Parameters<typeof someFunction> does not preserve type constraint or parameter inference.

[DrSammyD]

Given the following two types.

type Channel = {
  [key: string]: number;
};

type T<C extends Channel> = {
  channel: C;
  props: (keyof C)[];
};

The following function is able to correctly constrain the type of the second parameter given the following function declaration and call.

function createObject<C extends Channel>(channel: C, props: (keyof C)[]): T<C> {
  return {
    channel,
    props,
  };
}

const someObject = createObject(
  {
    Stuff: 1,
    Yarn: 2,
  },
  ['Stuff', 'Yarn', 'Thing'] // If you try to add a string that is not a key of 'channel', you'll get a type error. Thing is inferred invalid, and I don't have to specify a generic property
);

However, this constraint is not preserved in the Parameters utility type.

let thing: Parameters<typeof createObject> = [
  {
    "Stuff": 1,
    "Yarn": 2,
  },
  ["Yarn", "Stuff", "Thing"] // So... that constraint wasn't preserved.
]

At least not until the generic is explicitly set.

let thingAgain: Parameters<typeof createObject<{"Stuff":number;"Yarn":number}>> = [
  {
    "Stuff": 1,
    "Yarn": 2,
  },
  ["Yarn", "Stuff", "Thing"] // And we're back!
]

Ultimately, the desired functionality is to have types obtain the same type inference as functions without a runtime requirement to keep the typing/code dry.

Playground link

[fatcerberus]

This would first require implementing a new kind of a generic type that would have its type parameter(s) inferred on assignment, as well as the ability for type aliases to directly accept and return generic types (HKT). Neither of these are possible with the current type system.

[fatcerberus]

a new kind of a generic type that would have its type parameter(s) inferred on assignment

For this, see #47755.

the ability for type aliases to directly accept and return generic types (HKT)

For HKT, there are several issues, notably #45438, #44875, and the seminal issue #1213.

[DrSammyD]

Thanks @fatcerberus for linking the other issues.

[DrSammyD]

I'm going to reopen this, because even if those other suggestions are implemented, Parameters will probably still be unconstrained.

[RyanCavanaugh]

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