What is key remapping in mapped types? | Interview Question

Q: What is key remapping in mapped types?

**Target Audience:** Developers who can write a basic mapped type and want to rename or filter keys while mapping. **Difficulty:** Hard **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 assumes [mapped types](/interview-question/what-are-mapped-types). The string tricks used for new names (`` `get${...}` ``, `Capitalize`) are [template literal types](/interview-question/what-are-template-literal-types), and the filters use [conditional types](/interview-question/what-are-conditional-types); each has its own page. ## 1. Why This Even Matters — A Story First A label printer in a warehouse goes through every box on a shelf. A plain m

The problem

Question presented to candidate: "In a mapped type you can write as after the key. What does that do? Show me how you would generate getter names like getName from a Person type, and how you would drop some keys."

What a strong answer should cover:

The as clause, { [K in keyof T as NewKey]: ... }, computes the output key from each input key; added in TypeScript 4.1 together with template literal types
Renaming with template literal types and the intrinsic string helpers: `get${Capitalize<string & K>}`, and why string & K is needed (keys can be number or symbol)
Filtering: a key remapped to never is dropped, by key (Exclude<K, "kind">) or by value (T[K] extends V ? K : never)
Mapping over a union of objects rather than keys, e.g. an event union to an onClick / onKey handler map
Behaviour details: modifiers are still copied from keyof T, keys that collide merge into a union, and a filtering as turns an array or tuple into an object of all its members
The run-time object still has to be built by hand with matching names

Code / implementation expected: Yes — a Getters<T> type and a filter that keeps only properties of a given value type.

key-remapping

Try it yourself

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

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

Solution

Target Audience: Developers who can write a basic mapped type and want to rename or filter keys while mapping. Difficulty: Hard 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 assumes mapped types. The string tricks used for new names (`get${...}`, Capitalize) are template literal types, and the filters use conditional types; each has its own page.

1. Why This Even

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.

TypeScriptRenaming and filtering keys with as

Practice

TypeScriptstring & K, collisions, unions of objects and tuples

Practice

Discussion

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