ES6 Proxy Demo - Prepending a timestamp to console output

console-proxy.js

let pad = val => String(val).padStart(2, '0');

let getTimestamp = () => {
  let opts = ['day', 'month', 'year', 'hour', 'minute', 'second']
    .reduce((res, key) => (res[key] = '2-digit') && res, { hour12: false });
  return new Date().toLocaleString(undefined, opts);
};

let prependTimestamp = (fn) => ({
  apply(target, arg, args) {
    let tag = fn !== 'log' ? ` [${fn.toUpperCase()}]` : '';
    args = [`${getTimestamp()}${tag}`, ...args];
    return Reflect.apply(target, arg, args);
  }
});

['log', 'info', 'error', 'warn'].forEach(fn =>
  console[fn] = new Proxy(console[fn], prependTimestamp(fn)));

if (typeof require !== 'undefined' && require.main === module) {
  // Called directly (not via require).
  console.debug('Debug message (without proxy).')
  console.log('Log message.');
  console.info('Info message.');
  console.warn('Warn message.');
  console.error('Error message.');
}