loglevel-with-prefix.js

import log from 'loglevel';

function logger(name, prefix) {
  const result = log.getLogger(name);
  if (!prefix) return result;
  if (prefix === result.prefix) return result;
  result.prefix = prefix;
  const original = {
    trace: result.trace.bind(result),
    debug: result.debug.bind(result),
    info: result.info.bind(result),
    warn: result.warn.bind(result),
    error: result.error.bind(result),
  };
  result.trace = (...args) => original.trace((typeof prefix === 'function' ? prefix() : prefix), ...args);
  result.debug = (...args) => original.debug((typeof prefix === 'function' ? prefix() : prefix), ...args);
  result.info = (...args) => original.info((typeof prefix === 'function' ? prefix() : prefix), ...args);
  result.warn = (...args) => original.warn((typeof prefix === 'function' ? prefix() : prefix), ...args);
  result.error = (...args) => original.error((typeof prefix === 'function' ? prefix() : prefix), ...args);
  return result;
}

export default logger;

sample-usage.js

import logger from './loglevel-with-prefix';
const log = logger('my-logger', () => `${new Date().toISOString()} my-logger`);

log('> hey', 'you'); // 2020-01-22T16:08:11.361Z my-logger > hey you