How to get the return type of an overloaded function in typescript

Question

Scenario

So, I've the below ts file, where I'm trying to add the type annotation GetTokenResponse to a function getToken.

import { ConfigService } from '@nestjs/config';
import { google, GoogleApis } from 'googleapis';
import { AppConfig } from 'src/configurations/app.config';

type OAuth2Client = InstanceType<GoogleApis['auth']['OAuth2']>;

type GetTokenResponse = ReturnType<OAuth2Client['getToken']>;

getToken(code: string): GetTokenResponse {
    return this.oAuth2Client.getToken(code);
}

Problem

The below block of code (already mentioned in above snippet) returns void

type GetTokenResponse = ReturnType<OAuth2Client['getToken']>; //void

because

the function getToken has 3 overloads:

getToken(code: string): Promise<GetTokenResponse>;

getToken(options: GetTokenOptions): Promise<GetTokenResponse>;

getToken(code: string, callback: GetTokenCallback): void;

getToken(options: GetTokenOptions, callback: GetTokenCallback): void;

Question

So, how do I extract the type Promise<GetTokenResponse> from the getToken function?

Answer 1

@see https://github.com/sindresorhus/type-fest/issues/585
https://tsplay.dev/Nll5xN

type GetTokenResponse = { GetTokenResponse: true }
type GetTokenCallback = { GetTokenCallback: true }
type GetTokenOptions = { GetTokenOptions: true }

type x = {
    getToken(code: string): Promise<GetTokenResponse>;
    getToken(options: GetTokenOptions): Promise<GetTokenResponse>;
    getToken(code: string, callback: GetTokenCallback): void;
    getToken(options: GetTokenOptions, callback: GetTokenCallback): void;
}

// support 5 times overload at most
type OverloadedReturnType<T> = 
    T extends { (...args: any[]) : infer R; (...args: any[]) : infer R; (...args: any[]) : infer R ; (...args: any[]) : infer R; (...args: any[]) : infer R } ? R  :
    T extends { (...args: any[]) : infer R; (...args: any[]) : infer R; (...args: any[]) : infer R ; (...args: any[]) : infer R } ? R  :
    T extends { (...args: any[]) : infer R; (...args: any[]) : infer R; (...args: any[]) : infer R } ? R  :
    T extends { (...args: any[]) : infer R; (...args: any[]) : infer R } ? R  :
    T extends (...args: any[]) => infer R ? R : any;

type R = OverloadedReturnType<x['getToken']>
//   ^?
// type R = void | Promise<GetTokenResponse>