code refactor

src/fs.js

 /**
  * @name react-native-fetch-blob-fs
  * @author wkh237
- * @version 0.1.0
+ * @version 0.7.0
  * @flow
  */
 
  * @param  {'base64' | 'utf8' | 'ascii'} encoding Encoding of read stream.
  * @return {Promise<Array<number> | string>}
  */
-function readFile(path:string, encoding:string, bufferSize:number):Promise<any> {
+function readFile(path:string, encoding:string, bufferSize:?number):Promise<any> {
   if(typeof path !== 'string')
     return Promise.reject('Invalid argument "path" ')
   return RNFetchBlob.readFile(path, encoding)

src/index.js

 /**
  * @name react-native-fetch-blob
  * @author wkh237
- * @version 0.5.0
+ * @version 0.7.0
  * @flow
  */
 
   json : () => any;
   base64 : () => any;
   flush : () => void;
+  session : (name:string) => RNFetchBlobSession | null;
+  readFile : (encode: 'base64' | 'utf8' | 'ascii') => ?Promise;
   readStream : (
     encode: 'utf8' | 'ascii' | 'base64',
   ) => RNFetchBlobStream | null;
      * @return {Promise}
      */
     this.flush = () => {
-      return unlink(this.path())
+      let path = this.path()
+      if(!path)
+        return
+      return unlink(path)
     }
     /**
      * get path of response temp file
         return this.data
       return null
     }
-    this.session = (name) => {
+    this.session = (name:string):RNFetchBlobSession | null => {
       if(this.type === 'path')
         return session(name).add(this.data)
       else {