What are indexed access types?

Q: What are indexed access types?

**Target Audience:** Developers who know basic object types and want to reuse parts of existing types instead of copying them. **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. This page is the building block. The [`keyof` and `typeof`](/interview-question/what-do-keyof-and-typeof-do-in-typescript) operators that usually feed it have their own page, and the most common applied recipe, turning an object's values into a union, is in [How do you get a union of an object's values as a type?](/interview-question/how-do-you-get-a-union-of-an-object-s-values-as-a-type) ## 1. Why This Even Matters — A Story First Picture a bi

The problem

Question presented to candidate: "TypeScript lets you write Person["name"] in a type. What is that, what can you put inside the brackets, and where would you actually use it?"

What a strong answer should cover:

An indexed access type T[K] looks up the type of property K on type T, using element-access syntax at the type level (added with keyof in TypeScript 2.1)
The index must be a type: a literal key, a union of keys (gives a union of property types), keyof T (all value types), number for array elements, or a literal index or "length" on a tuple
T[K] is checked: a key that does not exist is error TS2339, and a value used as the key needs typeof
The generic pattern K extends keyof T with return type T[K], so getters and setters follow the key the caller passes
Practical use: drilling into large generated or third-party types (Api["data"]["users"][number]) instead of re-declaring them
Edge cases: optional properties include undefined, index signatures are indexed by their key type, a union of objects only exposes keys every member has

Code / implementation expected: Yes — derive a few types with T[K] and write a typed getProp<T, K extends keyof T>.

index-access

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 basic object types and want to reuse parts of existing types instead of copying them. 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.

This page is the building block. The keyof and typeof operators that usually feed it have their own page, and the most common applied recipe, turning an object's values into a union, is in [How do you get a union of an object's values as a type?](/interview-question/how-do-you-get

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.

TypeScriptThe forms of T[K] and a generic getter

Practice

TypeScriptEdge cases: optional keys, index signatures, unions and a typed setter