This gist is part of the book Learning Node by Shelley Powers, Novatec 2017

callback_example.js

const fb = (n) => {
  if(n<2) return n;
  return fb(n -1) + fb(n -2);
}

const Obj = function(){};

Obj.prototype.doSomething = function(arg1_){
  const callback_ = arguments[arguments.length - 1]; //This word 'arguments' just works with anonymous function
  callback = (typeof(callback_) == 'function' ? callback_ : null);
  const arg1 = typeof arg1_ === 'number' ? arg1_ : null;
  if(!arg1)
    return callback(new Error('first arg missing or not a number'));
    process.nextTick(() => {
      //this point will block the CPU
      const data = fb(arg1);
      callback(null, data);
    });
}


const test = new Obj();

test.doSomething(10, function(err, value){
  if(err) 
    console.error(err);
  else
    console.log('fibonaci value for %d is %d', 10, value);
});

console.log('called doSomething');