ReactJS / ES6 / ES7 logging - lightweight library with managed logging levels, similar to simple logging facades in other languages

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ReactJS/ES6/ES7 logging - lightweight library with managed logging levels, similar to simple logging facades in other languages


``npm install react-logger-lib --save``

Why one more library for logging?

This library is a tribute to simple logging facade from other languages and attempt to bring similar pattern into client side programming, ReactJS and other ES7 applications.


  • it allows to manipulate filtration without build tools.
  • filtration is based on components hierarchy
  • it is lightweight 2KB gzipped (~8KB original)
  • no external dependencies, basically this is a module that you can just copy to you project
  • useful for automated unit tests, where it can trigger warnings according to expectations
  • no performance issues (if used properly)

Typical Usage in React Component

SidePicker.jsx: ```javascript import React, { Component } from 'react'; import { Logger } from 'react-logger-lib'; class SidePicker extends Component {
// Let's imagine some useful component for random side picking
// One of 4 options should be chosen. And the same choice should not be repeated in the next attempt
values = ['left', 'right', 'top', 'bottom' ];
state = { side: 'left' };
componentDidMount() {
Logger.of('App.SidePicker.componentDidMount').info('state=', this.state);
pickSide = () => {
// set of possible values to exclude current side
const values = this.values.slice();
values.splice(this.values.indexOf(this.state.side), 1);
Logger.of('App.SidePicker.pickSide.values').info('Set of new values', values);
// new side is a random pick out of those values
const side = values[Math.floor(Math.random() * values.length)];
Logger.of('App.SidePicker.pickSide.side').info('Another side chosen', 'side=', side);
// save decision into the state
this.setState({ side });
pickWrongSide = () => {
// nothing happens. we produce just a warning, visible in Console
Logger.of('App.SidePicker.pickWrongSide').warn('Attempt to pick a wrong side');
render() {
return (
<div>Current Side: <strong>{this.state.side}</strong></div>
<button className='good-button' type='button' onClick={() => (this.pickSide())}>
<button className='bad-button' type='button' onClick={() => (this.pickWrongSide())}>
} export default SidePicker; ``` Once you mount this component into your application, nothing is shown in Console tab when launched and when you click the first button, as it is declared under INFO level - and suppressed by default (this is why it is basically acceptable for production). But when you click the second button, the wrong one, you will see console.warn ``` App.SidePicker.pickWrongSide | Attempt to pick a wrong side ``` stating that something wrong happened, though there was nothing critical for further application health. What is the key feature of this approach, is that once written, it is compiled and running, and completely forgotten, you can manage logging level at anytime - and describe what level is required for what component tree in terms of your business logic. This management can be done just by putting localStorage variables in your browser. For this particular example you can use one of the following to enable level at any depth: ```javascript localStorage.setItem('App', 'INFO'); localStorage.setItem('App.SidePicker', 'INFO'); localStorage.setItem('App.SidePicker.pickSide', 'INFO'); localStorage.setItem('App.SidePicker.componentDidMount', 'INFO'); ``` and then refresh the page to apply changes for these settings. When loaded and 2 buttons will be clicked, Console will show something like this: ``` App.SidePicker.componentDidMount | state= Object {side: "left"} App.SidePicker.pickSide.values | Set of new values (3) "right", "top", "bottom" App.SidePicker.pickSide.side | Another side chosen side= top App.SidePicker.pickWrongSide | Attempt to pick a wrong side ```

Levels of logging

There are 4 levels of logging currently. From lowest to highest these are TRACE-INFO-WARN-ERROR:
  • OFF suppresses the logging completely
  • ERROR level will show only messages with ERROR level.
  • WARN level will show all messages with ERROR level as well.
  • INFO level will show all messages with ERROR and WARN levels as well.
  • TRACE level will show all messages with INFO, WARN and ERROR levels as well.
TRACE is not calling console.trace, it is just a logical level in this library

Using in Unit Tests

Now let's see what we can benefit from our logging in unit tests: ```javascript import React from 'react'; import { shallow } from 'enzyme'; import { enableLogger, Logger } from 'react-logger-lib'; import SidePicker from './SidePicker'; const expectNoErrors = () => { expect(Logger.calls.error).toEqual(0); }; const expectNoWarn = () => { expect(Logger.calls.warn).toEqual(0); expectNoErrors(); }; const expectWarn = () => { expect(Logger.calls.warn).toBeGreaterThan(0); }; describe('App.SidePicker', () => { it('was mounted without warnings', () => {
enableLogger(() => {
shallow(<SidePicker />);
}); it('good button works without warnings', () => {
enableLogger(() => {
const elem = shallow(<SidePicker />);
// expect good button to be found
const goodButton = elem.find('button').at(0);
expect(goodButton.text()).toEqual('PICK ANOTHER SIDE');
// clicking on it
}); it('bad button triggers warnings', () => {
enableLogger(false); // we do not want to see the warning when it happens
const elem = shallow(<SidePicker />);
// expect bad button to be found
const badButton = elem.find('button').at(1);
expect(badButton.text()).toEqual('PICK WRONG SIDE');
// clicking on it
}); }); ``` Jest tests above are checking that there was no warnings during components mounting, that there was no warning when the good button is clicked, and some warning was expected on the bad button.
  • All logging is suppressed when running with Jest. Therefore enabledLogger() function is a way to display actual warnings and errors when they are happening during tests.
  • Typical case, as described above - we enable warnings when we do not expect them to happen. And we disable them when we actually expect a warning.
  • enableLogger() supports 2 ways of calling it. It's parameter could be a synchronous call with boolean flag or callback to be executed when warnings/errors logging is enabled.

Eliminating Logger using WebPack Loader

There is a natural desire to eliminate logging completely on certain production environments to get as small build size as possible. One of the ways it could be achieved - is by adding extra loader for your webpack configuration
  1. Create new file no-logger-loader.js with the following contents:
```javascript module.exports = function(source, map) { if (source.indexOf('react-logger-lib') !== -1) {
const cleanSource = source.replace(/Logger\.of\([^\)]+\)\.(trace|info|warn|error)/g, '');
this.callback(null, cleanSource, map);
} else {
this.callback(null, source, map);
} } ```
  1. Modify your production configuration It is important to add this loader before babel-loader.
```javascript ... module: {
rules: [
test: /\.(js|jsx)$/,
include: paths.appSrc,
loader: require.resolve('./no-logger-loader')
} ```
  1. Please make sure UglifyJsPlugin is also enabled in that webpack configuration.
This way you will get rid of all logging on this environment.


  • This library is more a pattern to be cloned and configured for your own needs. This is why there are no plugins and extensions like in other similar facades.
  • We did it just because we love Slf4J approach in Java, but there was no similar thing in React applications.
  • Unlike Java, where component path is taken to logs automatically, here naming is left on responsibility of a writer. Our advice is to keep it as hierarchy of your business logic, as clear and standard as possible.
  • Please use logging wisely on render() methods, as extensive logging during rendering is an easy way to downgrade your application performance.
  • There is no slowdown noticed when logs are suppressed or not matching their level.
  • Feel free to discuss this project under Github Issues.