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 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 3

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 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

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

Accepted Answer

**Target Audience:** Developers who use `Array.filter` and type guards daily and want to know exactly when TypeScript 5.5+ narrows the result for them.
**Difficulty:** Medium
**TypeScript version:** Inferred type predicates shipped in TypeScript 5.5 ([TypeScript 5.5 announcement](https://devblogs.microsoft.com/typescript/announcing-typescript-5-5/); the feature was implemented by Dan Vanderkam, see [Effective TypeScript: TS 5.5](https://effectivetypescript.com/2024/07/02/ts-55/)). The page runs

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 is the difference between void, undefined and never as return types?

Accepted Answer

**Target Audience:** Developers who annotate return types and want to know exactly what `void`, `undefined` and `never` promise to the caller.
**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

Question 10

What does `arethetypeswrong` (attw) detect and how do you fix false ESM/CJS packages?

Accepted Answer

**Target Audience:** Library authors who publish to npm, and app developers who hit a strange type error when importing a package that runs fine.
**Difficulty:** Medium
**TypeScript version:** The page examples were checked with TypeScript 5.4.5 (`--strict`). The package reproductions in sections 4 to 7 are small multi-file projects compiled with TypeScript 5.4.5 (and 5.9.3 where noted, `module` / `moduleResolution` `nodenext`) and run with Node.js 22.22. attw itself was **not** run here (it is

TYPESCRIPTDECODED.

Easy

How does type inference work in TypeScript?

What is the difference between interface and type?

What are the basic types in TypeScript?

What is tsconfig.json and which options matter most?

What are access modifiers in TypeScript classes?

Medium

What is the definite assignment assertion (!)?

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

What is declaration merging?

What is the difference between void, undefined and never as return types?

What does `arethetypeswrong` (attw) detect and how do you fix false ESM/CJS packages?

How do you debug type-resolution and ESM/CJS mismatches with `traceResolution` and `explain`?

What does the declare keyword do?

What are declaration files (.d.ts)?

Hard

How do you author library types that pass `isolatedDeclarations` for `oxc` / `tsdown` builds?

What is module augmentation?

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

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