Question 1

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 2

What is the definite assignment assertion (!)?

Accepted Answer

**Target Audience:** Developers who meet `field!: Type` in Angular, ORM or class-heavy code and want to know what it promises and what it costs.
**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

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 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 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 does the satisfies operator do?

Accepted Answer

**Target Audience:** Developers who know type annotations and `as`, and want to know what `satisfies` adds and when to use it.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 and `--strict`. `satisfies` arrived in TypeScript 4.9 ([Announcing TypeScript 4.9](https://devblogs.microsoft.com/typescript/announcing-typescript-4-9/), November 2022).

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

Question 8

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 9

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

Question 10

What is `isolatedDeclarations` and how does it enable single-file transpilation for declaration emit?

Accepted Answer

**Target Audience:** Developers who build or publish TypeScript libraries or large monorepos and want to understand why `.d.ts` generation is slow and what `isolatedDeclarations` changes.
**Difficulty:** Hard
**TypeScript version:** `isolatedDeclarations` arrived in TypeScript 5.5. The page compiler is TypeScript 5.4.5, which rejects the flag (`error TS5023: Unknown compiler option '--isolatedDeclarations'.`, checked here), so the page examples only show the underlying idea. Every `isolatedDecla

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