Question 1

What are optional properties and optional chaining in TypeScript?

Accepted Answer

**Target Audience:** Beginners modelling data where some fields are sometimes missing.
**Difficulty:** Easy
**TypeScript version:** Compiler behaviour checked with TypeScript 5.4.5 (`tsc --strict`, plus one check with `--exactOptionalPropertyTypes`); emitted JavaScript checked with the 5.4.5 `ts.transpile`.

> **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 me

Question 2

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 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

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 5

What is a polymorphic 'this' type?

Accepted Answer

**Target Audience:** Developers who write or use chainable APIs (builders, query builders, test helpers) and want subclass-friendly typing explained from first principles.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 and `--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. Type claims are checked by `tsc` (an `Equal<...>` line or a `

TYPESCRIPTDECODED.

Easy

What are optional properties and optional chaining in TypeScript?

Medium

What is type narrowing and what are type guards?

What are type assertions and when should you use them?

How do you safely work with JSON of unknown shape?

What is a polymorphic 'this' type?