Implement a callback function inside a function

Question

Say I have a function like the following.

loadXML('Method', requestString, function(callback){
   // The function as a callback parameter to loadXML goes here.

   // Here I am calling the callback function like this       
   if(callback != null){
       callback();
   }
});

But I want to define this callback function inside the loadXML function. So can I do this as following?

loadXML('Method', requestString, function(callback){
   // The function as a callback parameter to loadXML goes here.

   // Here I have to call the callback function like this, (do I?) which is a 
   // callback parameter to callback function of loadXML
   callback = function(){
       // The callback function implementation goes here
   }
});

Answer 1

Maybe this could help you to understand the nested callbacks mechanism:

var loadXML;
var outerCB;
var innerCB;     

loadXML = function(method, requestString, cb) {
  // pass the innerCB implementation as argument to the outer cb
  if('undefined' !== typeof innerCB) {
    cb(innerCB);
  } else {
    // in case innerCB would not be defined
    cb(function() {
      console.log('hi from anonymous cb')
    });
  }
};


innerCB = function() {
  console.log('hi from innerCB cb!')
};

outerCB = function(callback) {
  if('undefined' !== typeof callback) {
    callback(); // innerCB();    
  } else {
    console.log('no cb passed, do something else')
  }    
}

loadXML('Method', 'abcd', outerCB) // hi from innerCB cb!