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

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 3

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 4

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 5

What's new to type in TypeScript 5.5+ libs — `Array.fromAsync`, `Set` methods, `Iterator` helpers?

Accepted Answer

**Target Audience:** Developers who want to use the newest built-in JavaScript APIs from TypeScript and need to understand where the types come from and what they promise.
**Difficulty:** Medium
**TypeScript version:** The page runs TypeScript 5.4.5, whose libs have **none** of these three APIs, so the runnable examples use typed userland equivalents. The real lib signatures quoted below were read from, and checked with, **tsc 5.9.3** (`--strict --lib ESNext,DOM`) and **tsc 6.0.2**; that program

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

TYPESCRIPTDECODED.

Easy