Question 1

What is the difference between any and unknown?

Accepted Answer

**Target Audience:** Beginners who have seen `any` everywhere and want to know why reviewers keep asking for `unknown`.
**Difficulty:** Easy
**TypeScript version:** Compiler behaviour checked with TypeScript 5.4.5 (`tsc --strict`); the history of `unknown` checked against the release notes linked inline.

> **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 messa

Question 2

What are generic constraints?

Accepted Answer

**Target Audience:** Developers who know what `<T>` means and want to make generics that can actually do something with `T`.
**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 cod

Question 3

What are type assertions and when should you use them?

Accepted Answer

**Target Audience:** Developers who see `as` in code reviews and want a precise rule for when it is fine and when it hides a bug.
**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 produced by actually run

Question 4

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 5

What are indexed access types?

Accepted Answer

**Target Audience:** Developers who know basic object types and want to reuse parts of existing types instead of copying them.
**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 c

Question 6

What are utility types like Partial, Pick, and Omit?

Accepted Answer

**Target Audience:** Developers who write interfaces by hand and want to derive the variants (patch, create, preview) instead of copying them.
**Difficulty:** Medium
**TypeScript version:** Every type claim below was checked by TypeScript 5.4.5 with `--strict`; release versions are cited, not run.

> **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. Type claims are checked by

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

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 9

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

Question 10

What do keyof and typeof do in TypeScript?

Accepted Answer

**Target Audience:** Developers who know generics and want to derive types from existing types and values instead of writing them twice.
**Difficulty:** Medium
**TypeScript version:** Every compiler message and output below was produced with TypeScript 5.4.5 (`--strict`, target ES2022). `keyof` and indexed access types were introduced in TypeScript 2.1 ([TypeScript 2.1 release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-1.html)).

> **How to read this doc:** Ea

TYPESCRIPTDECODED.

Easy

What is the difference between any and unknown?

Medium