Example of using redux-saga eventChannel with axios/fetch etc

uploadFileSaga.js

import {eventChannel, END} from 'redux-saga'

function createUploaderChannel(key, files){
  return eventChannel(emit => {
    
    const onProgress = ({total, loaded}) => {
      const percentage = Math.round((loaded * 100) / total)
      emit(percentage)
    }  
   
    axios.post('url', files, {
      requestId: key,
      uploadProgress: onProgress,
    }).then(() => {
      emit(END)
    }).catch(err => {
      emit(new Error(err.message))
      emit(END)
    })
    
    const unsubscribe = () => {}
    return unsubscribe
  })
}

function* uploadProgressWatcher(fileName, channel) {
  while(true){
    try{
      const progress = yield take(channel)
      yield put(actions.uploadFiles.progress(progress))
    }
    catch(err){
      yield put(actions.uploadFiles.progress(err))
    }
    finally{
      if(yield cancelled()) channel.close()
    }
  }
}

function* uploadFile(fileName, index, files) {
  try{
    const formData = new FormData()
    formData.append('files', files[fileName])
    
    const uploadChannel = yield call(createUploaderChannel, fileName, formData)
    yield fork(uploadProgressWatcher, fileName, uploadChannel)
  }
  catch(err){
    yield* actions.uploadFiles.failure(err)
  }
}

function* uploadFiles({payload}) {
  const {files} = payload
  yield put(actions.uploadFiles.progress(payload))
  yield all(
    Object.keys(files).map(uploadFile)
  )
}

export default function*() {
  takeLatest(actions.uploadFiles, uploadFiles)
}

https://gist.github.com/jpgorman/f49501076a13cecfaa17d30e8d569be0#file-uploadfilesaga-js-L22

what does the return unsubscribe()do in this example?

in eventChannel subscribe should return a function to unsubscribe

Thanks !!