Question 1

What are type assertions and when should you use them?

Accepted Answer

**Target Audience:** Developers who see `as` in code reviews and want a precise rule for when it is fine and when it hides a bug.
**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 run

Question 2

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 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

What does the satisfies operator do?

Accepted Answer

**Target Audience:** Developers who know type annotations and `as`, and want to know what `satisfies` adds and when to use it.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 and `--strict`. `satisfies` arrived in TypeScript 4.9 ([Announcing TypeScript 4.9](https://devblogs.microsoft.com/typescript/announcing-typescript-4-9/), November 2022).

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

Question 6

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 7

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

Question 8

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

Accepted Answer

**Target Audience:** Library authors moving their build to tsdown, Rolldown or another oxc-based tool, and anyone asked to make an existing codebase pass `isolatedDeclarations`.
**Difficulty:** Hard
**TypeScript version:** The page examples were checked with TypeScript 5.4.5 `--strict` (which has no `isolatedDeclarations` flag). Every `isolatedDeclarations` error and `.d.ts` file below came from TypeScript 5.9.3 and from oxc's isolated-declaration transform as bundled in rolldown 1.0.3 (`isolate

Question 9

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

Accepted Answer

**Target Audience:** Developers who already know what each of the three features does on its own and now have to pick between them when designing a function or library API.
**Difficulty:** Hard
**TypeScript version:** `satisfies` arrived in TypeScript 4.9 ([4.9 release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-9.html)), `const` type parameters in 5.0 ([5.0 release notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-5-0.html)) and `NoI

TYPESCRIPTDECODED.

Medium