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 interface and type?

Accepted Answer

**Target Audience:** Beginners who have seen both keywords in the same codebase and want to know when the choice matters.
**Difficulty:** Easy
**TypeScript version:** Every error and output below comes from TypeScript 5.4.5 with `--strict`; release and guidance facts are cited.

> **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 output was produced

Question 3

What are the basic types in TypeScript?

Accepted Answer

**Target Audience:** Beginners writing their first TypeScript annotations.
**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 **Verified** section quotes it in full

Question 4

What is declaration merging?

Accepted Answer

**Target Audience:** Developers who have seen the same interface name declared twice, or `namespace` blocks next to a function, and want the rules.
**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 a

Question 5

What does the declare keyword do?

Accepted Answer

**Target Audience:** Developers who have seen `declare` in a codebase or a `.d.ts` file and want to know exactly what it does and does not do.
**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 actual

Question 6

What are declaration files (.d.ts)?

Accepted Answer

**Target Audience:** Developers who install packages, see "Could not find a declaration file", or need to publish a library with types.
**Difficulty:** Medium
**TypeScript version:** Every compiler message and output below was produced with TypeScript 5.4.5 (`--strict`, target ES2022). The multi-file projects in sections 3 to 6 were compiled separately with the same compiler, since the page runs a single file.

> **How to read this doc:** Each idea is explained in plain language first, then tagg

Question 7

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 8

What does the satisfies operator do?

Accepted Answer

**Target Audience:** Developers who know type annotations and `as`, and want to know what `satisfies` adds and when to use it.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 and `--strict`. `satisfies` arrived in TypeScript 4.9 ([Announcing TypeScript 4.9](https://devblogs.microsoft.com/typescript/announcing-typescript-4-9/), November 2022).

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you als

Question 9

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 10

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

TYPESCRIPTDECODED.

Easy