Is there some way to fix these types without casting?

[codingismy11to7]

Just picked up this library yesterday, and am trying to figure out why my code doesn't work without casting. I'm doing what I think would be a pretty standard thing for this lib, trying to use it for PATCHes.

i have a stored type and a patch type like (simplified)

type Stored = Readonly<{field1: ReqObj, field2?: OptObj}>;
type UpdateStored = Readonly<{field1?: ReqObj, field2?: OptObj}>;

where the update fields are all optional and the stored fields are a mixture of required and optional.

i'm using deepmerge in a patch function like

const DefaultStored: Stored = {...};

const updateStored = (patch: (old?: Stored) => Stored) => {...};

const doUpdate = (update: UpdateStored) => 
  updateStored(old => deepmerge(old ?? DefaultStored, update));

the code and functionality work fine, but unless i cast the update parameter or deepmerge return with as Stored, typescript complains that I'm returning an UpdateStored from the deepmerge call and refuses to compile. so right now i'm doing

const doUpdate = (update: UpdateStored) => 
  updateStored(old => deepmerge(old ?? DefaultStored, update) as Stored);

which compiles and works but makes me uneasy. just wondering if anybody else has run into anything like this or if i'm doing something obviously wrong

[RebeccaStevens]

Would this be a good representation of what you are trying to do?

import { deepmerge } from "deepmerge-ts";

type Stored = Readonly<{ field1: string; field2?: string }>;
type UpdateStored = Readonly<{ field1?: string; field2?: string }>;

const DefaultStored: Stored = {
  field1: "a"
};

const updateStored = (patch: (old?: Stored) => Stored) => patch({
  field1: "b"
});

const doUpdate = (update: UpdateStored) =>
  updateStored((old) => deepmerge(old ?? DefaultStored, update));

const result = doUpdate({
  field2: "c"
});

Because if so, I can't reproduce your issue. as Stored isn't need above.