Create Object type from Array type where keys are values of property in each object from array

Question

Given a type that is an array of objects, how do I create a new type that is an object, where the keys are values of a specific property in the array objects, and the values of the object are the type of one of the object generics.

With JS code, it would look like this:

const obj = arr.reduce((acc, element) => ({
  ...acc,
  [element.name]: element.bar
}))

So far I have this:

type ArrayToObject<Arr extends Array<CustomType<any>> = []> = {
  [K in keyof Arr]: ExtractCustomTypeGeneric<Arr[K]>
}

type ExtractCustomTypeGeneric<A> = A extends CustomType<infer T> ? T : never;

type CustomType<T> = {
  name: string,
  foo: number,
  bar: T
}

This gives me an array object (since K in keyof Arr are index numbers), whereas I would want the property keys to be name. Unfortunately something like [Arr[K]['name'] for K in keyof Arr] isn't something that works.

T.J. Crowder · Accepted Answer · 2020-09-27 16:47:34Z

3

You can get a union of all the keys in the array using Arr[number]['name'], then you can use Extract to extract from the union of all possible elements in the array (Arr[number]) just the items that have the currently mapped name:

type ArrayToObject<Arr extends Array<CustomType<any>> = []> = {
  [K in Arr[number]['name']]: ExtractCustomTypeGeneric<Extract<Arr[number], { name: K }>>
}

type ExtractCustomTypeGeneric<A> = A extends CustomType<infer T> ? T : never;

type CustomType<T, K = string> = {
  name: K,
  bar: T
}

function createItems<T extends Array<CustomType<any, K>>, K extends string>(...a: T) : T{
    return a;
}

let a = createItems({ name: "A", bar: ""}, { name: "B", bar: 0})

type R = ArrayToObject<typeof a>
//Same as 
type R = {
    A: string;
    B: number;
}

Play

edited Sep 27, 2020 at 16:47

T.J. Crowder

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answered Sep 23, 2019 at 16:55

Titian Cernicova-Dragomir

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3 Comments

T.J. Crowder Over a year ago

Wow. :-) Suppose I wanted to do this without a function? E.g., my starting point was const fields = [{ name: "A", bar: ""}, { name: "B", bar: 0}]; and I wanted to get the type { A: string; B: number; }? I tried but failed to make it work without the function, embarrassingly. :-) (Context: To use Amazon's RDS efficiently I need to have a fields-like object so I know what property name to use to get the value of a column. So I'd prefer not to have to maintain both runtime and compile-time type information for my business objects...)

Titian Cernicova-Dragomir Over a year ago

@T.J.Crowder as const on the constant initalizer will almost get you there, but that will preserve the actual literal type instead of widening to string or number ({ A: ""; B: 0;}). Functions are still the best way to have fine grained control over inference.

T.J. Crowder Over a year ago

Thanks! Yeah, that was indeed the stumbling block for me, that I ended up with "" and 0 rather than string and number. That's too bad. I wanted to avoid having to pass both a type and a schema object to the function. But I'm stuck with either let users: User[] = executeQuery("select ...", UserSchema); or let users = executeQuery<User>("select ...", UserSchema);. I guess since the latter doesn't require the type gymnastics above, I'll use it. So close! :-)

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