Question 1

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 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

How does `using` / `await using` with `Symbol.dispose` work in TypeScript 5.2+?

Accepted Answer

**Target Audience:** Developers who write cleanup code (closing connections, releasing locks, deleting temp files) and want to understand TypeScript's `using` declarations from zero.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 `--strict` (the page compiler; `using` has been supported since 5.2). The page compiles to ES2022, so it runs TypeScript's downlevel helpers. As a cross-check, both examples were also compiled to ESNext (native `using` syntax) and run in Bu

Question 5

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 6

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 7

How do you do exhaustiveness checking?

Accepted Answer

**Target Audience:** Developers who `switch` over unions and want the compiler to flag every place that must change when a new variant is added.
**Difficulty:** Medium
**TypeScript version:** Every error message below was produced by TypeScript 5.4.5 with `--strict`; the `satisfies` version fact is cited from the 4.9 release notes.

> **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 e

Question 8

What is declaration merging?

Accepted Answer

**Target Audience:** Developers who have seen the same interface name declared twice, or `namespace` blocks next to a function, and want the rules.
**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 a

Question 9

What does the declare keyword do?

Accepted Answer

**Target Audience:** Developers who have seen `declare` in a codebase or a `.d.ts` file and want to know exactly what it does and does not do.
**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 actual

Question 10

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

TYPESCRIPTDECODED.

Easy

How does type inference work in TypeScript?

What are tuple types?

What are access modifiers in TypeScript classes?

Medium

How does `using` / `await using` with `Symbol.dispose` work in TypeScript 5.2+?

What is the definite assignment assertion (!)?

What is type narrowing and what are type guards?

How do you do exhaustiveness checking?

What is declaration merging?

What does the declare keyword do?

How does the 'in' operator narrow types?

What are declaration files (.d.ts)?

Hard

What is module augmentation?

What does the infer keyword do?

What are covariance and contravariance?

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