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 literal types?

Accepted Answer

**Target Audience:** Beginners who know basic TypeScript types and want to model "only these exact values".
**Difficulty:** Easy
**TypeScript version:** Compiler behaviour checked with TypeScript 5.4.5 (`tsc --strict`).

> **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 every output shown was produced by actually running the code; the **Verified**

Question 4

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 5

What are generic constraints?

Accepted Answer

**Target Audience:** Developers who know what `<T>` means and want to make generics that can actually do something with `T`.
**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 cod

Question 6

What are union and intersection types?

Accepted Answer

**Target Audience:** Developers who know the basic types and want to combine them into real-world shapes.
**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 running the code; the **Ver

Question 7

What are enums and what are their trade-offs?

Accepted Answer

**Target Audience:** Developers deciding how to model a fixed set of values (statuses, roles, directions) and wanting the real behaviour of enums, not folklore.
**Difficulty:** Medium
**TypeScript version:** Behaviour checked with TypeScript 5.4.5 and `--strict`; the 5.0 and 5.8 changes are cited from release coverage.

> **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

Question 8

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 9

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 10

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

TYPESCRIPTDECODED.

Easy

How does type inference work in TypeScript?

What is the difference between interface and type?

What are literal types?

Medium

What is type narrowing and what are type guards?

What are generic constraints?

What are union and intersection types?

What are enums and what are their trade-offs?

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

How do you do exhaustiveness checking?

What are mapped types?

What are indexed access types?

Hard

What is key remapping in mapped types?

How would you build a DeepPartial type?

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

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

What are template literal types?

What are recursive types?

What does the infer keyword do?

What are conditional types?