muxing-logger.js

var EventEmitter = require('events').EventEmitter;
var es = require('event-stream');
var util = require('util');

function Logger() {
  Logger.super_.call(this);
}

util.inherits(Logger, EventEmitter);

['info', 'warn', 'error'].forEach(function(level) {
  Logger.prototype[level] = function(msg) {
    var line = util.format('[%s] %s', level, msg);
    this.emit('log', {level: level, line: line});
  };
});

Logger.prototype.getStream = function(levels) {
  var stream = es.through();
  var write = stream.write.bind(stream);
  stream.__end = stream.end; // necessary?
  stream.write = stream.end = function() {};
  this.on('log', function(data) {
    if (levels.indexOf(data.level) !== -1) {
      write(data.line + '\n');
    }
  });
  stream.bufferedStream = function() {
    var buffered = es.split();
    var self = this;
    buffered.pipe(es.through(function(line) {
      write(line + '\n');
    }));
    return buffered;
  };
  return stream;
};

var logger = new Logger();
// logger.on('log', function(data) {
//   console.log(data.line);
// });
// logger.info('test');
// logger.warn('123');
// logger.error('oops');

var logStdio = logger.getStream(['info', 'warn']);
logStdio.pipe(process.stdout);
var logStderr = logger.getStream(['error']);
logStderr.pipe(process.stderr);

logger.info('test');
logger.warn('123');
logger.error('oops');

var evil = es.through();
evil.pipe(logStdio); // pipe to the main stream? O NOES
evil.write('THIS SHOULD BREAK NOTHING'); evil.end();

var s1 = logStdio.bufferedStream();
s1.write('st');
logger.info('test');
s1.write('do');
logger.warn('123');
s1.write('ut'); s1.end();
logger.error('oops');

var s2 = logStdio.bufferedStream();
var t2 = es.through(function(s) { this.queue(s.toUpperCase()); });
t2.pipe(s2);
t2.write('st');
logger.info('test');
t2.write('do');
logger.warn('123');
t2.write('ut'); t2.end();
logger.error('oops');

var s3 = logStderr.bufferedStream();
var t2 = es.through();
t2.pipe(s3);
t2.write('st');
logger.info('test');
t2.write('de');
logger.warn('123');
t2.write('rr'); t2.end();
logger.error('oops');

output.txt

$ node muxing-logger.js
[info] test
[warn] 123
[error] oops
[info] test
[warn] 123
stdout
[error] oops
[info] test
[warn] 123
STDOUT
[error] oops
[info] test
[warn] 123
stderr
[error] oops

$ node muxing-logger.js >/dev/null
[error] oops
[error] oops
[error] oops
stderr
[error] oops

$ node muxing-logger.js 2>/dev/null
[info] test
[warn] 123
[info] test
[warn] 123
stdout
[info] test
[warn] 123
STDOUT
[info] test
[warn] 123