Function that creates a new object with properties set to the values of the input objects specific key in typescript

Consider the following code:

type namedObject = {
    name: string
}

declare function createObjectWithKey<TObjects extends namedObject[]>(...namedObject: TObjects): {
    [key in TObjects[number]['name']]: number
}

const a = createObjectWithKey({
    name: 'test'
}, {
    name: 'test2'
});

/** What I would want here, is that a be typed as an object that has a "test" and "test2" property, which values should be "number", like this
 * 
 * type createdObjectType = {
 *     'test': number,
 *     'test2': number
 * }
 */

How could I write the functions signature, so that the return type is an object which I want?

Link to playground

I made it work by slightly changing your function and adding as const.

type namedObject = {
    name: string
}

declare function createObjectWithKey<TObjects extends readonly namedObject[]>(namedObject: TObjects): {
    [key in TObjects[number]['name']]: number
}

const a = createObjectWithKey([{
    name: 'test'
}  , {
    name: 'test2'
}] as const);

It works because when you give as const, TS treats the whole array as an immutable thing. It knows that the name will not be any random string, it will only be these specific strings. That is, typeof namedObject[0].name is not string, it's "test".

playground