1-callback basics tutorial.js

the basics of programming with callbacks:
structure:
function simple1(cb) // the last argument is callback function
{

 if(cb)return cb();// the return is required to prevent continuation of the function
}
to make a function asynchronous: add cb as last argument.
put at the end: if(cb)return cb(); 

callback function:

simple form:
function(data){}

standard form:
function(error,data){} // if no error. error usually contains  null or maybe undefined

// with null as error
function simple2(a,b,cb)
{
 if(b==0)  return cb(new Error('error description'));
 var data=a/b;
 if(cb)return cb(null,data);
}

usage:
//
simple2(1, 3, function(err,data) {
 if(error){
  throw error;
  // or do something else like
  console.log(error.stack)
  return;
 }
 console.log('data=',data)
});

passing the callback on to next function:
function simple3(a,b,cb)
{
 a++;
 return simple2(a,b,cb);
}

using anonymous function to pass arguments as closure:
function simple3(a,b,cb)
{
 var preva=a;
 a++;
 return simple2(a,b,function(e,data){ if(cb) cb(e,data,preva)});
}

2-callback advanced tutorial.js

//use the name cb all over the place and it just works:
var async=require('async')

var items=[1,2,3]

function calc(a,cb)
{
  cb(a*a)
}

function simple4(items,cb)
{
 var results={};
 async.eachLimit(Object.keys(items),4,function(k,cb)
 {
   var item=items[k]
   result[k]=calc(item,cb)
 },function(){
  console.log('done all');
  cb(results);  
 });
}

// a short cut cb
function simple5(items,cb1)
{
 var results={};
 function cb(){cb1(results)} // sometimes people call the function next instead of cb

 async.eachLimit(Object.keys(items),4,function(k,cb)
 {
   var item=items[k]
   result[k]=calc(item,cb)
 },function(){
  console.log('done all');
  cb();  
 });
}


//while loop:

function simple6(times,cb)
{
  function loop()
  {
   times--;
   if(times==5)return loop();// use return iterator() as continue
   // do something here
   if(times>0)return loop();  else return cb()// use return cb() as break
  }
  if(times>0)loop();  else cb()
}


// retry
//from this:
function simple7(options,cb)
{
    do_async_request(options,function(err,data){
      if(err){
                console.log('simple7 - error, faild',err.stack,options);
                return;
      }
      //do work with data
    })
}
//to this:
function simple7_with_retry(options,cb)
{
  var retries=0;
  function retry()
  {
    do_async_request(options,function(err,data){
      if(err){
              if(retries<7)
              {
                retries++;
                console.log('simple7 - error, retrying',err.stack,options);
                setTimeout(retry,3000)
                return;
              }
              else {
                console.log('simple7 - error, faild',err.stack,options);
                return;
              }
      }
      //do work with data
    })
  }
  retry()
}