Q: What is a discriminated union?

**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 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

How does TypeScript handle null and undefined safely?

Accepted Answer

**Target Audience:** Beginners who have met "Cannot read properties of undefined" and want the compiler to catch it first.
**Difficulty:** Easy
**TypeScript version:** Compiler behaviour checked with TypeScript 5.4.5 (`tsc --strict`); version history 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 message and

Question 3

What is type narrowing and what are type guards?

Accepted Answer

**Target Audience:** Developers who use unions every day and want to know exactly when TypeScript lets them treat a value as one specific member.
**Difficulty:** Medium
**TypeScript version:** Every narrowing result and compiler message below comes from TypeScript 5.4.5 with `--strict`. The 5.5 feature mentioned at the end 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 int

Question 4

What are assertion functions (asserts)?

Accepted Answer

**Target Audience:** Developers who know type guards and want to write validation helpers that narrow types without wrapping everything in `if` blocks.
**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 pr

Question 5

What are union and intersection types?

Accepted Answer

**Target Audience:** Developers who know the basic types and want to combine them into real-world shapes.
**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 running the code; the **Ver

Question 6

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 7

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 8

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 9

What do Exclude, Extract, and NonNullable do?

Accepted Answer

**Target Audience:** Developers who know union types and want to transform them: remove members, keep members, strip `null`.
**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 `tsc` (an `Equal<.

Question 10

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

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