example.js

var myfunc1 = function(callback) {
  console.log('hello, world');
  callback();
}

var myfunc2 = function() {
  return 'hello, world';
}

myfunc1(myfunc2);
myfunc1(myfunc2());

example2.js

var myfunc1 = function(callback) {
  console.log('hello, world');
  callback();
}

myfunc1(function() { return 'hello, world'; });
myfunc2('hello, world');