File logger for loglevel

loglevel, logging
npm install @envoy/loglevel-file-logger@1.0.8



File logger for loglevel. This package is currently for React Native, it relies on react-native-fetch-blob. We didn't add react-native-fetch-blob in dependencies, as we are thinking maybe it's possible to open up this library to use different file library, so you need to install react-native-fetch-blob by yourself.


Install react-native-fetch-blob if it's not a dependency in your project yet.

yarn add react-native-fetch-blob

Then you can install loglevel-file-logger like this

yarn add loglevel-file-logger

Next, import it in the entry point file of your app.

import setupLogger, { FetchBlobWriter } from 'loglevel-file-logger'

// Notice: this is a hack, somehow
//     import * as Log from 'loglevel'
// Will clone the module.exports object, which is the default logger.
// As we want to change `methodFactory` of the original object instead of
// cloned one, we import the logger like this instead.
const defaultLogger: Log = require('loglevel')

export default class App extends Component {
  private readonly fileWriter: FetchBlobWriter

  constructor (props: any, context?: any) {
    super(props, context)
    this.fileWriter = new FetchBlobWriter()
    this.fileWriter.ensureFile().then(() => {
      // make linter not to complain about promise not subscribed
    // setup the file logger
      log: defaultLogger,
      write: this.fileWriter.write.bind(this.fileWriter)
    // enable all logger levels

Please notice that we use require('loglevel') to import the global root logger from loglevel, as if you use import * as Log from 'loglevel', somehow it will copy the export object and we won't be able to replace the original methodFactory of it. Also, you need to call setupLogger from loglevel-file-logger before calling enableAll on the root logger, as enableAll replaces logging methods for all logger, you need to setup file logger first otherwise it won't replace method with the one which writes to file.

After the logger setup, you can the logger from loglevel and write as much as log you want

const logger = Log.getLogger('my-logger')
logger.info('Hello world')


FetchBlobWriter is the only log writer we support now, it relies on react-native-fetch-blob. By default the log file path is

RNFetchBlob.fs.dirs.DocumentDir + '/log.txt'

To get the path in Objective C, you can write

NSArray<NSString *> *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentDir = paths[0];
NSString *logFile = [documentDir stringByAppendingPathComponent:@"log.txt"];

You can pass an alternative path into constructor. Like this

this.fileWriter = new FetchBlobWriter('/path/to/my/log')

The default log formatter we use is defaultFormatter from

import { defaultFormatter } from 'loglevel-file-logger'

You can define your own by implementing Formatter interface and pass it to constructor as well.

this.fileWriter = new FetchBlobWriter('/path/to/my/log', myFormatter)


  • FetchBlobWriter with file rotation, we can keep say like 7 days of logs