function(anInt, aString, cb, aBool=true){
   if(bool){...;}else{...;}
   cb();
}

function(1, 'no', function(){
  ...
}, false);

function(2, 'yes', function(){
  ...
});

var variable1 = typeof variable1  !== 'undefined' ?  variable1  : default_value;

function(anInt, aString, cb, aBool) {
  aBool = typeof aBool  !== 'undefined' ? aBool : true;
  if(bool){...;}else{...;}
  cb();
}

var sayMessage = function(message='This is a default message.') {
  console.log(message);
}

node --harmony_default_parameters --eval "const t = function(i = 42) { return i }; console.log(t());"

fn1 = fn.bind(fn, 1, 'no', function(){}, false);
fn1();
fn2 = fn.bind(fn, 2, 'yes', function(){});
fn2(true);

CoffeeScript:

fn = (bar='foo') ->

JavaScript:

fn = function(bar) {
  if (bar == null) {
    bar = 'foo';
  }
};