Recursion is fun! Node javascript factorial, sync and async.

async-factorial.js

module.exports = function (n,cb) {
  function helper (sum,i) {
  	if (i == 0) return cb(sum);
  	else process.nextTick( helper.bind(null,sum*i,i-1) );
  };
  return helper(1,n);
}

sync-factorial.js

module.exports = factorial;
function factorial (n) {
  if (n == 0) return 1;
  else return n * factorial (n - 1);
}

test.js

require('async').series([
    syncTestCase.bind(null,5,120),
    asyncTestCase.bind(null,5,120),
    syncTestCase.bind(null,0,1),
    asyncTestCase.bind(null,0,1),
    syncTestCase.bind(null,1,1),
    asyncTestCase.bind(null,1,1),
    syncTestCase.bind(null,4,24),
    asyncTestCase.bind(null,4,24),
  ], function(err,data) { if (err) throw 'tests failed'; })

function syncTestCase(input,output,callback) {
  process.nextTick(callback.bind(null,output!=(require('./sync-factorial.js')(input))));
}
function asyncTestCase(input,output,callback) {
  require('./async-factorial.js')(input,(function (result) { return callback(result!=output); }))
}

http://raganwald.com/2013/03/28/trampolines-in-javascript.html