is node.js' console.log asynchronous?

Are console.log/debug/warn/error in node.js asynchrounous? I mean will javascript code execution halt till the stuff is printed on screen or will it print at a later stage?

Also, I am interested in knowing if it is possible for a console.log to NOT display anything if the statement immediately after it crashes node.

Update: Starting with Node 0.6 this post is obsolete, since stdout is synchronous now.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

console.warn() and console.error() are blocking. They do not return until the underlying system calls have succeeded.

Yes, it is possible for a program to exit before everything written to stdout has been flushed. process.exit() will terminate node immediately, even if there are still queued writes to stdout. You should use console.warn to avoid this behavior.

My Conclusion , after reading Node.js 10.* docs (Attached below). is that you can use console.log for logging , console.log is synchronous and implemented in low level c . Although console.log is synchronic, it wont cause a performance issue only if you are not logging huge amount of data.

(The command line example below demonstrate, console.log async and console.error is sync)

Based on Node.js Doc's

> The console functions are synchronous when the destination is a terminal or a file (to avoid lost messages in case of premature exit) and asynchronous when it's a pipe (to avoid blocking for long periods of time). > > That is, in the following example, stdout is non-blocking while stderr is blocking:

$ node script.js 2> error.log | tee info.log > In daily use, the blocking/non-blocking dichotomy is not something you should worry about unless you > log huge amounts of data.

Hope it helps

Console.log is asynchronous in windows while it is synchronous in linux/mac. To make console.log synchronous in windows write this line at the start of your code probably in index.js file. Any console.log after this statement will be considered as synchronous by interpreter.

if (process.stdout._handle) process.stdout._handle.setBlocking(true);

You can use this for synchrounous logging:

const fs = require('fs')
fs.writeSync(1, 'Sync logging\n')