TypeScript type for "Object with key called ..."

Question

I want to make a function like

function sortByKey<T>(items: T[], key: string): T[] {
  return items.sort((a, b) => a[key] - b[key]);
}

I need T[key] to be a number, but I'm not sure how to express that.

If I knew the key ahead of time I could obviously just do {key: number} but that doesn't work here.

I tried something like sortByKey<K>(items: {[k: K]: number}[], key: K) but that gives the error "An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead." I looked into mapped types but they don't seem to do what I need. Is something like this possible in TypeScript?

Why wouldn't T extends { [x: string] : number } not work for you? Generics can be a part of index signature only if the signature is a template literal, and even that is a very recent addition to the language — 0Valt
– 0Valt, Commented Jan 7, 2022 at 19:07
@OlegValter that works if the object has only fields that are numbers, right? but what if T is like {i: number, name: string}? — QuinnF
– QuinnF, Commented Jan 7, 2022 at 19:10
Well, why not use a union then? :) or combine known props and an index signature? The gist of the idea is to use an index signature as a constraint for the generic type parameter — 0Valt
– 0Valt, Commented Jan 7, 2022 at 19:13

jcalz · Accepted Answer · 2022-01-07 19:12:43Z

4

You probably want the function to be generic both in T, the type of the items elements, and in K, the type of key. You can constrain K to be keylike (K extends PropertyKey) and constrain T to be a type with a number value at key K (T extends Record<K, number> using the Record<K, V> utility type):

function sortByKey<K extends PropertyKey, T extends Record<K, number>>(
  items: T[], 
  key: K
): T[] {
  return items.sort((a, b) => a[key] - b[key]);
}

You can't write {[k: K]: number} because that's an index signature which can't be generic. But you can write {[P in K]: number} using a mapped type. Mapped types are similar to but distinct from index signatures; see this answer for more information. Anyway, Record<K, number> is an alias for {[P in K]: number}, so you were getting close.

Playground link to code

answered Jan 7, 2022 at 19:12

jcalz

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2 Comments

QuinnF Over a year ago

Thanks, that works! I incorrectly assumed T extends Record<K, number> meant that T could only have keys of type K and they all had to be numbers

jcalz Over a year ago

A lot of people seem to think that TypeScript object types are "exact" or "sealed", but they're not. If A and B are object types and A extends B, it means that A must have all the known keys from B, but it doesn't mean that A can't have more. Indeed it would be a big problem if you couldn't add properties via extension like interface Foo extends Bar { baz: string }. See this for more info

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