nodejslearnerfiles/1.NodeJS-Intros/AsynchFunctions-Callbacks/callback.js

Very Good Beginnings if you want to Learn Node.js

var maxTime = 1000;

// if input is even, double it
// if input is odd, return error.
// ( call takes random amount of time < 1 sec )

var evenDoubler = function(v, callback) {
  var waitTime = Math.floor(Math.random()*(maxTime+1));
  if(v%2) {
      setTimeout(function (){
          callback(new Error("Odd Input"), null, waitTime);
      }, waitTime);
  } else {
      // Multiple parameters in a successful callback.
      setTimeout(function (){
          callback(null, v*2, waitTime);
      }, waitTime);
  }
};

// handleResults is a named function.
// a callback function usually have errors in the beginning and results and other additional parameters at the end of the argument list.

var handleResults = function(err, results, time) {
    if (err) {
        console.log("ERROR: "+ err.message +" ("+ time + "ms)" );
    } else {
        console.log("The results are : "+ results + " ("+ time + "ms)");
    }
};


// Calling the Asynchrnous function with callbacks, which means, once it is ready with "evenDoubling", it will invoke the "callback" function.
evenDoubler(2,handleResults);
evenDoubler(3,handleResults);
console.log("............");

/*output 

Debugger listening on [::]:15454
............
The results are : 4 (170ms)
ERROR: Odd Input (866ms)


*/