How to infer a typed array from a dynamic key array in typescript?

Question

I'm looking to type a generic object and have properties of that object return in a typed array. The ability to get a singular typed property from an object is documented and works, however i'm unable to get this to work with an array. Seems the 'union type'.

// from the documentation 
// @ http://www.typescriptlang.org/docs/handbook/advanced-types.html
function getProperty<T, K extends keyof T>(o: T, name: K): T[K] {
    return o[name];
}

const a = getProperty(person, 'age');
// a: number
const n = getProperty(person, 'name');
// n: string

const getProperties = <T>(obj: T, keys: Array<keyof T>) => keys.map((arg) => getProperty(obj, arg));

const [a2, n2] = getProperties(person, ['name', 'age']);
// result:
// a2: string | number
// n2: string | number

// what i want:
// a2: string
// n2: number

Titian Cernicova-Dragomir Titian Cernicova-Dragomir · Accepted Answer · 2018-06-09T17:06:19

The problem is that keys can be any key of T so when you call getProperty inside getProperties the result will be of T[keyof T] meaning any value of T. For getProperty(person, 'age') it works because name will be inferred as the string literal type 'age' and the result will thus be T['age'] which is number.

To accomplish what you want, we need a separate generic parameter for each key that will be passed. We can't support an arbitrary amount of keys but, using overloads, we can support a certain finite amount of keys which will be good enough for most cases (I added overloads for up to 3 keys, you can easily add more):

const person = {
    age: 1,
    name: ""
}
function getProperty<T, K extends keyof T>(o: T, name: K): T[K] {
    return o[name];
}


function getProperties<T, K extends keyof T, K2 extends keyof T, K3 extends keyof T>(obj: T, keys: [K, K2, K3]): [T[K], T[K2], T[K3]]
function getProperties<T, K extends keyof T, K2 extends keyof T>(obj: T, keys: [K, K2]): [T[K], T[K2]]
function getProperties<T, K extends keyof T>(obj: T, keys: [K]): [T[K]]
function getProperties<T>(obj: T, keys: Array<keyof T>) {
    return keys.map((arg) => getProperty(obj, arg));
} 

const [a2, n2] = getProperties(person, ['name', 'age']); // a2: string, n2: number

How to infer a typed array from a dynamic key array in typescript?

1 Answers