Create an object's type in Typescript where its keys are derived from an array of strings

Question

Given:

const keys = ["A1", "B1", "C1"]

const obj = {}

keys.forEach((key) => {
  const subKey = key[0]
  obj[subKey] = key
})

// obj = { A: A1, B: B1, C: C1 }

Question: For the given scenario above, how do I declare obj's type?

Something like:

type Obj<T in keys> = {
  T: string;
}

Answer 1

Something close to what you want is possible. Firstly we need to improve on the type of the array. By default the keys array will be of type string[] this does not help us as we need the string literal types for the string values. This helper function will help with that:

function arrayHelper<T extends string>(a: T[]) : T[] {
    return a;
}
var keys = arrayHelper(["A1", "B1", "C1"]); // The type of keys  will be ("A1" | "B1" | "C1")[]

Now that we have the string literal types, we can easily define a type based off them :

var object = (<T extends string>(o: T[]) => 
{
    const obj: { readonly [P in T]: P} = <any>{}
    keys.forEach((key) => {
    obj[key] = key
    });
    return obj;
})(keys);

The type of object will be

var object: {
    readonly A1: "A1";
    readonly B1: "B1";
    readonly C1: "C1";
}

An object that has the original values as keys and the value of the same type as the string literal.

The key is the full value not a substring (it is not possible to perform complex mutations on property names), but this might be something you can live with.