Question 1

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 2

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 3

What are access modifiers in TypeScript classes?

Accepted Answer

**Target Audience:** Beginners writing their first TypeScript classes, and developers from Java or C# who expect `private` to be enforced at run time.
**Difficulty:** Easy
**TypeScript version:** Every error and output below comes from TypeScript 5.4.5 with `--strict` (target ES2022, so `#private` fields run natively); version 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 i

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 mapped types?

Accepted Answer

**Target Audience:** Developers comfortable with `keyof` and indexed access types who want to transform whole object types.
**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:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. All output was produced by actually running the code

Question 6

What are utility types like Partial, Pick, and Omit?

Accepted Answer

**Target Audience:** Developers who write interfaces by hand and want to derive the variants (patch, create, preview) instead of copying them.
**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

Question 7

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

Question 8

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

Accepted Answer

**Target Audience:** Developers who want to use the newest built-in JavaScript APIs from TypeScript and need to understand where the types come from and what they promise.
**Difficulty:** Medium
**TypeScript version:** The page runs TypeScript 5.4.5, whose libs have **none** of these three APIs, so the runnable examples use typed userland equivalents. The real lib signatures quoted below were read from, and checked with, **tsc 5.9.3** (`--strict --lib ESNext,DOM`) and **tsc 6.0.2**; that program

Question 9

What is type widening and how do you prevent it?

Accepted Answer

**Target Audience:** Developers who have seen "Type 'string' is not assignable to type '"GET" | "POST"'" and want to know why it happens and which fix to use.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 and `--strict`. The widening rules date from TypeScript 2.1 ([PR #10676](https://github.com/microsoft/TypeScript/pull/10676)).

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

Question 10

How does `satisfies` validate shape without widening literal types?

Accepted Answer

**Target Audience:** Developers who already use `satisfies` and want to predict exactly which types it keeps, for config objects, route maps and lookup tables.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 and `--strict`; every "kept" or "widened" claim below is an `Equal` check that tsc verified.

> **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 expec

TYPESCRIPTDECODED.

Easy

How do you make values immutable in TypeScript?

What are tuple types?

What are access modifiers in TypeScript classes?

Medium

What is ReadonlyArray and readonly tuple?

What are mapped types?

What are utility types like Partial, Pick, and Omit?

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

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

What is type widening and how do you prevent it?

How does `satisfies` validate shape without widening literal types?

What are branded (nominal) types?

Hard

What are covariance and contravariance?

How do you choose between `NoInfer<T>`, `satisfies`, and `const` type params when designing APIs?