Simple logger for my Cycle.js apps. Writes pretty console output and exposes an observable message stream if further processing is desired.

example.js

import logger from './logger';

let log = logger('Category');

log.trace('Just tracing stuff');
log.debug('Here is a debug message');
log.success('The successful thing happened that we wanted to happen');
log.info('Information makes the world go around, and here is that string:', str);
log('An info message can be logged using short form');
log.warn('You better be careful about this kind of thing');
log.error('Argh, something bad happened');
log.error(new Error('Your code has an error'));
log.fatal('Oh no! The world is ending!', new Error('Your code has a fatal error'));

logger.js

'use strict';

import _ from 'lodash';
import {Rx} from '@cycle/core';

function makeLogMessage(level, category, args) {
  return {
    level: level,
    category: category,
    args: args
  };
}

function decomposeMessage(msg) {
  // todo: introduce serilog-flavoured log arguments, rather than discarding extra args
  let message, error = null, args = msg.args;
  if(args[0] instanceof Error) {
    error = args[0];
    message = error.message;
    args = args.slice(1);
  }
  else if(args[args.length - 1] instanceof Error) {
    message = args[0];
    error = args[args.length - 1];
    args = args.slice(0, args.length - 1);
  }
  else if(_.isObject(args[0])) {
    message = '';
  }
  else {
    message = args[0];
    args = args.slice(1);
  }
  return [msg.level, msg.category, message, error, args];
}

function writeConsole([level, category, message, error, args]) {
  let catbg, catfg, msgbg, msgfg, levelLabel;
  switch(level) {
    case 'trace':    [levelLabel, catbg, catfg, msgbg, msgfg] = [' TRACE ', '#eee', '#999',  'default', '#bbb']; break;
    case 'debug':    [levelLabel, catbg, catfg, msgbg, msgfg] = [' DEBUG ', '#63c', 'white', 'default', '#63c']; break;
    case 'info':     [levelLabel, catbg, catfg, msgbg, msgfg] = [' INFO  ', '#333', '#eee',  'default', 'default']; break;
    case 'success':  [levelLabel, catbg, catfg, msgbg, msgfg] = ['SUCCESS', '#9d9', '#262',  'default', '#090']; break;
    case 'warn':     [levelLabel, catbg, catfg, msgbg, msgfg] = ['WARNING', '#ee0', '#933',  '#ffe',    '#933']; break;
    case 'error':    [levelLabel, catbg, catfg, msgbg, msgfg] = [' ERROR ', '#900', '#ff3',  'default', '#c00']; break;
    case 'fatal':    [levelLabel, catbg, catfg, msgbg, msgfg] = [' FATAL ', '#c00', '#ff0',  '#ff3',    '#c00']; break;
    default: return;
  }
  console.log(`%c ${levelLabel} %c %c[${category||'General'}] ${message}`,
              `background: ${catbg}; color: ${catfg};`, '',
              `background: ${msgbg}; color: ${msgfg};`, ...args);
  if(error) {
    console.error(error);
  }
}

let subject = new Rx.Subject();
subject.subscribe(msg => {
  let data = decomposeMessage(msg);
  writeConsole(data);
});

function createLogger(category) {
  function logger(...args) {
    subject.onNext(makeLogMessage('info', category, args));
  }
  logger.trace = function(...args) { subject.onNext(makeLogMessage('trace', category, args)); }
  logger.debug = function(...args) { subject.onNext(makeLogMessage('debug', category, args)); }
  logger.success = function(...args) { subject.onNext(makeLogMessage('success', category, args)); }
  logger.info = logger;
  logger.warn = function(...args) { subject.onNext(makeLogMessage('warn', category, args)); }
  logger.error = function(...args) { subject.onNext(makeLogMessage('error', category, args)); }
  logger.fatal = function(...args) { subject.onNext(makeLogMessage('fatal', category, args)); }
  return logger;
}
createLogger.message$ = subject.asObservable();

export default createLogger;