typescript `Parameters` when calling method by name

Question

I am trying to type correctly following code:

interface SomeObject {
  fn1: (params: { a: string, b: object }) => void,
  fn2: (params: { c: number }) => number,
  fn3: () => number
}

type Methods = keyof SomeObject;

let someObject: SomeObject = {
  fn1: ({ a, b }) => console.log(a, b),
  fn2: ({ c }) => c,
  fn3: () => 1
}

const fn = <T extends Methods>({ method, parameters }: {
  method: T,
  parameters: Parameters<SomeObject[T]>
}) => {
  const _method = someObject[method]

  _method(parameters[0]);
}

fn({ method: 'fn1', parameters: [{a:'str', b:{}}] });

The problem that I encountered is that _method gets a following type:

const _method: (params: {
    a: string;
    b: object;
} & {
    c: number;
}) => number | void

So it takes an intersection of all possible parameters and thus making it impossible to use the fn({ method, parameters }) notation. Is there any workaround? (without using as or any)

You can find TypeScript playground here.

captain-yossarian from Ukraine · Accepted Answer · 2021-02-16 16:35:59Z

1

Here you have a small trick how to make it work:

interface SomeObject {
  fn1: (params: { a: string, b: object }) => void,
  fn2: (params: { c: number }) => number,
  fn3: () => number
}

type Methods = keyof SomeObject;

const someObject: SomeObject = {
  fn1: ({ a, b }) => console.log(a, b),
  fn2: ({ c }) => c,
  fn3: () => 1
};

type Args<T extends Methods> = {
  method: T,
  parameters: Parameters<SomeObject[T]>
}
const fn = <T extends Methods>(method: T, callback: (method: SomeObject[T]) => any) => callback(someObject[method]);

fn('fn1', (method) => method({ a: 'sdf', b: {} })) // ok
fn('fn1', (method) => method({ a: 2, b: {} })) // expected error

fn('fn2', (method) => method({ c:42})) // ok
fn('fn2', (method) => method({ c:'42'})) // expected error

fn('fn3', (method) => method()) //ok
fn('fn3', (method) => method(2)) //expected error

Playground link

I'm not specialist in type system theory, but I'd willing to bet the the problem caused by contravariance.

I will happy to hear any explanations )

answered Feb 16, 2021 at 16:35

captain-yossarian from Ukraine

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1 Comment

Konstantin Over a year ago

thank you for your response. I decided to open a bug github.com/microsoft/TypeScript/issues/42818 to seek the official explanation

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