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 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 3

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 4

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

Question 5

How do you get a union of an object's values as a type?

Accepted Answer

**Target Audience:** Developers who want a single constant object to be the source of truth for both run-time values and 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

Question 6

What does as const do?

Accepted Answer

**Target Audience:** Developers who have seen `as const` in codebases and want to know exactly what it changes and when to use it.
**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 ru

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