I want to write a array to object function, which extracts one property as the index, like:
export const arrayToObject
= <T, K extends keyof T, V extends T[K] & string>(arr: T[], key: K): { [v: V]: T } =>
Object.assign({}, ...arr.map(item => ({[item[key]]: item})));
But it gives a compile error saying An index signature parameter type must be 'string' or 'number' for V, although I constraint the V type to be string. Why is the error and what is the proper way to do it?
V may extend T[K]. But since T is not constrained in any way when you index using item[key] all the compiler will know is that item[key] is of type T[K]. You need to constrain T so that for the key K the value will be of a type compatible with a property key.
export const arrayToObject
= <T extends Record<K, V>, K extends keyof T, V extends PropertyKey>(arr: T[], key: K): { [v in T[K]]: T } =>
Object.assign({}, ...arr.map(item => ({[item[key]]: item})));
let r = arrayToObject([{ key: "A", value: 0 }, { key: "B", value: 0 }], "key");
r.A.value //ok
r.B.value //ok
r.C.value //err
I also found an working solution, which seems a little bit lighter:
export const arrayToObject =
<T extends {[k in K]: number | string}, K extends keyof T>(arr: T[], key: K): { [k in T[K]]: T } =>
Object.assign({}, ...arr.map(item => ({ [item[key]]: item })));
