my-great-type-test.ts

interface GreatClientConfig {
    url: string
}

interface GreatClientResponse {
    data: {
        datetime: string
        isGreat: boolean
    }
}

const myGreatClientFactory = (config: GreatClientConfig) => {
    return {
        getConfig: (): GreatClientConfig => config,
        doSomethingAsync: async (path: string): Promise<GreatClientResponse> => {
          const response = await fetch(`${config.url}/${path}`)

          if (!response.ok) {
              throw new Error(`Something failed: ${response.status}`)
          }

          return await response.json()
        }
    }

}

const myGreatClient = myGreatClientFactory({ url: 'https://it.is-great.com/api' })

type doSomethingAsyncParams = Parameters<typeof myGreatClient.doSomethingAsync>
type doSomethingasyncReturnType = ReturnType<typeof myGreatClient.doSomethingAsync>

Why you default to 4 spaces, VS Code 🤦‍♂️