기존 프로젝트의 axiosInstance의 타입들이 Definite Assigment Aeesrtions 또는 Non-null assertion opertator를 통해 단언이 된 것을 확인하여 커스텀 타입을 정의하였다.

import {
  AxiosHeaders as OriginAxiosHeaders,
  AxiosInstance as OriginAxiosInstance,
  AxiosInterceptorManager,
  AxiosResponse,
  InternalAxiosRequestConfig,
} from 'axios';

type AxiosHeaders = OriginAxiosHeaders & {
  [key: string]: string | undefined;
};
export interface AxiosRequestConfig extends InternalAxiosRequestConfig {
  headers: AxiosHeaders;
}
export interface CustomInstance extends OriginAxiosInstance {
  interceptors: {
    request: AxiosInterceptorManager<AxiosRequestConfig>;
    response: AxiosInterceptorManager<AxiosResponse>;
  };
}