You could use the buffer library to decode base64 encoded data in a react native app.

npm i buffer 

Once installed you can import the buffer library into your react native app

import { Buffer } from 'buffer';

and then decode base64 encoded data

Buffer.from(data, 'base64');

The line above returns the decoded data as bytes and

Buffer.from(data, 'base64').toString('ascii');

returns the decoded data as an ascii encoded string.

RNFS.writeFile ?

import RNFS from 'react-native-fs';

RNFS.writeFile(RNFS.CachesDirectoryPath + '/a.jpeg', base64, 'base64')

// RNFS.CachesDirectoryPath + '/a.jpeg'

I use rn-fetch-blob, basically it provides lot of file system and network functions make transferring data pretty easy.

react-native-fetch-blob is deprecated

import RNFetchBlob from "rn-fetch-blob";
const fs = RNFetchBlob.fs;
let imagePath = null;
RNFetchBlob.config({
  fileCache: true
})
  .fetch("GET", "http://www.example.com/image.png")
  // the image is now dowloaded to device's storage
  .then(resp => {
    // the image path you can use it directly with Image component
    imagePath = resp.path();
    return resp.readFile("base64");
  })
  .then(base64Data => {
    // here's base64 encoded image
    console.log(base64Data);
    // remove the file from storage
    return fs.unlink(imagePath);
  });

source Project Wiki

I find some simple way worked for me, the same api as node.

Install buffer

yarn add buffer

Usage:

console.log(Buffer.from("Hello World").toString('base64'));
console.log(Buffer.from("SGVsbG8gV29ybGQ=", 'base64').toString('ascii'));

I ended up using react-native-blob-util

const res = await DocumentPicker.pickSingle({
              type: [
                DocumentPicker.types.images,
                DocumentPicker.types.pdf,
                DocumentPicker.types.docx,
                DocumentPicker.types.zip,
              ],
            });
            const newUploadedFile: IUploadedFile[] = [];

            const fileType = res.type;
            if (fileType) {
              const fileExtension = fileType.substr(fileType.indexOf('/') + 1);
              const realURI = Platform.select({
                android: res.uri,
                ios: decodeURI(res.uri),
              });
              if (realURI) {
                const b64 = await ReactNativeBlobUtil.fs.readFile(
                  realURI,
                  'base64',
                );
                const filename = res.name.replace(/\s/g, '');
                const path = uuid.v4();
                newUploadedFile.push({
                  name: filename,
                  type: fileType,
                  size: res.size as number,
                  extension: fileExtension,
                  blob: b64,
                  path: Array.isArray(path) ? path.join() : path,
                });
              } else {
                throw new Error('Failed to process file');
              }
            } else {
              throw new Error('Failed to process file');
            }