Interface-handling regression in 5.4.7?

[jstasiak]

Hey, first of all thanks for developing this library.

We bumped into an interesting TS compilation problem when trying to upgrade from 5.4.6 to 6.0.0, I narrowed it down to version 5.4.7 and this commit: 3ab58ee.

I'm not entirely sure if we're doing something wrong or is there an actual regression.

A piece of code to reproduce the problem:

interface Camel {
    userName: string
}

function f1(c: Camel): {user_name: string} {
    const snake = snakecaseKeys(c)
    return snake
}

function f2(c: Camel[]): {user_name: string}[] {
    const snake = snakecaseKeys(c, { deep: true })
    return snake
}

With snakecase-keys 5.4.6 it compiles as expected.

With 5.4.7 it fails to compile:

file.ts:1496:33 - error TS2345: Argument of type 'Camel' is not assignable to parameter of type 'readonly any[] | Record<string, unknown>'.
  Type 'Camel' is not assignable to type 'Record<string, unknown>'.
    Index signature for type 'string' is missing in type 'Camel'.

1496     const snake = snakecaseKeys(c)
                                     ~

file.ts:1497:5 - error TS2322: Type 'readonly any[] | { [x: string]: unknown; }' is not assignable to type '{ user_name: string; }'.
  Property 'user_name' is missing in type 'readonly any[]' but required in type '{ user_name: string; }'.

1497     return snake
         ~~~~~~

  src/apiModels/v1Adapters.ts:1495:25
    1495 function f1(c: Camel): {user_name: string} {
                                 ~~~~~~~~~
    'user_name' is declared here.

file.ts:1502:5 - error TS2322: Type 'Camel[]' is not assignable to type '{ user_name: string; }[]'.
  Property 'user_name' is missing in type 'Camel' but required in type '{ user_name: string; }'.

1502     return snake
         ~~~~~~

  src/apiModels/v1Adapters.ts:1500:27
    1500 function f2(c: Camel[]): {user_name: string}[] {
                                   ~~~~~~~~~
    'user_name' is declared here.

TS version: 5.3.2

Happy to provide more details if that helps.

[guansss]

We are getting this problem too.

microsoft/TypeScript#50087 (comment) likely explains this. Maybe the Record<string, any> shouldn't be changed to Record<string, unknown>?

[yutak23]

The following problems recur when using any.
#116

I think the essence of the error lies in the index signature part.
sindresorhus/camelcase-keys#114 (comment)

The following methods may help.
sindresorhus/camelcase-keys#114 (comment)

[guansss]

Hi @yutak23 , thanks for the solution, it works for my case!

However, after diving deep into this issue, I think the fix for #116 in #117 is inaccurate because the use of Record<string, unknown> is just skipping the case conversion for anything that's not assignable to it, which includes interfaces and classes.

As a result, it breaks nested classes:

class Foo {
  someVar = 1
  someMethod() {}
}

const a = snakecaseKeys({ foo: new Foo() })
// the type is { foo: Foo }
// the value is { foo: { some_var: 1 } }

As well as nested interfaces:

interface Bar {
  someVar: number
}

const bar: Bar = { someVar: 1 }
const b = snakecaseKeys({ bar })
// the type is { bar: Bar }
// the value is { bar: { some_var: 1 } }

So I still think Record<string, any> should be used to ensure type correctness.

As for removing functions from the object (a fix for #116), I believe a simple condition will do the trick:

type WithoutFunctions<T extends Record<string, any>> = {
  [K in keyof T as T[K] extends Function ? never : K]: T[K]
}

type _Date = WithoutFunctions<Date>
// {}

type _Error = WithoutFunctions<Error>
// { name: string; message: string; stack?: string | undefined; }

[yutak23]

I believe that using this library to change the case of object other than plain object, such as instances of a class or function, is not the intended use of the library.

Although the readme simply mentions object, I think it specifically refers to plain object. This can be understood by looking at the readme of camelcase-keys, a similar library to snakecase-keys.