Samples for hooking into STDOUT for unit testing in Node.js

hook_stdout.coffee

exports = module.exports

exports.setup = (callback) ->
  write = process.stdout.write
	
  process.stdout.write = ((stub) ->
    (string, encoding, fd) ->
      stub.apply process.stdout, arguments
      callback string, encoding, fd)(process.stdout.write)
			
    return -> process.stdout.write = write

hook_stdout.js

var exports = module.exports;

exports.setup = function(callback) {
  var write = process.stdout.write;

  process.stdout.write = (function(stub) {
    function(string, encoding, fd) {
      stub.apply(process.stdout, arguments);
      callback(string, encoding, fd);
    };
  })(process.stdout.write);

  return function() {
    process.stdout.write = write;
  };
};

server.coffee

exports = module.exports

# server logic omitted for brevity

exports.listen = (port, host, callback) ->
  try
    server.listen port, host, callback
    util.log "Server listening at http://#{host or "127.0.0.1"}:#{port}/"
  catch err
    util.log err.message

server.js

var exports;
exports = module.exports;

exports.listen = function(port, host, callback) {
  try {
    server.listen(port, host, callback);
    return util.log("Server listening at http://" + (host || "127.0.0.1") + ":" + port + "/");
  } catch (err) {
    return util.log(err.message);
  }
};

spec.coffee

server = require '../js/server'
hook = require '../js/hook_stdout'

describe 'chat server', ->

   it 'should listen at localhost on a port I specify', ->
     unhook = hook.setup((string, encoding, fd) ->
       expect(string).toContain 'Server listening at http://127.0.0.1:3001/'
     )
     server.listen '3001'  	
     unhook()

# this isn't necessary:
# exports = module.exports

exports.setup = (callback) ->
  write = process.stdout.write

  # use `do` to immediately call a function
  process.stdout.write = do (stub = write) ->
    (string, encoding, cb) ->
      stub.apply process.stdout, arguments
      callback string, encoding, cd
  
  # fix the indentation here, add an empty return
  return -> process.stdout.write = write ; return

Alternate version:

exports.setup = (callback) ->

  write = process.stdout.write

  # no need to do the double step above, this works:
  process.stdout.write = (string, encoding, cb) ->
    write.apply process.stdout, arguments
    callback string, encoding, cb

  -> process.stdout.write = write ; return