Classifying Union Types based on their distinct characteristics

There is a specific type with its own unique property (method)

type Functions = {
  method: "connect",
  request: number,
  response: number,
} | {
  method: "remove",
  request: string,
  response: string,
}

I aim to create a function that can handle input and output values of the specified type based on the distinct field.

type Func = <T extends Functions>(input: Pick<T, 'method' | 'request'>) => Pick<T, 'method' | 'response'>; 

However, when I use this implementation, for example:

const result = func({ method: "connect", request: 10 })
result.response 

The resulting type for the response field shows as:

string || number

Is there a way to refine the type without additional verification of the method property? (Possibly by altering the Func type)

Answer №1

The issue stems from the function call

fn({ method: "connections", request: 10 })

having this type:

const fn: <Actions>(a: Pick<Actions, "method" | "request">)

This means that the type parameter A is inferred as Actions, which encompasses all possible types of Action, rather than the specific Action type being used. This can lead to unexpected behaviors beyond just a generalized return type. For example:

fn({ method: "connections", request: "weird" })

will compile without error, because the type of request, A['request'], resolves to Action['request'], resulting in number | string, allowing for values like "weird" ...

Unfortunately, there doesn't seem to be a straightforward solution with the provided action specification. However, if we define the actions more explicitly like this:

type Actions = {
  connections: {
    request: number,
    response: number,
  },
  delete: {
    request: string,
    response: string,
  }
}

then the following code works as intended:

type Fn = <M extends keyof Actions>(a: {method: M, request: Actions[M]['request']}) => {method: M, response: Actions[M]['response']};

declare const fn: Fn;

fn({ method: "connections", request: "weird" }); // Error: Type 'string' is not assignable to type 'number'
const x = fn({ method: "connections", request: 42 }); // ok
x.response // type number

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