I'm trying to define a function typed with FlowJS using generics and I don't understand why it's raising an error in this function since I've got another one similar which works just fine.

For example, this is the one that is correct. Takes a map object and returns an array of objects.

const toArray = <T: { code: string }, U: { [code: string]: T }>(mapObject: U): T[] =>
  Object.keys(mapObject).map((code: string): T => mapObject[code]);

Usage example

const ITEM1 = {code: "code1", otherObjProp: "otherProp"};
const ITEM2 = {code: "code2", otherObjProp: "otherProp"};

const object = { code1: ITEM1, code2: ITEM2 };

toArray(object); // [ITEM1, ITEM2] 

Well, in the order hand I've this other function, supposed to do the inverse job. Received an array of objects and gets a map object. In getting error message Cannot call array.reduce because object literal [1] is incompatible with U

const toMapObject = <T: { code: string }, U: { [code: string]: T }>(array: T[]): U =>
  array.reduce(
    (acc: U, item: T): U => ({
      ...acc,
      [item.code]: item,
    }),
    {}
  );

Usage example

const ITEM1 = { code: "code1", otherObjProp: "otherProp" };
const ITEM2 = { code: "code2", otherObjProp: "otherProp" };

const array = [ ITEM1, ITEM2 ];

toMapObject(array); // { code1: ITEM1, code2: ITEM2 } 

Any idea why this could be happening? Thx!