What are generics and why are they useful? | Interview Question

Q: What are generics and why are they useful?

**Target Audience:** Developers who can write basic TypeScript types and keep meeting ` ` in library code. **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; the **Verified** section quotes it in full. ## 1. Why This Even Matters — A Story First Think of a parcel locker at a train station. The locker does not care what you put inside: a book, a cake, a laptop. But it does make one promise: **whatever you put in is exactly what you get back out**. Put in a cake, take out a cake, never a surprise laptop. Now imagine two ways to describe that locker in code: - "It holds *anything* and returns *anything*." That is `any`. True, but useless: when you open it, nobod

The problem

Question presented to candidate: "What are generics in TypeScript? Show me a function where a generic is better than any, and tell me where you see generics in everyday code."

What a strong answer should cover:

A generic is a type parameter (<T>): a placeholder filled in per call or per use, so one piece of code works for many types without losing type information
Why not any: any switches checking off for the result, while T links the input type to the output type (first<T>(items: T[]): T | undefined)
Type arguments are usually inferred from the call, and can be passed explicitly (first<string>([])); a const result keeps a literal type
Generics appear on functions, type aliases, interfaces and classes: Array<T>, Promise<T>, Map<K, V>, Result<T, E>, a Stack<T>
Generics are erased: they do not exist at run time, so you cannot new T() or check typeof T
Rule of thumb: a type parameter should appear at least twice (relate two things); otherwise a plain type is simpler

Code / implementation expected: Yes — a small generic function (identity, first, pair) and ideally a generic interface or class.

generics

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Developers who can write basic TypeScript types and keep meeting `` in library code. 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; the Verified section quotes it in full.

1. Why This Even Matters — A Story First

Think of a parcel locker at a train station. The locker does not care what you put inside: a book, a cake, a laptop. But it does make one promise: whatever you put in is exactly what you get back out. Put in a cake, take out a cake, never a surprise laptop.

Now imagine two ways to describe that locker in code:

"It ho

The solution is hidden so you can try first

10 min read · 10 sections

Run the code

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

TypeScriptany versus a generic: inference and explicit type arguments

Practice

TypeScriptGeneric type aliases, interfaces and classes

Practice

Discussion

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