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
   }
});

I think that the callback is a parameter to loadXML() and isn't passed anywhere else. Did you try console.log(arguments); inside the function? — Ismael Miguel
– Ismael Miguel, Commented Oct 22, 2014 at 9:22
Why would you define a function and then not use it? And if you aren't passing it into the function, it isn't really a callback anyway. What are you actually trying to achieve? — Quentin
– Quentin, Commented Oct 22, 2014 at 9:28
There is also a callback function for loadXML() which is defined there and I want to have another callback function to that function. Let me update the code then. — vidulaJ
– vidulaJ, Commented Oct 22, 2014 at 9:28
"Here I have to call the callback function like this" — That overwrites the callback function with a new function and doesn't call it at all. What are you trying to achieve? — Quentin
– Quentin, Commented Oct 22, 2014 at 9:36
I am trying to implement a callback function to a callback function inside the former callback function. I just want to know whether I am doing it right or what the right method is? — vidulaJ
– vidulaJ, Commented Oct 22, 2014 at 9:39

Viliam · Accepted Answer · 2014-10-22 11:02:44Z

3

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!

answered Oct 22, 2014 at 11:02

Viliam

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1 Comment

vidulaJ Over a year ago

Thanx Villiam. I'd go with this & try this.

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