Question 1

How do you make values immutable in TypeScript?

Accepted Answer

**Target Audience:** Developers who want to stop accidental mutation of config, state and shared data, and who need to know which guarantee each tool really gives.
**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 sh

Question 2

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 3

How does `using` / `await using` with `Symbol.dispose` work in TypeScript 5.2+?

Accepted Answer

**Target Audience:** Developers who write cleanup code (closing connections, releasing locks, deleting temp files) and want to understand TypeScript's `using` declarations from zero.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 `--strict` (the page compiler; `using` has been supported since 5.2). The page compiles to ES2022, so it runs TypeScript's downlevel helpers. As a cross-check, both examples were also compiled to ESNext (native `using` syntax) and run in Bu

Question 4

What are assertion functions (asserts)?

Accepted Answer

**Target Audience:** Developers who know type guards and want to write validation helpers that narrow types without wrapping everything in `if` blocks.
**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 pr

Question 5

How do you debug type-resolution and ESM/CJS mismatches with `traceResolution` and `explain`?

Accepted Answer

**Target Audience:** Developers fighting "Cannot find module", TS1479 or types that compile but crash at run time, and anyone asked how they would debug module resolution.
**Difficulty:** Medium
**TypeScript version:** Every trace, explanation and error below came from TypeScript 5.4.5 and Node.js 22 on scratch projects (paths shortened to `/work/...`). `--explainFiles` arrived in TypeScript 4.2 ([Visual Studio Magazine on TypeScript 4.2](https://visualstudiomagazine.com/articles/2021/02/24/type

Question 6

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 7

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 8

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 9

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 10

How does `satisfies` validate shape without widening literal types?

Accepted Answer

**Target Audience:** Developers who already use `satisfies` and want to predict exactly which types it keeps, for config objects, route maps and lookup tables.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 and `--strict`; every "kept" or "widened" claim below is an `Equal` check that tsc verified.

> **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 expec

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