Error when trying to use `pick` utility on a Struct constructed with `Describe` utility function

Hi @ianstormtaylor, I followed your suggestion here https://github.com/ianstormtaylor/superstruct/issues/526#issuecomment-733209466 and everything works Ok except when I tried to use pick<Struct, ['field-1', 'field-2', ... , 'field-x']> where the Struct is defined using the Describe type utility function that helps to ensure that structs match types. For example, the following:

type User = {
  id: number
  name: string
  age: number
  address: string
}

const User: Describe<User> = object({
  id: number(),
  name: string(),
  age: number(),
  address: string()
})

const basicDetails = pick<User, ['name', 'age']>

will throw the error Argument of type 'Describe<User>' is not assignable to parameter of type 'Struct<{ [x: string]: any; }, Record<string, Struct<any, any>>>'. But if I remove : Describe<User> then it works Ok as expected.

[nwaughachukwuma]

Thanks @ianstormtaylor, you’re patient and helpful.

[ianstormtaylor]

@nwaughachukwuma I don’t think you’re using the pick utility correctly, at least based on that snippet. The <...> brackets are for typings, not for runtime values. You likely want:

import {string, number, Describe, object, nullable, pick} from 'superstruct'

type Order = {
  id: string 
  pid: string | null
  userId: string
  quantity: number
}

const Order: Describe<Order> = object({
  id: string(),
  pid: nullable(string()),
  userId: string(),
  quantity: number()
})

const basicDetails = pick(Order, ['pid', 'userId'])

Notice the (...) parentheses instead. (And you need to import pick as well.)

If you were looking to use the typing-level utility that’s built in to TypeScript, you’d do:

type BasicDetails = Pick<Order, 'pid' | 'userId'>

Notice <...>, uppercase Pick, and no need to import it. And this would be creating a new type not a struct or other runtime value.

I realize the difference between compile-time typings and runtime values can often be confusing, I get tripped up on that every once in a while too. That’s the mental model complexity cost paid to have things named the same, which has other benefits.