TypeScript dynamic Function definition based on functions argument

Question

is it posible to change the type of something based on the value of an argument passed to a function. I need this kind of type for an event emiter with callback. Example:

interface IUser {
  name: string
}

type CallBack = /* the Type for the Callback that should be dynamic */

//for event1 the type CallBack should be (user: IUser) => any
use("event1", (user) => { /* check that event1 is used, so only add 1 argument to the callback type  */ })

//for event2 the type CallBack should be (email: string, password: string) => any
use("event2", (email, password) => { /* check that event12is used, so add 2 argument to the callback type */ })

so is there a way to detect which event is used and apply the right type to the callback?

Answer 1

You could use types like this:

type CallBackEvent1 = (event: "event1", cb: (user: IUser) => void) => void;
type CallBackEvent2 = (
  event: "event2",
  cb: (email: string, pass: string) => void
) => void;

type CallBack = CallBackEvent1 | CallBackEvent2;

const callBack1: CallBack = (event: "event1", cb: (user: IUser) => void) =>
  undefined;
const callBack2: CallBack = (
  event: "event2",
  cb: (email: string, pass: string) => void
) => undefined;

const wrongCallBack: CallBack = (event: "event2", cb: (user: IUser) => void) =>
  undefined;
// Type '(event: "event2", cb: (user: IUser) => void) => undefined' is not assignable to type 'CallBack'.
//   Type '(event: "event2", cb: (user: IUser) => void) => undefined' is not assignable to type 'CallBackEvent1'.
//     Types of parameters 'event' and 'event' are incompatible.
//       Type '"event1"' is not assignable to type '"event2"'.ts(2322)