers who already know what each of the three features does on its own and now have to pick between them when designing a function or library API.
**Difficulty:** Hard
**TypeScript version:** `satisfies` arrived in TypeScript 4.9 ([4.9 release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-9.html)), `const` type parameters in 5.0 ([5.0 release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-5-0.html)) and `NoI

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Easy