typescript function discrimination

Question

typescript function discrimination

const enum Tag {
  Friday = 'Friday',
  Planning = 'Planing',
}
const test = (tag: Tag, task:/* ??? */): string => {/* some logic */}

If tag is set to Tag.Friday, then the function task should expect a parameter of type (tour: string) => string

If tag is set to Tag.Planning, then the function task should expect a parameter of type (date: Date) => string

I attempted to use function discriminated union and function overloads, but neither was able to infer the type of task. You can check out my code in the TS Playground

typescript overloading discriminated-union

Answer 1

Answer №1

If you want to utilize function overloading in TypeScript, you can achieve this by defining your function as an interface.

Here's an example:

const enum Tag {
  Friday = 'Friday',
  Planning = 'Planing',
}
type TourFunc = (tour: string) => string
type DateFunc = (date: Date) => string
interface Fn {
  (tag: Tag.Friday, todo: TourFunc): string;
  (tag: Tag.Planning, todo: DateFunc): string;
}

const test: Fn = (tag: Tag, todo: TourFunc | DateFunc): string => {
  return 'a';
}

const dateFunc : DateFunc = (a: Date) => a.toDateString()
const tourFunc : TourFunc = (b: string) => b
test(Tag.Planning, (s: string) => 'a'); // Error
test(Tag.Planning, dateFunc); // Ok 
test(Tag.Friday, tourFunc); // Ok
test(Tag.Planning, tourFunc); // Error

Try it out on the Playground!

Answer 2

If you want to utilize function overloading in TypeScript, you can achieve this by defining your function as an interface.

Here's an example:

const enum Tag {
  Friday = 'Friday',
  Planning = 'Planing',
}
type TourFunc = (tour: string) => string
type DateFunc = (date: Date) => string
interface Fn {
  (tag: Tag.Friday, todo: TourFunc): string;
  (tag: Tag.Planning, todo: DateFunc): string;
}

const test: Fn = (tag: Tag, todo: TourFunc | DateFunc): string => {
  return 'a';
}

const dateFunc : DateFunc = (a: Date) => a.toDateString()
const tourFunc : TourFunc = (b: string) => b
test(Tag.Planning, (s: string) => 'a'); // Error
test(Tag.Planning, dateFunc); // Ok 
test(Tag.Friday, tourFunc); // Ok
test(Tag.Planning, tourFunc); // Error

Try it out on the Playground!

typescript function discrimination

Answer №1

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