Question 1

What are enums and what are their trade-offs?

Accepted Answer

**Target Audience:** Developers deciding how to model a fixed set of values (statuses, roles, directions) and wanting the real behaviour of enums, not folklore.
**Difficulty:** Medium
**TypeScript version:** Behaviour checked with TypeScript 5.4.5 and `--strict`; the 5.0 and 5.8 changes are cited from release coverage.

> **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

Question 2

What is declaration merging?

Accepted Answer

**Target Audience:** Developers who have seen the same interface name declared twice, or `namespace` blocks next to a function, and want the rules.
**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 a

Question 3

What does the declare keyword do?

Accepted Answer

**Target Audience:** Developers who have seen `declare` in a codebase or a `.d.ts` file and want to know exactly what it does and does not do.
**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 actual

Question 4

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 5

What are branded (nominal) types?

Accepted Answer

**Target Audience:** Developers who have mixed up two IDs, two units or a raw and a sanitized string, and want the compiler to stop it — explained from first principles.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 and `--strict`; `unique symbol` dates from TypeScript 2.7 ([TypeScript 2.7 release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-7.html)).

> **How to read this doc:** Every idea is explained in plain language first, the

Question 6

What does as const do?

Accepted Answer

**Target Audience:** Developers who have seen `as const` in codebases and want to know exactly what it changes and when to use it.
**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 ru

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