What is the type obtained through Typescript's keyof operator?

I am looking to achieve something similar to the following:

interface StateItem<T extends StateItemType>{
    id: string;
    values: {
        [key in keyof T]: Provider<corresponding typeof T>
    }
}

type Primitive = number | string | Position; 

interface StateItemType {
    [key: string] : Primitive; 
}

interface Mover extends StateItemType  {
    center: Position; 
    speed: number; 
    vector: Position; 
    accruedVector: Position;
}

const mover : StateItem<Mover> = {
    id: "123", 
    values: {
         center: createRandomPosition(), 
         speed: createRandomNumber(), 
         vector: createRandomPosition(), 
         accruedVector: createRandomPosition(), 
    }
}


In essence, I want an interface T that represents a simple key:type mapping. When creating a StateItem<T>, all instances of it should include a values object containing all those keys and Provider<type> instances.

Could someone provide guidance on how I can achieve this?

Answer №1

If you wish to retrieve the property value type for each key in T, you can utilize the following method:

interface StateItem<T extends StateItemType>{
    id: string;
    values: {
        [key in keyof T]: Provider<T[key]>>
    }
}

The usage of

{[key in keyof T]: Provider<T[key]>}
constitutes a mapped type. It generates a fresh type, assigning a new property value type to each key in T.

The variable key refers to the presently utilized key in the mapped type and resembles the functionality of a for...in loop, enabling sequential access to individual properties via an index variable. One can also incorporate key within the definition of the property value type in the mapped type. Typically, brief names such as P or K are employed to differentiate these index type variables from genuine types.

Within the mapped type structure, T[key] would retrieve the property value type linked to the currently accessed property key preserved by the key variable inside T (the kind being mapped). Therefore, something akin to this is essentially a NOOP:

type T1 = { foo: string; bar: number }
type T2 = {[K in keyof T1]: T1[K]} // an unremarkable type...

This explanation hopefully offers some clarity on the concept!

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