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

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 3

What do ReturnType and Parameters do?

Accepted Answer

**Target Audience:** Developers who keep writing the same type twice — once on a function, once next to it — and want to derive it instead.
**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 `ts

Question 4

What is ReadonlyArray and readonly tuple?

Accepted Answer

**Target Audience:** Developers who type function parameters and want to promise callers that their arrays will not be changed.
**Difficulty:** Medium
**TypeScript version:** Every compiler message and output below was produced with TypeScript 5.4.5 (`--strict`, target ES2022, lib ES2022 + DOM).

> **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 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 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 7

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 8

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 9

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

Question 10

What are `const` type parameters (`<const T>`) and why are they better than `as const`?

Accepted Answer

**Target Audience:** Developers writing helpers such as `defineConfig`, `defineRoutes` or typed event lists, who want callers to get exact literal types without extra syntax.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 and `--strict`. `const` type parameters arrived in TypeScript 5.0 ([PR #51865](https://github.com/microsoft/TypeScript/pull/51865), released March 2023).

> **How to read this doc:** Every idea is explained in plain language first, then tagged with

TYPESCRIPTDECODED.

Easy

What is the difference between any and unknown?

What are the basic types in TypeScript?

Medium

What do ReturnType and Parameters do?

What is ReadonlyArray and readonly tuple?

What are type assertions and when should you use them?

How do you type async functions and Promises?

How does the 'in' operator narrow types?

How do you migrate a JavaScript codebase to TypeScript?

What does the 'strict' flag in tsconfig enable?

What are `const` type parameters (`<const T>`) and why are they better than `as const`?

Hard

How do you use `infer` to build deep `Awaited`, `ReturnType` and `Unwrap` utilities?

What does `NoInfer<T>` do and when should you block inference?

Why are conditional types distributive and how do you control it with `infer`?

What are recursive types?