uppy/examples/react-native-expo/tusFileReader.js

import base64 from 'base64-js'
import * as FileSystem from 'expo-file-system'

export default function getTusFileReader(file, chunkSize, cb) {
  FileSystem.getInfoAsync(file.uri, { size: true })
    .then((info) => {
      cb(null, new TusFileReader(file, info.size))
    })
    .catch(cb)
}

class TusFileReader {
  constructor(file, size) {
    this.file = file
    this.size = size
  }

  slice(start, end, cb) {
    const options = {
      encoding: FileSystem.EncodingType.Base64,
      length: Math.min(end, this.size) - start,
      position: start,
    }
    FileSystem.readAsStringAsync(this.file.uri, options)
      .then((data) => {
        cb(null, base64.toByteArray(data))
      })
      .catch(cb)
  }
}