In TypeScript, how to qualify the index value must be the same as the id field of the corresponding property

Here is a type definition in TypeScript:

interface Obj {
id:string
}

I want to define a collection of Obj called ObjCollection, where the index of ObjCollection must be the same as the id field.

const objcollection: ObjCollection = {
  key1: { id: "key1" },
  key2: { id: "key2" },
  key3: { id: "key33333" } // Error, id should be key3
}

So how should ObjCollection be defined?

>Solution :

There is no specific ObjCollection type in TypeScript that meets your requirement. You can instead define a generic type like

type ObjCollection<K extends PropertyKey> =
    { [P in K]: { id: P } }

which expresses this idea, but it depends on supplying a type argument corresponding to a set of keys. So your objcollection would be of type ObjCollection<"key1" | "key2" | "key3">. To save you from having to write that yourself, you can write a generic helper function to have the compiler infer it:

const asObjCollection = <K extends PropertyKey>(
    oc: ObjCollection<K>) => oc;

Then instead of writing const objcollection: ObjCollection = {⋯}, you’d write const objcollection = asObjCollection({⋯}). It’s not the same, but it’s pretty close, and it gives you the desired behavior:

const objcollection = asObjCollection({
    key1: { id: "key1" },
    key2: { id: "key2" },
    key3: { id: "key33333" } // error!
});

Playground link to code