Types not inferred correctly for Arrays with additional properties anymore

[doepnern]

What version of Hono are you using?

6.6.10

What runtime/platform is your app running on? (with version if possible)

Browser

What steps can reproduce the bug?

import type { JSONParsed } from 'hono/utils/types';

type TestWorking = { foo: 'bar' }[];
type TestBroken = { foo: 'bar' }[] & { meta: 'v1' };
type ResultWorking = JSONParsed<TestWorking>;
type ResultBroken = JSONParsed<TestBroken>;

What is the expected behavior?

type ResultWorking = {
    foo: "bar";
}[]

What do you see instead?

type ResultBroken = {
    [x: number]: {
        foo: "bar";
    };
    length: number;
    toString: null;
    toLocaleString: null;
    pop: null;
    push: {};
    concat: {};
    join: {};
    reverse: null;
    shift: null;
    slice: {};
    sort: {};
    splice: {};
    unshift: {};
    indexOf: {};
    ... 27 more ...;
    meta: "v1";
}

Additional information

We are using postgres.js and sometimes want to return a RowList, which is an Array, extended with some additional properties.
When returning an Array with additional properties in hono, it will not be resolved as a simple Array (even though the JSON transformation will remove the additional properties).

So in this case, the frontend code will expect the returned Response to have a property meta, even though it will be stripped in transport. This was working as expected in an earlier version. See https://github.com/honojs/hono/blob/2d0135956c96e00fee48fb5faf287dbc9daf7f4f/src/utils/types.ts

[yusukebe]

Hi @doepnern

Sorry, I can't fully understand your problem. If the following:

type T = JSONParsed<TestBroken>

What do you expect the type of T?

And I think JSON string can't have { foo: 'bar' }[] & { meta: 'v1' } structure. Do you mean it should be { foo: 'bar' }[]?

[yusukebe]

@doepnern

This means if you have the following object and you do JSON.stringify, the value will be [1, 2, 3], which means the type should also be number[], right?

const data = Object.assign([1, 2, 3], { foo: 'bar' })
console.log(JSON.stringify(data)) // [1,2,3] => number[]

@m-shaka I'll investigate it later but, can you take a look at this?

[doepnern]

@yusukebe Yeah, I would expect it to be the same as the the one without the additional properties, a simple Array. As long as your code (and the typescript) detects the type to be an array the output will loose all additional properties anyway when being stringified.