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()

Great stuff, thanks for sharing; there are problems, though:

• hook_stdout.coffee won't compile, because it contains an unexpected indent on the last line
• hook_stdout.js won't compile, because it is missing two return statements to make it the direct translation of its .coffee source

While your technique works great for writes performed by the process itself, it doesn't for writes from child processes created with spawn and { stdio: 'inherit' }. Any idea how to fix that?

@mklement0 I'm also looking for solutions to stdio: 'inherit'. Did you find any?

@satazor and @mklement0 it is missing a return

var exports = module.exports;

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

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

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

# 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