interface AxiosRequestTransformer {
    (this: InternalAxiosRequestConfig, data: any, headers: AxiosRequestHeaders): any;
}

interface AxiosRequestConfig<D = any> {
    ....
    transformRequest?: AxiosRequestTransformer | AxiosRequestTransformer[];
   ....
    data?: D;

type ResultOfChainAxiosRequestTransformer = Stream | string | Buffer | ArrayBuffer;

interface FirstAndOnlyAxiosRequestTransformer<D> {
    (this: InternalAxiosRequestConfig, data: D, headers: AxiosRequestHeaders):  ResultOfChainAxiosRequestTransformer;
  }
  
  
  
interface FirstOfChainAxiosRequestTransformer<D> {
    (this: InternalAxiosRequestConfig, data: D, headers: AxiosRequestHeaders): D | ResultOfChainAxiosRequestTransformer;
  }
  
  
interface NotLastOfChainAxiosRequestTransformer<D> {
    (this: InternalAxiosRequestConfig, data: D | ResultOfChainAxiosRequestTransformer, headers: AxiosRequestHeaders): D | ResultOfChainAxiosRequestTransformer;
  }
  
interface LastOfChainAxiosRequestTransformer<D> {
    (this: InternalAxiosRequestConfig, data: D | ResultOfChainAxiosRequestTransformer, headers: AxiosRequestHeaders):  ResultOfChainAxiosRequestTransformer;
  }

interface AxiosRequestConfig<D = any> {
    ....
        transformRequest?: FirstAndOnlyAxiosRequestTransformer | [FirstOfChainAxiosRequestTransformer, ...NotLastOfChainAxiosRequestTransformer[], LastOfChainAxiosRequestTransformer];
   ....
    data?: D;