Question 1

What are literal types?

Accepted Answer

**Target Audience:** Beginners who know basic TypeScript types and want to model "only these exact values".
**Difficulty:** Easy
**TypeScript version:** Compiler behaviour checked with TypeScript 5.4.5 (`tsc --strict`).

> **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 every output shown was produced by actually running the code; the **Verified**

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 is the difference between void, undefined and never as return types?

Accepted Answer

**Target Audience:** Developers who annotate return types and want to know exactly what `void`, `undefined` and `never` promise to the caller.
**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

Question 4

What is a discriminated union?

Accepted Answer

**Target Audience:** Developers who model state, events or API results and want the most useful pattern in everyday TypeScript explained from first principles.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 and `--strict`; the 4.4 and 4.6 improvements are cited from their 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 expects. Type cla

Question 5

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 6

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 7

What is the never type?

Accepted Answer

**Target Audience:** Developers who have seen `never` in error messages and utility types and want to understand what it is and how to use it on purpose.
**Difficulty:** Medium
**TypeScript version:** Every type claim below was checked by TypeScript 5.4.5 with `--strict`; that `never` arrived in TypeScript 2.0 is 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 expe

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Easy