deep-pick-object.ts

// NaN also, but `typeof NaN` === number
type Falsy = false | null | undefined | 0 | -0 | 0n | "";

type AnyArray<T> = Array<T> | ReadonlyArray<T>

type DeepReplaceKeysPartialObj<Obj extends object, T> = {
    [key in keyof Obj]:
        Obj[key] extends AnyArray<infer Q> ?
                DeepReplaceKeys<Q, T> :
                    Obj[key] extends object ?
                        DeepReplaceKeys<Obj[key], T> :
                        T;
};

type DeepReplaceKeys<Obj, T> = Obj extends object ? DeepReplaceKeysPartialObj<Obj, T> : Obj;

type DeepPartial<T> = {
    [P in keyof T]?: DeepPartial<T[P]>;
};

type PickDeepObj<ReplaceType, ToPick extends object, PickObj extends DeepPartial<DeepReplaceKeys<ToPick, ReplaceType>>> = {
    [key in keyof PickObj]: 
         key extends keyof ToPick ? 
            ToPick[key] extends AnyArray<infer Q> ?
                Array<PickDeep<ReplaceType, Q, PickObj[key]>> :
                ToPick[key] extends object ? 
                    PickDeep<ReplaceType, ToPick[key], PickObj[key]> :
                    PickObj[key] extends Falsy ? 
                        never : 
                        ToPick[key] :
                            never
}

type PickDeep<ReplaceType, Obj, T> = Obj extends object ? PickDeepObj<ReplaceType, Obj, T> : Obj;

declare function pickDeep<Obj extends object, ObjDec extends DeepPartial<DeepReplaceKeys<Obj, true | false>>>
    (objToPick: Obj, pickObjDeclare: ObjDec): PickDeepObj<true | false, Obj, ObjDec>;

const pickedObject = pickDeep({
    test: {
        other: 900, 
        ignored: 9, 
        explicitNo: 77, 
        arr: [
            {
                other: 3000,
                ignored: 999
            }
        ]
    },
    one: 1
} as const, {
        test: {
            other: true,
            arr: {
                other: true
            },
            explicitNo: false
        }
} as const);

// No errors
pickedObject['test']['arr'][0]['other']; // Type should be 3000
pickedObject['test']['other']; // Type should be 900
pickedObject['test']['explicitNo']; // type should be "never"
// This should error
pickedObject['test']['arr']['other']
pickedObject['test']['arr'][0]['ignored'];