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

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 4

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 5

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 6

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 7

How do you type async functions and Promises?

Accepted Answer

**Target Audience:** Developers who use `async`/`await` daily and want the types to be precise — return types, errors, and concurrent calls.
**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 `t

Question 8

What are declaration files (.d.ts)?

Accepted Answer

**Target Audience:** Developers who install packages, see "Could not find a declaration file", or need to publish a library with types.
**Difficulty:** Medium
**TypeScript version:** Every compiler message and output below was produced with TypeScript 5.4.5 (`--strict`, target ES2022). The multi-file projects in sections 3 to 6 were compiled separately with the same compiler, since the page runs a single file.

> **How to read this doc:** Each idea is explained in plain language first, then tagg

Question 9

How do you migrate a JavaScript codebase to TypeScript?

Accepted Answer

**Target Audience:** Developers and tech leads who need to move a working JavaScript codebase to TypeScript while the team keeps shipping.
**Difficulty:** Medium
**TypeScript version:** Every compiler message below was produced with TypeScript 5.4.5, both on the page's examples (`--strict`, target ES2022) and on a small mixed JavaScript/TypeScript project built with `tsc -p` and run with Node 22, step by step, as quoted.

> **How to read this doc:** Each idea is explained in plain language first

Question 10

What does the 'strict' flag in tsconfig enable?

Accepted Answer

**Target Audience:** Developers who have `"strict": true` in their config and want to know exactly what it switched on.
**Difficulty:** Medium
**TypeScript version:** The flag list and every error below come from TypeScript 5.4.5 (its option table and real `tsc` runs); the 5.6 and 6.0 changes are cited from 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 a

TYPESCRIPTDECODED.

Easy

What are optional properties and optional chaining in TypeScript?

How do you make values immutable in TypeScript?

What is the difference between any and unknown?

What is tsconfig.json and which options matter most?

How does TypeScript handle null and undefined safely?

Medium

What is the definite assignment assertion (!)?

How do you type async functions and Promises?

What are declaration files (.d.ts)?

How do you migrate a JavaScript codebase to TypeScript?

What does the 'strict' flag in tsconfig enable?

How do `noUncheckedIndexedAccess`, `exactOptionalPropertyTypes` and TS 5.6+ checks harden 2025 codebases?

What's new to type in TypeScript 5.5+ libs — `Array.fromAsync`, `Set` methods, `Iterator` helpers?

What is an index signature?

Hard

How would you build a DeepPartial type?

What are covariance and contravariance?