nodejslearnerfiles/1.NodeJS-Intros/AsynchFunctions-Callbacks/callback2.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 {
      setTimeout(function (){
          callback(null, v*2, waitTime);
      }, waitTime);
  }
};


// handleResults is a named function.
// a callback function usually have errors in the beginning and results at the end.

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.
for (var i=0; i<10 ; i++) {
    console.log('calling evenDoubler for value: '+ i);
    evenDoubler(i, handleResults);
}
console.log("............");

/* output 


calling evenDoubler for value: 0
calling evenDoubler for value: 1
calling evenDoubler for value: 2
calling evenDoubler for value: 3
calling evenDoubler for value: 4
calling evenDoubler for value: 5
calling evenDoubler for value: 6
calling evenDoubler for value: 7
calling evenDoubler for value: 8
calling evenDoubler for value: 9
............
The results are : 12 (28ms)
ERROR: Odd Input (221ms)
The results are : 8 (311ms)
The results are : 16 (343ms)
The results are : 0 (397ms)
ERROR: Odd Input (535ms)
ERROR: Odd Input (550ms)
ERROR: Odd Input (562ms)
The results are : 4 (694ms)
ERROR: Odd Input (812ms)


Process exited with code: 0


*/