Question 1

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 2

How do you do exhaustiveness checking?

Accepted Answer

**Target Audience:** Developers who `switch` over unions and want the compiler to flag every place that must change when a new variant is added.
**Difficulty:** Medium
**TypeScript version:** Every error message below was produced by TypeScript 5.4.5 with `--strict`; the `satisfies` version fact is cited from the 4.9 release notes.

> **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 e

Question 3

What are mapped types?

Accepted Answer

**Target Audience:** Developers comfortable with `keyof` and indexed access types who want to transform whole object types.
**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 actually running the code

Question 4

How does the 'in' operator narrow types?

Accepted Answer

**Target Audience:** Developers who narrow unions of object types and want to know the exact rules of `in` narrowing, including the cases that differ between the type checker and the running code.
**Difficulty:** Medium
**TypeScript version:** Every narrowing result and error below was checked with TypeScript 5.4.5 and `--strict`; the 4.9 change is cited from its release notes.

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

Question 5

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 6

How do you safely work with JSON of unknown shape?

Accepted Answer

**Target Audience:** Developers who parse API responses, request bodies, config files or `localStorage` and want those values typed without lying to the compiler.
**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 s

Question 7

What is the Record utility type?

Accepted Answer

**Target Audience:** Beginners who have met `Record<string, ...>` in code and want to know what it really is and when it helps.
**Difficulty:** Medium
**TypeScript version:** Every error and output below comes from TypeScript 5.4.5 with `--strict`; the definition is quoted from the 5.4.5 `lib.es5.d.ts`.

> **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 messag

Question 8

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

Question 9

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

Question 10

How do you get a union of an object's values as a type?

Accepted Answer

**Target Audience:** Developers who want a single constant object to be the source of truth for both run-time values and types.
**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 actually running the

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