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
 }
});

• Ismael Miguel: 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?
• Quentin: 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?
• vidulaJ: 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.
• Quentin: "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?
• vidulaJ: 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?

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! 

• vidulaJ: Thanx Villiam. I'd go with this & try this.