Question 1

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 2

How does TypeScript handle null and undefined safely?

Accepted Answer

**Target Audience:** Beginners who have met "Cannot read properties of undefined" and want the compiler to catch it first.
**Difficulty:** Easy
**TypeScript version:** Compiler behaviour checked with TypeScript 5.4.5 (`tsc --strict`); version history checked against the release notes linked inline.

> **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

Question 3

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 4

What are assertion functions (asserts)?

Accepted Answer

**Target Audience:** Developers who know type guards and want to write validation helpers that narrow types without wrapping everything in `if` blocks.
**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 pr

Question 5

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 6

How do you type async functions and Promises?

Accepted Answer

**Target Audience:** Developers who use `async`/`await` daily and want the types to be precise — return types, errors, and concurrent calls.
**Difficulty:** Medium
**TypeScript version:** Every type claim below was checked by TypeScript 5.4.5 with `--strict`; release versions are 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 interviewer expects. Type claims are checked by `t

Question 7

How do `noUncheckedIndexedAccess`, `exactOptionalPropertyTypes` and TS 5.6+ checks harden 2025 codebases?

Accepted Answer

**Target Audience:** Developers on a codebase that already has `strict: true` and want to know which extra checks are worth turning on, and what newer TypeScript versions catch for free.
**Difficulty:** Medium
**TypeScript version:** The page examples were checked with TypeScript 5.4.5 `--strict`. The flag-on errors in sections 3 and 4 were produced by running TypeScript 5.4.5 on the same files with `--noUncheckedIndexedAccess --exactOptionalPropertyTypes`. Everything marked 5.5+, 5.6+, 5.7 or 5

Question 8

How do you safely work with JSON of unknown shape?

Accepted Answer

**Target Audience:** Developers who parse API responses, request bodies, config files or `localStorage` and want those values typed without lying to the compiler.
**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 s

Question 9

What is a polymorphic 'this' type?

Accepted Answer

**Target Audience:** Developers who write or use chainable APIs (builders, query builders, test helpers) and want subclass-friendly typing explained from first principles.
**Difficulty:** Medium
**TypeScript version:** Checked with TypeScript 5.4.5 and `--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. Type claims are checked by `tsc` (an `Equal<...>` line or a `

Question 10

What is an index signature?

Accepted Answer

**Target Audience:** Beginners who need to type a dictionary-like object, and anyone who has been bitten by `obj[key]` being `undefined` at run time.
**Difficulty:** Medium
**TypeScript version:** Every error and output below comes from TypeScript 5.4.5 with `--strict` (two flag-specific errors were produced by adding that one flag, as noted); 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 l

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