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

How do you make values immutable in TypeScript?

Accepted Answer

**Target Audience:** Developers who want to stop accidental mutation of config, state and shared data, and who need to know which guarantee each tool really gives.
**Difficulty:** Easy
**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 sh

Question 3

How does type inference work in TypeScript?

Accepted Answer

**Target Audience:** Beginners who wonder how much TypeScript they actually have to write by hand.
**Difficulty:** Easy
**TypeScript version:** Every inferred type below is proven by a compile-time check that TypeScript 5.4.5 (`--strict`) accepts; outputs are from a real 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. Inferred types are not guessed from memory: each

Question 4

What is the difference between interface and type?

Accepted Answer

**Target Audience:** Beginners who have seen both keywords in the same codebase and want to know when the choice matters.
**Difficulty:** Easy
**TypeScript version:** Every error and output below comes from TypeScript 5.4.5 with `--strict`; release and guidance facts are cited.

> **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 output was produced

Question 5

What are the basic types in TypeScript?

Accepted Answer

**Target Audience:** Beginners writing their first TypeScript annotations.
**Difficulty:** Easy
**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 **Verified** section quotes it in full

Question 6

What is TypeScript and why use it over JavaScript?

Accepted Answer

**Target Audience:** Beginners who know some JavaScript and keep hearing "just use TypeScript".
**Difficulty:** Easy
**TypeScript version:** Compiler behaviour checked with TypeScript 5.4.5 (`tsc --strict`); release history checked against the sources 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 every output shown was produced

Question 7

What are tuple types?

Accepted Answer

**Target Audience:** Beginners who know arrays and want to describe fixed-shape data such as pairs and records.
**Difficulty:** Easy
**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 *

Question 8

What is tsconfig.json and which options matter most?

Accepted Answer

**Target Audience:** Beginners who have just run `tsc --init` and are staring at a wall of options.
**Difficulty:** Easy
**TypeScript version:** All compiler runs below used TypeScript 5.4.5; changes in later releases (5.9, 6.0) are cited from their release coverage and were not run here.

> **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 emitted file and compiler mess

Question 9

How do you type functions: optional, default and rest parameters?

Accepted Answer

**Target Audience:** Beginners writing their first typed functions with flexible arguments.
**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** section quotes i

Question 10

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

TYPESCRIPTDECODED.

Easy

What are optional properties and optional chaining in TypeScript?

How do you make values immutable in TypeScript?

How does type inference work in TypeScript?

What is the difference between interface and type?

What are the basic types in TypeScript?

What is TypeScript and why use it over JavaScript?

What are tuple types?

What is tsconfig.json and which options matter most?

Medium

What is the definite assignment assertion (!)?

What are union and intersection types?

What do ReturnType and Parameters do?

What is ReadonlyArray and readonly tuple?

What are type assertions and when should you use them?

What are enums and what are their trade-offs?

How does TypeScript 5.5 infer type predicates for `Array.filter` automatically?

What is declaration merging?

Hard

How would you build a DeepPartial type?

What is module augmentation?

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