How to get the type of a property by name using generics

Question

I need a way to get the type of any property of a class or a type.

For example if I had the PersonClass and PersonType I want to get the nationalId type. I can do this:

class PersonClass{
    name: string
    nationalId: number
}

type PersonalType={
    name: string
    nationalId: string
}

type GetNationalIdType<T> = T extends {nationalId: infer U} ? U : never;
var nId3: GetNationalIdType<PersonClass>
var nId4: GetNationalIdType<PersonalType>

And it works ok nId3 is a number and nId4 is a string . But If I dont know how to do if I want to get any property. I try these:

// type GetProp<T, K> = T extends {[key: K]: infer U} ? U : never;
type GetProp<T, K extends string> = T extends {[key: K]: infer U} ? U : never;

var nId1: GetProp<PersonClass, "nationalId">
var nId2: GetProp<PersonalType, "nationalId">

And I get:

Answer 1

You just want lookup types . If T is an object type and K is the type of one of its keys (or a union of such keys), then T[K] is the type of the value for that key (or the union of the value types for those keys):

var nId1: PersonClass["nationalId"]; 
var nId2: PersonalType["nationalId"];

If you feel the need to define GetProp , it is pretty straightforward without conditional types:

type GetProp<T, K extends keyof T> = T[K];

or if you must allow K that is not assignable to keyof T :

type GetProp<T, K extends keyof any> = K extends keyof T ? T[K] : never;

or if you really want to use infer and conditional types , you need a mapped type like Record :

type GetProp<T, K extends keyof any> = T extends Record<K, infer V> ? V : never;

But really the simple lookup type is the way to go, in my opinion.

Hope that helps. Good luck!

Answer 2

You need to use the mapped type syntax to do this with conditional types:

type GetProp<T, K extends string> = T extends {[key in K]: infer U} ? U : never;

But I think you are really looking for a type query :

var nId1: PersonClass["nationalId"]
var nId2: PersonalType["nationalId"]