What are abstract classes? | Interview Question

Q: What are abstract classes?

**Target Audience:** Developers who know basic classes and want to know what `abstract` adds, when it beats an interface, and what it does not do. **Difficulty:** Medium **TypeScript version:** Checked with TypeScript 5.4.5 and `--strict`; version facts (4.2, 4.3) are cited from their release material. > **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` (a `// @ts-expect-error` line fails the build if the error does not happen), and the **Verified** section quotes the real output in full. ## 1. Why This Even Matters — A Story First A franchise hands every new restaurant a binder. Most pages are finished: how to greet guests, how to take payment, how to close the till at night. But two pages are blank, with a note in bold: **"Your menu goes here"** and **"Your opening hours go here."** Nobody can open a restaurant with the binder al

The problem

Question presented to candidate: "What is an abstract class in TypeScript? When would you reach for one instead of an interface, and what does the abstract keyword actually do at run time?"

What a strong answer should cover:

An abstract class cannot be instantiated with new; it exists to be extended, and it can mix finished code (fields, constructor, concrete methods) with abstract members that have no body
Every non-abstract subclass must implement all abstract members, or tsc reports an error; a subclass can also stay abstract and pass the duty down
The template method pattern: a concrete base method calls abstract hooks that each subclass fills in
Abstract class vs interface: the class carries shared code and state and survives to run time (instanceof works), an interface is a pure shape with no code and is erased
abstract itself is compile-time only: the emitted JavaScript is an ordinary class, so the "cannot instantiate" rule is a type check, not a run-time guard
Constructor types: new (...) => T rejects an abstract class, abstract new (...) => T (TypeScript 4.2) accepts it, which is what mixins over abstract bases need

Code / implementation expected: Yes — a small abstract base class with one abstract method, two subclasses, and a concrete method that uses the abstract one.

abstract

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Solution

Target Audience: Developers who know basic classes and want to know what abstract adds, when it beats an interface, and what it does not do. Difficulty: Medium TypeScript version: Checked with TypeScript 5.4.5 and --strict; version facts (4.2, 4.3) are cited from their release material.

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 (a // @ts-expect-error line fails the build if the error does not happen), and the Verified section quotes the real output in full.

1. Why This Even Matters — A Story First

A franchise hands every new restaurant a binder. Most pages are finished: how to greet guests, how to take payment, how to close the till at night. But two pages are blank, with a note in bold: **"Your men

The solution is hidden so you can try first

11 min read · 11 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

TypeScriptAn abstract base class with a template method

Practice

TypeScriptAbstract class vs interface, and abstract constructor types

Practice

TypeScriptThe constructor-order trap with abstract hooks

Practice

Discussion

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