What are generic constraints? | Interview Question

Q: What are generic constraints?

**Target Audience:** Developers who know what ` ` means and want to make generics that can actually do something with `T`. **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. If generics themselves are new, start with [What are generics and why are they useful?](/interview-question/what-are-generics-and-why-are-they-useful); this page assumes you can read `function first (items: T[]): T | undefined`. ## 1. Why This Even Matters — A Story First A car-rental desk will rent you *any* car, which sounds very flexible. But to rent it to you, they need one thing: **a driving licence**. They do not care whether you are a st

The problem

Question presented to candidate: "What is a generic constraint? Write a function that only accepts things with a length, and a getProperty(obj, key) that rejects keys the object does not have."

What a strong answer should cover:

T extends X sets an upper bound: T can be X or any subtype of it, and inside the function you may use the members X guarantees
Without a constraint, x.length on a T is an error; with T extends { length: number } it is allowed, and the caller type (string, array) is still preserved in the return
Constraints can refer to other type parameters: K extends keyof T, returning T[K]
Why a constraint beats a plain parameter type: (x: HasId) => HasId forgets extra fields, <T extends HasId>(x: T) => T keeps them
The classic error: returning { id: 0 } as T fails because T "could be instantiated with a different subtype of constraint"
Variants: T extends object, unions (T extends string | number), intersections for several requirements, constructor types (new () => T)

Code / implementation expected: Yes — a longest or getProperty with a constraint, and the error it prevents.

generics
constraints

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 know what `` means and want to make generics that can actually do something with T. 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.

If generics themselves are new, start with What are generics and why are they useful?; this page assumes you can read function first(items: T[]): T | undefined.

1. Why This Even Matters — A Story First

A car-rental desk will rent you any car, which sounds very flexible. But to ren

The solution is hidden so you can try first

10 min read · 11 sections

Run the code

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

TypeScriptConstraints: length, K extends keyof T and unions

Practice

TypeScriptKeeping the caller type and the different subtype error

Practice

Discussion

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