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

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?

CodePudding user response：

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.

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