enum Channel {
  GITHUG = 'github',
  GITLAB = 'gitlab',
}

interface Github {}

interface Gitlab {}

function foo (a, b) {}

The first parameter a is an enum (Channel) value

If parameter a is github, then the type of b is GitHub

If parameter a is gitlab, then the type of b is Gitlab

How to define the type of function foo?

enum Channel {
  GITHUG = 'github',
  GITLAB = 'gitlab',
}

interface Github {}

interface Gitlab {}

function foo (a, b) {}

The first parameter a is an enum (Channel) value

If parameter a is github, then the type of b is GitHub

If parameter a is gitlab, then the type of b is Gitlab

How to define the type of function foo?

• javascript
• typescript

Share Improve this question Follow edited May 21, 2021 at 11:18 loveyunk asked May 21, 2021 at 10:31 loveyunkloveyunk 37833 silver badges1313 bronze badges 1

• typescriptlang/docs/handbook/2/…? – jonrsharpe Commented May 21, 2021 at 10:33

Add a ment  | 

4 Answers 4

Sorted by: Reset to default 4

Here you have some alternative ways:

enum Channel {
  GITHUB = 'github',
  GITLAB = 'gitlab',
}

interface Github {
  type: 'Github'
}

interface Gitlab {
  type: 'Gitlab'
}

/**
 * First approach
 */
type Params = [Channel.GITHUB, Github] | [Channel.GITLAB, Gitlab]

function foo(...args: Params) {
  if (args[0] === Channel.GITHUB) {
    const x = args[1] // Github
  }
}


type Overloading =
  & ((a: Channel.GITHUB, b: Github) => void)
  & ((a: Channel.GITLAB, b: Gitlab) => void)


/**
 * Second approach
 */

const foo2: Overloading = (a: Channel, b: Github | Gitlab) => null as any

const result = foo2(Channel.GITHUB, { type: 'Github' }) // ok


/**
 * Third approach
 */
function foo3(a: Channel.GITLAB, b: Gitlab): void
function foo3(a: Channel.GITHUB, b: Github): void
function foo3(a: Channel, b: Github | Gitlab) {

}
foo3(Channel.GITLAB, { type: 'Gitlab' }) // ok

Playground

Use conditional types technique. Look at example below or go to online playground in order to test in action (thanks for help to Joonas)

enum Channel {
  GITHUG = 'github',
  GITLAB = 'gitlab',
}

interface Github {
  _foo: number
}

interface Gitlab {
  _bar: number
}

type ParamType<C> = C extends Channel.GITHUG ? Github : Gitlab;
function foo<C extends Channel>(a: C, b: ParamType<C>): void {}

foo(Channel.GITLAB, { _foo: 1 }); // error
foo(Channel.GITLAB, { _bar: 1 }); // success

---

Please, let me know if it works or not )

Explain look at the code ments:

enum Channel {
  GITHUG = 'github',
  GITLAB = 'gitlab',
}

interface Github {}

interface Gitlab {}

// Map enum value and type
interface Foo {
  github: Github
  gitlab: Gitlab
}

// `${Channel}` => 'github' | 'gitlab'
// Get the type K of a through the passed parameter
// if K is `github`, Foo[K] is Github
// if k is `gitlab`, Foo[K] is Gitlab
function foo<K extends `${Channel}`>(a: K, b: Foo[K]) {}

Playground

I had a very similar problem, in my case, the second parameter type es from an interface:

interface ArgTypes { [id: string]: any }
interface ChannelArgs extends ArgTypes{
  github: {
    hubhub: number
  },
  gitlab: {
    lablab: number
  },
  ...
}
type Arg<Args extends ArgTypes, Id extends keyof Args> = Args[Id]

My foo function has to work with all types implementing the interface, so it is generic itself:

foo<ChannelArgs>('github', {hubhub: 12})

Turns out, declaration of a generic type and an inferred type does not mix, but it is possible to put them in different brackets:

type Foo<Args extends ArgTypes> = <Id extends keyof Args>(
  id: Id,
  arg: Arg<Args, Id>
) => void

const gitFoo: Foo<ChannelArgs> = function(a,b): void {}

gitFoo('github', {hubhub: 12}) // no error
gitFoo('github', {lablab: 12}) // error