Question 1

What is tsconfig.json and which options matter most?

Accepted Answer

**Target Audience:** Beginners who have just run `tsc --init` and are staring at a wall of options.
**Difficulty:** Easy
**TypeScript version:** All compiler runs below used TypeScript 5.4.5; changes in later releases (5.9, 6.0) are cited from their 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 an interviewer expects. Every emitted file and compiler mess

Question 2

What does `arethetypeswrong` (attw) detect and how do you fix false ESM/CJS packages?

Accepted Answer

**Target Audience:** Library authors who publish to npm, and app developers who hit a strange type error when importing a package that runs fine.
**Difficulty:** Medium
**TypeScript version:** The page examples were checked with TypeScript 5.4.5 (`--strict`). The package reproductions in sections 4 to 7 are small multi-file projects compiled with TypeScript 5.4.5 (and 5.9.3 where noted, `module` / `moduleResolution` `nodenext`) and run with Node.js 22.22. attw itself was **not** run here (it is

Question 3

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 4

How do you migrate a JavaScript codebase to TypeScript?

Accepted Answer

**Target Audience:** Developers and tech leads who need to move a working JavaScript codebase to TypeScript while the team keeps shipping.
**Difficulty:** Medium
**TypeScript version:** Every compiler message below was produced with TypeScript 5.4.5, both on the page's examples (`--strict`, target ES2022) and on a small mixed JavaScript/TypeScript project built with `tsc -p` and run with Node 22, step by step, as quoted.

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

Question 5

When should you use `moduleResolution: "bundler"` versus `"nodenext"`?

Accepted Answer

**Target Audience:** Developers choosing compiler options for a new app, Node service or library, and anyone who has seen "Relative import paths need explicit file extensions" and wondered why.
**Difficulty:** Medium
**TypeScript version:** Every compiler message, trace and run result below came from TypeScript 5.4.5 and Node.js 22 on scratch projects. `node16` and `nodenext` arrived in TypeScript 4.7 ([Announcing TypeScript 4.7](https://devblogs.microsoft.com/typescript/announcing-typescript-4-

Question 6

How does `allowImportingTsExtensions` work with `moduleResolution: "bundler"` and `.ts` imports?

Accepted Answer

**Target Audience:** Developers using Vite, Bun, Deno or Node's built-in TypeScript support who want to write the real file name in imports, and anyone puzzled by TS5096.
**Difficulty:** Medium
**TypeScript version:** Compiler messages and output below came from TypeScript 5.4.5 and Node.js 22.22 on a scratch project. `allowImportingTsExtensions` arrived in TypeScript 5.0 alongside `moduleResolution: "bundler"` ([TypeScript 5.0 RC announcement](https://devblogs.microsoft.com/typescript/announcin

Question 7

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