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 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 3

How do `noUncheckedIndexedAccess`, `exactOptionalPropertyTypes` and TS 5.6+ checks harden 2025 codebases?

Accepted Answer

**Target Audience:** Developers on a codebase that already has `strict: true` and want to know which extra checks are worth turning on, and what newer TypeScript versions catch for free.
**Difficulty:** Medium
**TypeScript version:** The page examples were checked with TypeScript 5.4.5 `--strict`. The flag-on errors in sections 3 and 4 were produced by running TypeScript 5.4.5 on the same files with `--noUncheckedIndexedAccess --exactOptionalPropertyTypes`. Everything marked 5.5+, 5.6+, 5.7 or 5

Question 4

What is an index signature?

Accepted Answer

**Target Audience:** Beginners who need to type a dictionary-like object, and anyone who has been bitten by `obj[key]` being `undefined` at run time.
**Difficulty:** Medium
**TypeScript version:** Every error and output below comes from TypeScript 5.4.5 with `--strict` (two flag-specific errors were produced by adding that one flag, as noted); version facts are cited.

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

TYPESCRIPTDECODED.

Easy

What is tsconfig.json and which options matter most?

Medium

What does the 'strict' flag in tsconfig enable?

How do `noUncheckedIndexedAccess`, `exactOptionalPropertyTypes` and TS 5.6+ checks harden 2025 codebases?

What is an index signature?