is node.js’ console.log asynchronous?

Update: Starting with Node 0.6 this post is obsolete, since stdout is synchronous now.

Well let’s see what console.log actually does.

First of all it’s part of the console module:

exports.log = function() {
  process.stdout.write(format.apply(this, arguments) + '\n');
};

So it simply does some formatting and writes to process.stdout, nothing asynchronous so far.

process.stdout is a getter defined on startup which is lazily initialized, I’ve added some comments to explain things:

.... code here...
process.__defineGetter__('stdout', function() {
  if (stdout) return stdout;                            // only initialize it once 

  /// many requires here ...

  if (binding.isatty(fd)) {                             // a terminal? great!
    stdout = new tty.WriteStream(fd);
  } else if (binding.isStdoutBlocking()) {              // a file?
    stdout = new fs.WriteStream(null, {fd: fd});
  } else {
    stdout = new net.Stream(fd);                        // a stream? 
                                                        // For example: node foo.js > out.txt
    stdout.readable = false;
  }

  return stdout;
});

In case of a TTY and UNIX we end up here, this thing inherits from socket. So all that node bascially does is to push the data on to the socket, then the terminal takes care of the rest.

Let’s test it!

var data="111111111111111111111111111111111111111111111111111";
for(var i = 0, l = 12; i < l; i++) {
    data += data; // warning! gets very large, very quick
}

var start = Date.now();
console.log(data);
console.log('wrote %d bytes in %dms', data.length, Date.now() - start);

Result

....a lot of ones....1111111111111111
wrote 208896 bytes in 17ms

real    0m0.969s
user    0m0.068s
sys  0m0.012s

The terminal needs around 1 seconds to print out the sockets content, but node only needs 17 milliseconds to push the data to the terminal.

The same goes for the stream case, and also the file case gets handle asynchronous.

So yes Node.js holds true to its non-blocking promises.