generic_flow_control.js

var fs = require('fs');

function cascade(callbacks, callback) {

  // clone the array
  var functions = callbacks.slice(0);

  function processNext(err) {

    if (err) {
      return callback(err);
    }

    var args = Array.prototype.slice.call(arguments);

    var func = functions.shift();
    if (func) {
      // remove first argument containing the error
      args.shift();
    } else {
      func = callback;
    }

    args.push(processNext);

    func.apply(this, args);
  }

  processNext.call(this);

}


function append_some_a_to_b(callback) {

  var aFd, bFd,
      buffer = new Buffer(10);

  cascade([
    function open_a(next) {
      fs.open(__dirname + '/a.txt', 'r', next);
    },

    function read_from_a(fd, next) {
      aFd = fd;
      fs.read(aFd, buffer, 0, buffer.length, 0, next);
    },

    function close_a(bytesRead, buf, next) {
      fs.close(aFd, next);
    },

    function open_b(next) {
      fs.open(__dirname + '/b.txt', 'a', next);
    },

    function stat_b(fd, next) {
      bFd = fd;
      fs.fstat(bFd, next);
    },

    function write_b(bStats, next) {
      fs.write(bFd, buffer, 0, buffer.length, bStats.size, next);
    },

    function close_b(bytesWritten, buf, next) {
      fs.close(bFd, next);
    }
  ], callback);
}

console.log('starting...');

append_some_a_to_b(function done(err) {

  if (err) {
    throw err;
  }

  console.log('done');
});