concat-promises.js

var when = require('when');
var fs = require('fs');

var concat = {};

concat.each = function(src, dest, options) {
  if (!options) { options = {}; }
  if (!options.separator) { options.separator = ''; }
  return when.reduce(src, function(output, srcpath) {
    var dfd = when.defer();
    fs.readFile(srcpath, function(err, data) {
      if (err) { dfd.reject(err); }
      dfd.resolve(output.concat(data));
    });
    return dfd;
  }, []).then(function(combined) {
    var dfd = when.defer();
    fs.writeFile(dest, combined.join(options.separator), function(err) {
      if (err) { dfd.reject(err); }
      dfd.resolve('File written.');
    });
    return dfd;
  });
};

concat.each(
  ['test/fixtures/concat/a.txt', 'test/fixtures/concat/b.txt', 'test/fixtures/concat/c.txt'],
  'tmp/abc.txt',
  {separator: '\n'}
).then(function(status) {
  console.log(status);
});

map-ex1.js

var when = require('when');
var nodefn = require('when/node/function');
var fs = require('fs');

var readFile = nodefn.bind(require('fs').readFile);
var writeFile = nodefn.bind(require('fs').writeFile);

var concat = {};

concat.each = function(src, dest, options) {
  if (!options) { options = {}; }
	if (!options.separator) { options.separator = ''; }

	// Yeah, this is everything
	// Map each filename to its contents
	// Join the contents synchronously
	// write the result
	return when.map(src, readFile).then(function(array) {
		return array.join(options.separator);
	}).then(writeFile.bind(fs, dest));
};

concat.each(
  ['test/fixtures/concat/a.txt', 'test/fixtures/concat/b.txt', 'test/fixtures/concat/c.txt'],
  'tmp/abc.txt',
  {separator: '\n'}
).then(function(status) {
  console.log(status);
});

reduce-ex1.js

var when = require('when');
var nodefn = require('when/node/function');
var fs = require('fs');

var readFile = nodefn.bind(require('fs').readFile);
var writeFile = nodefn.bind(require('fs').writeFile);

var concat = {};

concat.each = function(src, dest, options) {
  if (!options) { options = {}; }
	if (!options.separator) { options.separator = ''; }

	// Map each filename to its contents
	// Reduce the resulting promised array of contents
	// (Note: when.map and when.reduce are composable)
	// write the result
	return when.reduce(src, function(result, file) {
		return readFile(file).then(function(contents) {
			return result + contents;
		});
	}, '').then(writeFile.bind(fs, dest));
};

concat.each(
  ['test/fixtures/concat/a.txt', 'test/fixtures/concat/b.txt', 'test/fixtures/concat/c.txt'],
  'tmp/abc.txt',
  {separator: '\n'}
).then(function(status) {
  console.log(status);
});