Does `console.log` buffer output in node.js?

The documentation for console.log() probably doesn't specify whether it buffers output because it delegates that decision to an underlying stream:

Console.prototype.log = function() {
  this._stdout.write(util.format.apply(this, arguments) + '\n');
};

The writable.write() it's using documents the general possibility of buffering:

The return value indicates if you should continue writing right now. If the data had to be buffered internally, then it will return false. Otherwise, it will return true.

This return value is strictly advisory. You MAY continue to write, even if it returns false. However, writes will be buffered in memory, so it is best not to do this excessively. Instead, wait for the drain event before writing more data.

Though, the global console uses process.stdout that's more likely to block, buffering only under certain circumstances:

process.stderr and process.stdout are unlike other streams in Node in that writes to them are usually blocking.

• They are blocking in the case that they refer to regular files or TTY file descriptors.
• In the case they refer to pipes:
  • They are blocking in Linux/Unix.
  • They are non-blocking like other streams in Windows.

To see whether any lines are buffered, you could .write() them to stdout yourself and capture the return value:

var util = require('util');
var buffered = [];

array.forEach(function(line){
  buffered.push(!process.stdout.write(util.format(line) + '\n'));
});

console.log(buffered);