How to save a file with react-native?

let try this.

import RNFetchBlob from 'rn-fetch-blob'
import Share from 'react-native-share'
import RNFS from 'react-native-fs'
import {Alert, Platform} from 'react-native'

const download = (url) => {
  let dirs = RNFetchBlob.fs.dirs
  try {
    if (Platform.OS === 'android') {
      const configOptions = { fileCache: true }
      RNFetchBlob.config(configOptions)
      .fetch('GET', url, {
      'Authorization': '', //yourTokenIfHave
      'Content-Type': '' // 'application/octet-stream'
    })
    .then(resp => {
      return resp.readFile('base64')
    })
    .then(async base64Data => {
      base64Data = `data:application/pdf;base64,` + base64Data
      await Share.open({ url: base64Data })
      // remove the image or pdf from device's storage
      await RNFS.unlink(filePath)
    })
} else {
  RNFetchBlob
    .config({
      fileCache: true,
      path: dirs.DocumentDir + `/${itemPDF.fileName}`
    })
    .fetch('GET', url, {
      'Authorization': '',
      'Content-Type': '' // 'application/octet-stream'
    })
    .then(async (res) => {

      // the temp file path
      if (res && res.path()) {
        const filePath = res.path()
        let options = {
          type: 'application/pdf',
          url: filePath
        }
        await Share.open(options)
        await RNFS.unlink(filePath)
      }
    })
    .catch((error) => {
      console.log(error)
    })
    }
  } catch (error) {
   console.log('download: ', error)
 }
}

I did not try again rn-fetch-blob, I ended up using react-native-fs and it works. react-native-fs seems more popular and more maintained.

(I did a react-native version upgrade before trying again to save a file, so maybe it wasn't entirely due to a bug in rn-fetch-blob but also due to using an old react-native version.)

You problem appears to be with the permissions access to that dir, .config({ path: RNFetchBlob.fs.dirs.DocumentDir + '/medias/foo' }), probably because /medias/foo does not exist, try to do it without a specific path.

I always use it in this way, as explained in the docs:

RNFetchBlob
  .config({
    // add this option that makes response data to be stored as a file,
    // this is much more performant.
    fileCache : true,
  })
  .fetch('GET', 'http://www.example.com/file/example.zip', {
    //some headers ..
  })
  .then((res) => {
    // the temp file path
    console.log('The file saved to ', res.path())
  })

I had same issue, if you are using "rn-fetch-blob": 0.10.14 switch to "0.10.13", check https://github.com/joltup/rn-fetch-blob/issues/266 & https://github.com/wonday/react-native-pdf/issues/286