Fixing an issue that type 'null' and 'undefined' is not assignable to validateStatus when typescript strict option is enabled (#3200)

index.d.ts

@@ -62,7 +62,7 @@ export interface AxiosRequestConfig {
   onUploadProgress?: (progressEvent: any) => void;
   onDownloadProgress?: (progressEvent: any) => void;
   maxContentLength?: number;
-  validateStatus?: ((status: number) => boolean | null);
+  validateStatus?: ((status: number) => boolean) | null;
   maxBodyLength?: number;
   maxRedirects?: number;
   socketPath?: string | null;

test/typescript/axios.ts

@@ -44,6 +44,14 @@ const config: AxiosRequestConfig = {
   cancelToken: new axios.CancelToken((cancel: Canceler) => {})
 };
 
+const nullValidateStatusConfig: AxiosRequestConfig = {
+  validateStatus: null
+};
+
+const undefinedValidateStatusConfig: AxiosRequestConfig = {
+  validateStatus: undefined
+};
+
 const handleResponse = (response: AxiosResponse) => {
   console.log(response.data);
   console.log(response.status);