Typescript make the second parameter type depends on the first parameter type

Question

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?

Answer 1

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

Answer 2

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

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Please, let me know if it works or not )